Section 1 Clinical Mycology

**3**

**Chapter 1**

**1. Introduction**

**2. Aspergillosis**

(facilitated by low CD4+

Introductory Chapter:

Infection - An Overview

*Erico S. Loreto and Juliana S.M. Tondolo*

Epidemiology of Invasive Fungal

Invasive fungal infections (IFIs) are a significant cause of morbidity and mortality

in hospitalized patients and the immunocompromised populations. Candidemia, invasive aspergillosis, mucormycosis, cryptococcosis, and *Pneumocystis* pneumonia (PCP) are IFIs associated with the highest incidence and mortality. The broader use of more aggressive treatment modalities, such as hematopoietic stem cell transplantation (HSCT) and solid organ transplantation (SOT), as well as chemotherapy for cancer patients and prolonged corticosteroid therapy, has increased the population of immunocompromised patients at risk for IFIs. Other groups at risk include individuals who have HIV/AIDS in which PCP is an AIDS-defining disease [1]. In this chapter,

we aim to overview the epidemiology of the leading causes of IFIs in humans.

The genus *Aspergillus* contains more than 300 species described and is divided into 20 sections [2]. However, only a few are known to cause human disease. Human aspergillosis is primarily caused by *Aspergillus fumigatus* (the most common species described in aspergillosis cases), *A. flavus*, *A. niger*, *A. terreus*, and *A. nidulans*. *Aspergillus* species are ubiquitous, are found in soil and several organic debris, and produce conidia that are easily aerosolized. These conidia, when inhaled, can colonize the host's lungs, which can develop various clinical syndromes depending on their degree of immunocompetence. Ingestion of spores via the gastrointestinal tract or direct inoculation via skin injuries is an uncommon way of inoculation [3–5]. The major risk factors for infection include prolonged neutropenia, HSCT, SOT, corticosteroid therapy, chronic granulomatous disease, immunosuppressive treatment for malignancies, hematologic malignancy, myelodysplastic syndrome or aplastic anemia, advanced stage of human immunodeficiency virus (HIV) infection

rus infection), and patients with critical illness [4, 6]. The spectrum of disease is determined by the host's immune status and the virulence of *Aspergillus* species. In immunocompetent hosts, aspergillosis causes mainly allergic symptoms without invasion and destruction of the host's tissues and chronic pulmonary aspergillosis. Allergic bronchopulmonary aspergillosis (ABPA) is a syndrome that arises from a hypersensitivity reaction to antigens from *Aspergillus* and may be developed in patients with asthma and cystic fibrosis [7]. In the chronic pulmonary aspergillosis, a preexisting pulmonary condition is generally observed. Chronic cavitary

cell counts), previous infections (such as cytomegalovi-

#### **Chapter 1**

## Introductory Chapter: Epidemiology of Invasive Fungal Infection - An Overview

*Erico S. Loreto and Juliana S.M. Tondolo* 

#### **1. Introduction**

Invasive fungal infections (IFIs) are a significant cause of morbidity and mortality in hospitalized patients and the immunocompromised populations. Candidemia, invasive aspergillosis, mucormycosis, cryptococcosis, and *Pneumocystis* pneumonia (PCP) are IFIs associated with the highest incidence and mortality. The broader use of more aggressive treatment modalities, such as hematopoietic stem cell transplantation (HSCT) and solid organ transplantation (SOT), as well as chemotherapy for cancer patients and prolonged corticosteroid therapy, has increased the population of immunocompromised patients at risk for IFIs. Other groups at risk include individuals who have HIV/AIDS in which PCP is an AIDS-defining disease [1]. In this chapter, we aim to overview the epidemiology of the leading causes of IFIs in humans.

#### **2. Aspergillosis**

The genus *Aspergillus* contains more than 300 species described and is divided into 20 sections [2]. However, only a few are known to cause human disease. Human aspergillosis is primarily caused by *Aspergillus fumigatus* (the most common species described in aspergillosis cases), *A. flavus*, *A. niger*, *A. terreus*, and *A. nidulans*. *Aspergillus* species are ubiquitous, are found in soil and several organic debris, and produce conidia that are easily aerosolized. These conidia, when inhaled, can colonize the host's lungs, which can develop various clinical syndromes depending on their degree of immunocompetence. Ingestion of spores via the gastrointestinal tract or direct inoculation via skin injuries is an uncommon way of inoculation [3–5].

The major risk factors for infection include prolonged neutropenia, HSCT, SOT, corticosteroid therapy, chronic granulomatous disease, immunosuppressive treatment for malignancies, hematologic malignancy, myelodysplastic syndrome or aplastic anemia, advanced stage of human immunodeficiency virus (HIV) infection (facilitated by low CD4+ cell counts), previous infections (such as cytomegalovirus infection), and patients with critical illness [4, 6]. The spectrum of disease is determined by the host's immune status and the virulence of *Aspergillus* species.

In immunocompetent hosts, aspergillosis causes mainly allergic symptoms without invasion and destruction of the host's tissues and chronic pulmonary aspergillosis. Allergic bronchopulmonary aspergillosis (ABPA) is a syndrome that arises from a hypersensitivity reaction to antigens from *Aspergillus* and may be developed in patients with asthma and cystic fibrosis [7]. In the chronic pulmonary aspergillosis, a preexisting pulmonary condition is generally observed. Chronic cavitary

pulmonary aspergillosis (aspergilloma or fungus ball) is the best-recognized form of pulmonary involvement due to *Aspergillus*, usually occurring in a preformed cavity in the lung (due to tuberculosis, sarcoidosis, or other necrotizing pulmonary processes) or in the paranasal sinuses [8, 9]. Subacute invasive aspergillosis (also called chronic necrotizing pulmonary aspergillosis) is a locally destructive invasion of lung parenchyma without invasion or dissemination to other organs [9, 10].

 In immunocompromised patients, invasive aspergillosis (IA) can be a rapidly, progressive and frequently fatal disease. Invasive pulmonary aspergillosis (IPA) and rhinocerebral aspergillosis are the most common clinical forms of IA. Other clinical conditions included tracheobronchitis, invasive *Aspergillus* infection of the eye or heart, gastrointestinal invasive aspergillosis, cutaneous aspergillosis, and disseminated invasive aspergillosis [5]. Data from the Transplant-Associated Infection Surveillance Network (TRANSNET) [11] described that in HSCT recipients, invasive aspergillosis was the most common IFI (425 cases, 43%), followed by invasive candidiasis (276 cases, 28%) and zygomycosis (77 cases; 8%). One-year overall mortality rate reaches 75% [11]. In the Prospective Antifungal Therapy Alliance (PATH Alliance®) registry, from a cohort study of 960 cases of proven/probable IA, 48.3% of patients had hematologic malignancy, 29.2% received SOT, 27.9% were HSCT recipients, and 33.8% were neutropenic. The lung was the organ most frequently affected (76% of cases). The tracheobronchial tree, sinuses, skin, soft tissues, and the central nervous system were the most common extrapulmonary sites of infections. The most predominant species was *A. fumigatus* (72.6%), followed by *A. flavus* (9.9%), *A. niger* (8.7%), and *A. terreus* (4.3%). Overall Kaplan-Meier survival (12-week post-diagnosis) among all patients with IA was 64.4%.

#### **3. Candidiasis**

*Candida* species are ubiquitous yeasts, being frequent colonizers of the skin and normal flora of mucocutaneous membranes of humans. Also, it was also recovered from soil, hospital environment, food, inanimate objects, and nonanimal environments [12]. *Candida albicans*, *Candida dubliniensis*, *Candida glabrata*, *Candida guilliermondii*, *Candida intermedia*, *Candida kefyr*, *Candida krusei*, *Candida lusitaniae*, *Candida parapsilosis*, *Candida pseudotropicalis*, *Candida stellatoidea*, and *Candida tropicalis* are the main species associated with candidiasis, although more than 200 species of *Candida* have been identified.

 *Candida albicans* remains the predominant species in most studies [13]. However, a shift in the etiology can be observed in different regions of the world [14]. For example, in northwestern Europe and the United States, *Candida glabrata*  is generally recovered as the most common species, whereas in Southern Europe, some Asian countries and Latin America, *Candida parapsilosis* and *Candida tropicalis* are more frequently recovered than *Candida glabrata*. Of notable concern is the emergence of *Candida auris*, a multiresistant species associated with outbreaks of candidemia in many countries that presents a serious global health threat [12, 14–16].

As opportunistic pathogens, *Candida* infections can occur due to factors related to the host, the microorganism, or both. The three major conditions that predispose the human infection are: (i) the use of broad-spectrum antibiotics (long-term and/or repeated use), (ii) mucosal barrier breakdown, such as those induced by cytotoxic chemotherapy and medical interventions, and (iii) iatrogenic immunosuppression, such as corticosteroid therapy or chemotherapy-induced neutropenia [15]. Long hospital or intensive care unit (ICU) stay is the most common health

#### *Introductory Chapter: Epidemiology of Invasive Fungal Infection - An Overview DOI: http://dx.doi.org/10.5772/intechopen.85955*

care-associated risk [17]. Among the several virulence factors described for *Candida*, (i) the ability of most species to switch between yeast, pseudohypha, and hyphae morphotypes; (ii) the secretion of a variety of factors, such as secreted aspartyl proteases, phospholipases and candidalysin toxin; and (iii) the effective capacity of adherence (mediated by proteins such as agglutinin-like protein 3) and biofilm formation are the main microorganism-related factors that contribute to candidiasis [15].

 The incidence of *Candida* infections varies according to several epidemiological and geographic characteristics. *Candida* species are among the top four main pathogens causing health care-associated bloodstream infections, particularly in ICU, affecting 250,000 people and causing more than 50,000 deaths worldwide every year, based on conservative estimates [18–20]. In an international study of prevalence and outcomes of infection in ICU, *Candida* was the third most common cause of infection (17%), after *Staphylococcus aureus* (20.5%) and *Pseudomonas*  species (19.9%) [21].

 *Candida* was the most common fungal pathogen that causes invasive infection in SOT population [22]. In bone marrow transplantation (BMT) under fluconazole prophylaxis, *Aspergillus* species replaced *Candida* as main cause of IFI [11]. Newborn infants [23], HIV-infected patient (without the use of antiretroviral therapy) [24], and patients who underwent abdominal surgery [25] are other populations at increased risk for *Candida* infections. Unadjusted mortality rates vary widely (from 29 to 76%) for candidemia. In the United States, the attributed mortality rate ranges from >30 to 40% and the median cost for inpatient care was \$46,684 [15, 19, 26, 27].

#### **4. Cryptococcosis**

*Cryptococcus neoformans* and *Cryptococcus gattii* are the two species that commonly cause cryptococcosis in humans. Historically, these species were classified into three varieties, five serotypes, and eight molecular subtypes. However, based on phylogenetic and genotyping studies, it was proposed to split *Cryptococcus neoformans* into two species (*Cryptococcus deneoformans* and *Cryptococcus neoformans*) and *Cryptococcus gattii* into five species (*Cryptococcus bacillisporus*, *Cryptococcus decagatti*, *Cryptococcus deuterogattii*, *Cryptococcus gattii*, and *Cryptococcus tetragattii*) [28]. Nonetheless, considering that more data about the genetic diversity of *Cryptococcus* were recently described and the absence of defined biological and clinical differences between the seven new species, some authors recommend the use of "*Cryptococcus neoformans* species complex" and "*Cryptococcus gattii* species complex" as a practical intermediate step until this species differentiation is clinically relevant [29].

*Cryptococcus neoformans* has been isolated in decaying material within hollows of several tree species, fruit, and soil enriched by avian excreta (such as feral pigeons) and is globally distributed. *Cryptococcus gattii* is classically associated with eucalyptus tree and limited to tropical and subtropical regions. However, recent outbreaks in Canada, Northern Europe, and Northern USA suggest that the ecological range of this species may not be fully recognized. Both species can survive and replicate in environmental scavengers such as free-living amoebae and nematodes [30, 31]. The respiratory tract is the main portal of entry for the aerosolized infectious particles from the disrupted and contaminated environment (soil, tree, or bird droppingsenriched areas). Lung and the central nervous system (CNS) are the primary sites of infection, but eyes, prostate, and skin can be frequently involved. Traumatic inoculation may occur but is infrequent [31–33].

 HIV infection, idiopathic CD4<sup>+</sup> lymphopenia, corticosteroid treatment, SOT, malignant and lymphoproliferative disorders, sarcoidosis, treatment with some monoclonal antibodies (such as alemtuzumab, infliximab, etanercept, adalimumab, or anti-GM CSF), rheumatologic diseases (such as systemic lupus erythematosus and rheumatoid arthritis), chronic liver disease, renal failure and/or peritoneal dialysis, hyper-IgM syndrome or hyper-IgE syndrome are the main risk factors for cryptococcosis [31, 33, 34].

 *Cryptococcus* infections in humans were considered uncommon before the 1970s. Cryptococcosis incidence increased significantly in the HIV epidemics in the 1980s. The overall incidence of 0.8 cases per million persons per year in the pre-AIDS era reached almost five cases per 100,000 persons per year in the peak of the AIDS epidemic. The incidence of cryptococcosis declined and stabilized from the mid-1990s with the use of fluconazole for the treatment of oral candidiasis and with the widespread use of active antiretroviral therapy (ART) [34–36]. However, HIV-associated cryptococcosis mortality remains unacceptably high, and globally, cryptococcal meningitis accounts for 15% of AIDS-related deaths. Cryptococcal infection-related deaths were estimated at 181,100 globally, with 75% (135,900) occurring in sub-Saharan Africa [37–39].

 In HIV-negative individuals, cryptococcosis occurs in transplant recipients and other patients with primary or acquired defects in cell-mediated immunity [32]. In a recently multicenter, longitudinal cohort study in the United States [40], the demographics of 145 HIV-negative patients with cryptococcosis demonstrated that SOT (49 cases, 33.8%) was the main underlying disease, followed by autoimmune syndromes (15.9%), hematologic malignancy (11.7%), decompensated liver disease (9.7%), solid tumor (5.6%), primary immunodeficiency (2.1%), and HSCT (2.8%). Glucocorticoid therapy and cytotoxic chemotherapy were the immunosuppressive medications described for more than 40% of patients. CNS involvement was observed in 71 patients (49%).

#### **5. Mucormycosis**

*Rhizopus* is the most common genera causative of human disease, followed by *Mucor*, *Lichtheimia*, *Apophysomyces*, *Rhizomucor*, and *Cunninghamella* species*.* Less frequently, members include *Actinomucor*, *Cokeromyces*, *aksenaea*, and  *yncephalastrum* [41–43]. These members from Mucorales family are ubiquitous in the environment, are taken by the host via inhalation of spores or ingestion of contaminated food, but rarely cause infection without obvious predisposing host factors [44].

 Uncontrolled diabetes, hematological malignancy, malnutrition, solid organ transplantation, hematopoietic stem cell transplant, and liver disease are the primary underlying conditions associated with mucormycosis. Predisposing factors include corticosteroid use, neutropenia, trauma, anticancer therapy, use of calcineurin inhibitors, biological and renal replacement therapies, prior antifungal prophylaxis (e.g., voriconazole), iron overload and deferoxamine therapy [41, 42, 44].

Rhinocerebral, pulmonary, cutaneous, gastrointestinal, and disseminated mucormycosis are the common types of disease described. The mortality and morbidity rates are dependent on affected organ, Mucorales species, and medical status of the patient. Mucormycosis can be an extremely aggressive disease, and mortality rates can reach 46% in sinus infection, 73% in mucormycosis after exposure to voriconazole, 76% in pulmonary disease, and 96% in disseminated infections [42, 45].

Based on autopsy reports [46], mucormycosis is the third most common cause of invasive fungal infection, after candidiasis and aspergillosis. In developed countries, hematologic malignancies and hematopoietic stem cell transplantation *Introductory Chapter: Epidemiology of Invasive Fungal Infection - An Overview DOI: http://dx.doi.org/10.5772/intechopen.85955* 

are the leading underlying conditions in mucormycosis cases while in developing countries, particularly in India, the major causes of the disease are associated with uncontrolled diabetes or trauma [43, 47]. Data from Transplant-Associated Infection Surveillance Network show that mucormycosis (formerly zygomycosis) was the third most common IFI (8%) in HSCT [11] and sixth most common IFI (2%) among organ transplant recipients [22].

#### **6. Pneumocystis**

*Pneumocystis jirovecii* (previously *Pneumocystis carinii* f. sp. hominis) is an opportunistic pathogen causing pneumonia in patients with immunodeficiencies and can colonize the lung of healthy individuals. Initially classified as a protozoan species, it is now recognized as a fungus based on phylogenetic data and the genus comprising a group of highly diversified species with a high degree of hosts-species specificity [48]. The environmental reservoir was not identified so that the mammalian hosts can be considered as reservoirs. Indeed, it was demonstrated that close person-toperson contact could facilitate the transmission, and nosocomial transmission has been reported [48, 49].

Despite the genus *Pneumocystis* being known for years, its life cycle remains poorly understood, principally by the lack of a reliable continuous culture system. The hypothesized life cycle comprises different morphologic forms: trophozoites, cysts, and intracystic bodies (sporozoites) and all these forms reside in the alveoli of the lung with the cyst being considered the infectant and transmissible form [48, 50]. Evidence suggests that the gateway to infection is through inhalation since controlled studies in different animal models have demonstrated airborne transmission [48, 51]. As the organism is host specific, transmission from animals to humans is unlikely [51].

 The occurrence of *Pneumocystis* pneumonia (PCP) is related to severely immunocompromised people, principally in HIV/AIDS patients, and with other immunosuppressed conditions, that is, cancers, autoimmune disorders, transplantation, chronic lung disease, especially obstructive pulmonary disease (COPD) [48]. Colonization rates have been reported on the order of 20–69% for HIV patients, from 0 to 20% for healthy adults, and in 6% of organ transplant recipients if no prophylaxis is given [51]. Primary exposure appears to occur at early childhood as demonstrated by the seroconversion seen in 85% of children up to 20 months of age [52]. Colonization of both children and adults may be a source of transmission of *Pneumocystis jirovecii*, serving as potential reservoirs. Guidelines for prevention and treatment of opportunistic infections in HIV-infected adults and adolescents include: treating patients with PCP together with prophylaxis of susceptible individuals (HIV patients with CD4 counts of <200 cells/μl or CD4 percentages of <14%); it is also recommended that a patient with PCP should not be placed in the same room with an immunodeficient patient. The prophylaxis among transplant recipients has been proved to be the most effective approach for ending outbreaks of PCP [48, 53].

#### **7. Conclusions**

The changes in the spectrum of the fungal infections associated with new risk factors and the emergence of resistant fungi highlight the necessity of a continuous update on knowledge of the epidemiology of fungal infections. Besides, the reduction of mortality among patients with IFIs must be accompanied by research that allows the development of new antifungal treatment strategies and earlier diagnosis by traditional and non-culture-based molecular tests.

*Fungal Infection* 

#### **Author details**

Erico S. Loreto\* and Juliana S. M. Tondolo Sobresp Faculdade de Ciências da Saúde, RS, Brazil

\*Address all correspondence to: erico.loreto@gmail.com

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Introductory Chapter: Epidemiology of Invasive Fungal Infection - An Overview DOI: http://dx.doi.org/10.5772/intechopen.85955* 

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[45] Trifilio SM, Bennett CL, Yarnold PR, McKoy JM, Parada J, Mehta J, et al. Breakthrough zygomycosis after voriconazole administration among patients with hematologic malignancies who receive hematopoietic stem-cell transplants or intensive chemotherapy. Bone Marrow Transplantation. 2007;**39**:425-429. DOI: 10.1038/ sj.bmt.1705614

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#### **Chapter 2**

## *Cryptococcus neoformans*-Host Interactions Determine Disease Outcomes

*Jintao Xu, Peter R. Wiliamson and Michal A. Olszewski* 

#### **Abstract**

The fungal pathogen *Cryptococcus neoformans* can infect the central nervous system (CNS) and cause fatal meningoencephalitis, which accounts for an estimated 180,000 deaths per year. Cryptococcal meningoencephalitis (CM) occurs mainly in the individuals with compromised immune systems. Thus, cryptococcal disease in the CNS has been predominantly attributed to insufficient immune responses and subsequent uncontrolled fungal growth. However, evidence has emerged that an inappropriate immune response, as much as an insufficient response, may promote clinical deterioration and pathogenesis. In this chapter, we will review the different types of immune responses to *C. neoformans* and their contribution to tissue damage and diseases.

**Keywords:** *Cryptococcus neoformans*, pathogenesis, immune pathology

#### **1. Introduction**

 The human fungal pathogen *Cryptococcus neoformans* causes substantial morbidity and mortality worldwide, with an estimated 1 million infections and 180,000 deaths per year [1–3]. Although the primary route of infection is through inhalation of yeast into the lungs, fungal dissemination to the central nervous system (CNS) leads to severe meningoencephalitis that can cause death or long-lasting neurological sequelae, including memory loss, vision deficiencies, hearing and speech impairments, and motor deficits [4–6]. Treatment options for cryptococcal meningoencephalitis (CM) are limited and often unsuccessful due to the increasing development of drug resistance, the high toxicity of the antifungal drugs and the poor permeability of the blood brain barrier [7–9]. Unsuccessful treatments are often accompanied with high mortality rates up to 15% and relapse rates of 30–50% [10–12]. Thus, there is a pressing need for understanding the pathogenesis of *C. neoformans* infection to develop more effective therapeutic strategies.

Cryptococcal infections usually manifest in patients who are immunocompromised secondary to HIV infection, cancer therapies, or organ transplantation [3]. This has led to the characterization of *C. neoformans* as an "opportunistic pathogen" that causes disease only when the immune system cannot control its growth. Prior studies have established a central role for T cell mediated immunity in fungal clearance from the lungs and suggested that T-cell mediated immunity is also beneficial in the CNS [13–25]. These studies also support a paradigm that clinical failures are predominantly due to a deficiency in microbiological control. However, attempts

to develop immunotherapies that enhance the immune responses in CM have been largely unsuccessful, indicating other factors may also participate in the disease pathology [26]. Furthermore, clinical and experimental studies increasingly show that an exaggerated host immune response can promote cryptococcal pathogenesis. For example, a common complication of CM in HIV/AIDS patients is the immune reconstitution inflammatory syndrome (cIRIS) that develops after initiation of anti-retroviral therapy [27, 28]. A parallel syndrome occurs among non-HIV patients with severe cryptococcal CNS infection, termed post infectious immune inflammatory syndrome (PIIRS) [29–31]. These patients develop severe neurological sequela and morbidity with persisting inflammatory responses, often despite fungal eradication [28, 32–36]. A detrimental role of host inflammation is further supported by the therapeutic effects of corticosteroids in ameliorating IRIS and PIIRS symptoms and by observations that premature or abrupt steroid-weaning may result in the recurrence of CNS lesions and clinical relapse [29, 30, 37].

This evidence challenges the view that cryptococcal disease is a consequence of a compromised immune system. Instead, the outcomes of cryptococcal disease can be better understood as a balance of *C. neoformans*-host interactions. The effect of *C. neoformans* on host disease can be explained by the damage-response framework (DRF), a theory for microbial pathogenesis proposed in 1999 [38]. The DRF theory incorporates the contributions of host-microbe interaction, rather than presenting microbial pathogenesis as a singular outcome of either microbial factors or host factors. The results of host-microbe interaction can be visualized with a single parabola depicting host damage as a function of the strength of the immune response [26]. Weak host immune responses due to HIV infection or immunosuppressive therapies fail to control fungal growth, which results in fungus-mediated host damage. However, strong immune responses elicited by *C. neoformans* can also lead to host damages and diseases. In this chapter, we will review recent human clinical and experimental animal studies that have enhanced our understanding of the complex mechanisms involved in immunopathogenesis during *C. neoformans* infection. Uncovering the mechanisms that are involved in anticryptococcal host defense or in immunopathogenesis will facilitate the discovery of new intervention strategies to treat cryptococcal infections.

#### **2. Cryptococcal immune reconstitution inflammatory syndrome**

*C. neoformans* can cause infection in both the meninges and the Virchow-Robin channels surrounding the penetrating vessels within the brain parenchyma [39]. Although the exact mechanism by which this encapsulated pathogen migrates into the CNS is currently unclear, studies have found that circulating *C. neoformans* was trapped in the brain capillary and can actively transmigrate the microvasculature with contributions from urease and metalloprotease [40–42]. After migration, *C. neoformans* causes fatal meningoencephalitis which accounts for 15–20% of AIDSrelated deaths [1, 43, 44]. The high fungal burdens during CM in AIDS are associated with mortality, suggesting a prominent role of the fungal pathogen for host damage [45]. Thus, in HIV infection/AIDS, susceptibility to CM is thought to occur due to lack of T cell-mediated fungal clearance. Indeed, studies have shown that presence of CSF cytokine and chemokine responses consisting primarily of IL-6, IFN-γ, IL-8, IL-10, IL-17, CCL5 and TNF-α, are associated with increased macrophage activation, more rapid fungal clearance from the CSF, and patient survival [45]. The overall low levels of cytokine production in AIDS patients and insufficient activation of resident or recruited macrophages in the absence of T cells producing IFN-γ/TNF-α lead to uncontrolled fungal growth [45, 46].

#### *Cryptococcus neoformans-Host Interactions Determine Disease Outcomes DOI: http://dx.doi.org/10.5772/intechopen.83750*

 Antiretroviral therapy (ART) in AIDS patients rapidly restores host T cell responses. However, in a portion of patients it leads to a highly lethal complication, cIRIS, which is defined as a paradoxical clinical deterioration after initiation of ART, despite efficient control of fungal infection [28]. cIRIS occurs in 15–30% of HIV-infected individuals with cryptococcosis [28, 47]. Similarly, patients who undergo immune suppressive regiments during bone marrow transplantation or autoimmune diseases can develop cIRIS like syndromes once the host immune response is restored when immunosuppressive therapy is tapered [48]. Previous studies have found that paucity of initial CSF inflammation, low IFN-γ levels, and high fungal loads are risk factors for the development of IRIS [27, 45]. During cIRIS, the immune response in the brain is characterized by excessive activation of Th1 CD4+ subsets with elevated production of cytokines including IFN-γ and TNF-α [27, 33, 49].

 While the exact pathogenic mechanisms of IRIS have not been unraveled, the lymphopenic environment during HIV infection may result in abnormal function of residual CD4 T cells, rendering them more pathogenic as the population expands after ART [50]. Furthermore, it has also been proposed that there exists a decoupling of innate and adaptive immune responses in AIDS patients prior to ART due to deficient T cell responses, which sets the stage for excessive inflammation after T cell reconstitution. Indeed, several lines of evidence show that mononuclear immune cells are implicated in cIRIS. Predisposition to cIRIS has been shown to be associated with higher CCL2/MCP-1, CCL3/MIP-1α, and GMCSF production in the CSF, which promotes trafficking and activation of macrophages in the infection sites [45]. Patients with cIRIS had increased numbers of proinflammatory intermediate monocytes (CD14highCD16+) which produce reactive oxygen species [51, 52]. Although macrophages can be primed by fungal pathogens in AIDS patients prior to ART, they never become fully activated in the absence of T cell help to exert their effector functions in fungal clearance. This results in high levels of pathogen replication as the disease progresses. Nevertheless, increasing numbers of primed macrophages accumulate and create a state of immunological hyperresponsiveness to the subsequently CD4+ T cell help. ART rapidly restores Th1 type response in the host with high level of IFN-γ production. Large numbers of primed macrophages then become fully activated to produce an acute spike in proinflammatory mediators, which may drive immunopathology during cIRIS. Thus, macrophage activation in cIRIS may act in concert with T-cell responses resulting in tissue-destructive inflammatory responses.

The mechanisms of tissue damage by host inflammation during fungal infections are still under active research. Macrophage or T cell production of TNF-α, IL-1β, reactive oxygen species (ROS) and nitrogen species (RNS), may contribute to irreversible tissue damage and/or lead to neuronal apoptosis [53–55]. *C. neoformans*  infection also induces cerebral edema and raised CSF pressure that are associated with symptoms including headache, nausea, and mental status deterioration.

#### **3. Postinfectious inflammatory response syndrome**

Another example showing that strong host immune responses during *C. neoformans* infection can induce immunopathology is post-infectious inflammatory response syndrome (PIIRS). It is characterized in non-HIV patients during initial therapy by severe mental deficits despite antifungal therapy and their apparent immunocompetent state. Reports showed that up to 25% of cases in the United States and 60% in the Far East occur in apparently immune competent patients [56, 57]. Despite antifungal therapy and negative CSF-*C. neoformans* cultures, PIIRS patients

many times require ventricle-peritoneal shunts to relieve the high CSF pressure caused by inflammation. Recent studies have shown that patients with PIIRS exhibit strong intrathecal Th1 responses with high levels of IFN-γ production and a relatively lack of Th2 responses [30]. Importantly, elevated levels of CSF neurofilament light chain (NFL), a marker of axonal injury, indicate ongoing immunological host neuron damage. Interestingly, macrophages recruited to the CNS infection site are often alternatively activated (M2) and exhibit poor phagocytic effect during PIIRS [30]. This apparent Th1-M2 discrepancy suggests that PIIRS patients may have downstream defects in monocyte activation. New therapies that consider immune-mediated host injury may decrease mortality in these severe or refractory clinical cases [43].

#### **4. Animal models of IRIS and PIIRS**

Detrimental roles for immune responses in the pathogenesis of cryptococcusassociated IRIS or PIIRS have also been recently demonstrated in experimental mouse models. A recent study in our lab established a reproducible mouse model of CM using C57BL/6 mice infected intravenously with 106 CFU of *C. neoformans*  strain 52D [58]. Using this model, we found that infected mice displayed overt and severe symptoms similar to that of human patients, including increased cranial pressure, ataxia, and limb paralysis after 21 days post infection (dpi). Importantly, over 50% of animals succumbed to infection between 21 and 35 dpi, despite apparent fungal control in the CNS (**Figure 1**). Thus, we showed that the magnitude of CNS fungal burden does not directly correlate with the intensity of disease symptoms or mortality during CM.

Brain cellular inflammation, marked by leukocyte accumulation after 21 dpi and dominated by CD4+ T cell infiltration, plays an important role in the pathology of the CNS in cryptococcal-infected mice. Similar to human patients with IRIS and PIIRS, infiltrating CD4+ T cells in brains of cryptococcal-infected mice exhibit a Th1-type bias and produce high levels of IFN-γ. Critically, the influx of immune cells into the CNS after 21 dpi was synchronized with the onset of fungal clearance, development of neurological symptoms, and mortality. The depletion of CD4 + T cells leads to a reduction in mortality and inflammatory pathology, providing conceptual evidence that CD4 + T cells are a principal mediator of inflammation and pathology in this model. Notably, over the course of the study, the survival of CD4+

#### **Figure 1.**

*Depletion of CD4+ T cells resulted in improved survival despite higher fungal burden during CM. C57BL/6 mice were infected with 106 CFU of C. neoformans 52D via retro-orbital intravenous inoculation. (A) Fungal burdens were measured in whole-brain homogenates at the indicated time points. Naive mice and animals that succumbed to infection (†) are indicated. (B) Representative images of severe cranial swelling and CNS tissue injury in infected mice. (C) Survival of infected CD4-depleted (broken line) and isotype-treated mice through 35 dpi. (D) Brain fungal burdens were calculated on day 21 and 35 dpi. \*, P < 0.05; \*\*, P < 0.01; \*\*\*\*, P < 0.0001. Reproduced from Neal et al., [58].* 

#### *Cryptococcus neoformans-Host Interactions Determine Disease Outcomes DOI: http://dx.doi.org/10.5772/intechopen.83750*

T cell depleted mice significantly improved despite having higher fungal loads in the CNS compared to mice with sufficient CD4+ T cells (**Figure 1**). Depletion of CD4+ T cells during CM also broadly inhibited all other aspects of the CNS inflammatory response, including accumulation of CD8+ T cells and CD11b + Ly6C+ myeloid effector cells. Taken together, these data strongly support the idea that CD4+ T cells exert dual but opposing roles during CM: promoting the elimination of the fungal pathogen in the CNS but simultaneously driving tissue damage, neurological deterioration, and death.

 Another animal model has also demonstrated the pathological role of CD4+ T cells in cIRIS. Eschke and colleagues reconstituted RAG<sup>−</sup>/<sup>−</sup> mice, which are deficient in T and B cells, with WT CD4+ T cells after infection with *C. neoformans* [59]. They found that mice receiving CD4 T cells displayed high levels of Th1-type cytokines such as TNF-α and IFN-γ compared to mice not receiving CD4+ T cells. These results suggested that CD4+ T cell reconstitution in mice infected with *C. neoformans* may lead to syndromes similar to IRIS in HIV-infected patients [59]. These animal models provide important tools for further investigating the mechanism of cryptococcal pathology.

#### **5. Host immunity to** *C. neoformans* **infection: protective or nonprotective, the yin and yang**

 Protective immunity is conferred by a fine balance between immune responses that eliminate the pathogen and those that limit host damages. However, an immune response induced by the pathogen may be non-protective for any one or combination of the following reasons: (1) it could occur in the wrong location or timeframe, promoting inflammatory injury without effective clearance of pathogens; (2) it could be too strong and cause immunopathology despite control of pathogen burden; 3) regulatory mechanisms meant to maintain host tissue integrity may lead to microbial survival and persistence and thus result in chronic inflammation. Below, we describe cellular and molecular mechanisms by which dysregulation of immune responses contribute to host disease during infection with *C. neoformans*.

#### **5.1 Host immune responses contribute to fungal clearance but also tissue damage**

 Upon infection, *C. neoformans* is sensed by a variety of innate receptors including Toll-like receptors [60–62], mannose receptors [60], and β-glucan receptors [63–65]. Macrophages [66, 67], DCs [68], natural killer cells [69–72] and neutrophils [73] have been shown to mediate killing *C. neoformans*, however, the development of the adaptive immune response is required for controlling the fungal growth in the host [74–76]. Specifically, the development of Th1 and Th17 responses that are associated with classical activation of macrophages (M1) promotes fungal clearance in both humans and experimental mouse models [14, 77]. DCs and macrophages function as the potential sensors for infection through PRRs or inflammasomes, and produce cytokines such as IL-12, IL-23, IL-6, IL-18, TNF-α and IL-1β, which have been shown to promote the Th1/Th17 response during *C. neoformans* infection [78–84]. During this response, Th1 and Th17 cells produce cytokines such as IFN-γ, IL-17 and IL-22, which act on macrophages, neutrophils or epithelial cells and induce robust antimicrobial and phagocytic responses, including the production of reactive oxygen and nitrogen species [16, 85–90]. As a result, resident and/or recruited macrophages and DCs can become highly activated and function as the main effector cells for controlling fungal infection [91, 92].

**Figure 2.** 

*RIPK3 and FADD modulate host responses against C. neoformans infection. (A) Ripk3<sup>−</sup>/<sup>−</sup> and Ripk3<sup>−</sup>/<sup>−</sup>Fadd<sup>−</sup>/<sup>−</sup> mice were more susceptible to C. neoformans infection. (B) Diminished fungal clearance in lungs of Ripk3<sup>−</sup>/<sup>−</sup> and Ripk3<sup>−</sup>/<sup>−</sup>Fadd<sup>−</sup>/<sup>−</sup> mice. (C) RIPK3 or FADD deletions altered Th-polarizing and pro-inflammatory cytokine profiles in pulmonary response to cryptococcal infection. \*, P < 0.05; \*\*, P < 0.01; \*\*\*\*, P < 0.0001. Reproduced from Fa et al., [93].* 

Although generation of the Th1/Th17 response and subsequent M1 activation play a critical role in controlling fungal growth, excessive immune responses can become destructive and cause lung immunopathology following fungal infection. Recent studies demonstrated that FADD and RIPK3 proteins, which are mediators of death receptor-triggered extrinsic apoptosis, play a crucial immune regulatory role in preventing excessive inflammation during *C. neoformans* infection [93]. Deletion of RIPK3 and FADD led to a robust Th1-biased response with M1-biased macrophage activation, which is accompanied by marked upregulation of cytokines like TNF-α, IL-1α, IL-1β, IL6, and IFN-γ (**Figure 2**). Rather than being protective, this robust host response was deleterious and is associated with paradoxical fungal growth and rapid clinical deterioration (**Figure 2**). These findings showed that excessive inflammation can mediate tissue damage and host disease during cryptococcal infection [93]. Furthermore, the balance between Th1 and Th17 immune responses plays important roles in optimizing clearance and minimizing inflammatory damage to the host tissues during fungal infections. For example, it has been shown that IL-23 and Th17 pathway act as a negative regulator of Th1 response and thus contribute to fungal growth during *C. albicans* and *A. fumigatus* infection [94]. Recent studies show that the Th1, Th2, Th17 responses and cytokines co-exist and evolve during different time points in a chronic fungal infection [13], while fungus adapts to and exploit the dysregulation of this immune balance. Thus, therapeutic cytokines and vaccines may create a new therapeutic mean to restore protective host responses and fungal control, but would need to be introduced with extreme caution not to induce an excessive immune bias.

 DCs play a critical role in modulating host antifungal responses. Distinct PRRs and intracellular signaling pathways in DCs help to define the immune response to fungal pathogens [95]. Studies from Bonifazi *et al.* showed that *C. albicans*  exploits multiple, functionally distinct, receptor-signaling pathways in DCs ultimately affecting the local inflammatory/anti-inflammatory state in the gut [96]. Furthermore, depletion of DC through administration of diphtheria toxin to transgenic mice resulted in rapid clinical deterioration and death of mice infected with *C. neoformans* [97]. Early mortality in DC-depleted mice was related to increased neutrophil accumulation accompanied with histopathologic evidence of alveolar damage, including hemorrhagic and proteinaceous exudates [97]. Similar changes mediated by neutrophils were associated with respiratory failure and death [98]. Collectively, these data define an important role for DC in regulating the initial innate and adaptive response following fungal infections.

#### **5.2 Host immune responses normally associated with homeostasis can contribute to fungal persistence**

 Cryptococcal virulence includes evasion of immune recognition, interference with phagocytosis, and modulation of host immune responses [56, 99]. Many fungal factors have been shown to promote allergic Th2 or Treg responses. These types of responses are characterized by alternatively activated macrophages and may promote uncontrolled fungal growth [56]. However, the regulatory immune response is also crucial for maintaining host tissue homeostasis and limiting the inflammatory responses that can cause tissue damage.

 *Th2*: In murine models, *C. neoformans* exhibits a remarkable ability to induce Th2 response, which is associated with fungal growth, fungus-associated allergic responses and disease relapse. Although rare for *C. neoformans* infection, other fungal pathogens such as *Aspergillus fumigatus* can induce devastating allergic bronchopulmonary mycosis in human patients that is accompanied by a Th2 response [100, 101]. Additionally, enhancing the Th2 response in a mouse model has been shown to exacerbate pulmonary disease during cryptococcal infection, supporting a causal role of Th2 response in pathology [102].

 IL-4 and IL-13 provide the most potent proximal signals for Th2 cell polarization [13, 17, 103]. The epithelial-derived cytokines thymic stromal lymphopoietin (TSLP), IL-25, and IL-33 have been shown to regulate the development of Th2 response during asthma [104, 105]. A time-dependent increase in IL-33 expression in the lungs has been found during *C. neoformans* infection, and IL-33 signaling can promote Th2 response and facilitates cryptococcal growth in the lungs [106]. In addition, chitin recognition via host chitotriosidase promotes harmful Th2 cell differentiation by CD11b + conventional DCs in response to pulmonary fungal infection [102]. However, Th2 polarization may play beneficial roles at certain stage of infection. IL-4Ra has been shown to afford protection early upon infection associated with increased IFN-γ and nitric oxide production. More importantly, Th2 response plays important roles in wound healing to tissue destructive pathogens [107] and down regulating inflammatory responses [108]. Many of the proteins produced in response to IL-4 and IL-13, such as arginase, MMP12, and TREM-2, are associated with injury [109]. Th2-activated macrophages also produce TGF-β which can suppress pro-inflammatory responses while at the same time serving as a potent pro-fibrotic mediator [110].

 *Treg*: CD4+ CD25+ Treg cells expressing the transcription factor forkhead box protein 3 (FoxP3) play critical roles in down-regulating immune responses and promoting homeostasis [111, 112]. Accumulation of antigen specific Treg has been shown during infection with fungal pathogens [113–116]. Multiple studies have shown that Treg can suppress effector cells and lead to fungal persistence. For example, Treg in mice infected with *C. albicans* were shown to be capable of inhibiting Th1 activity, thereby limiting protective responses. However, the roles of Treg in modulating Th17 activity are still controversial, with both positive and negative effects reported [117, 118]. Similar enhancement of effector function in the absence of Treg can be found in multiple other models of viral, bacterial, and parasitic infection [119].

 While Treg may lead to pathogen persistence, they can actually be beneficial in protecting against immune-mediated damage to the host. This has been demonstrated in diseases caused by *Pneumocystis pneumonia*, HSV, *Schistosoma mansoni* where depletion of Treg leads to enhanced pathology [120–123]. It has been shown that depletion of Tregs enhanced Th2 response during pulmonary cryptococcal infection as evidenced by increased mucus production, enhanced eosinophilia, and increased IgE production [113]. Interestingly, Treg-depleted mice exhibited elevated fungal burden compared to control mice, suggesting that Treg mediated enhanced fungal control by inhibiting non-protective Th2

 responses [113]. Confirming these observations, therapeutic expansion of Tregs during pulmonary cryptococcal infection has been shown to limit allergic airway inflammation, as demonstrated by reduced production of IgE and Th2 cytokines [116, 124]. Since laboratory mice show a strong tendency to develop a detrimental Th2 response during *C. neoformans* infection*,* Tregs may be protective in this context by inhibiting the tissue-damaging Th2 response. Furthermore, Tregs have also been demonstrated to be required for resistance to reinfection with *C. albicans* [120].

IL-10 is a critical effector molecule involved in the immunoregulatory functions of Treg cells [125]. IL-10 has been reported to inhibit production of cytokines such as IL-1, IL-6, IL-23, IFN-γ, TNF-α and chemokines including CCL2(MCP-1), CCL12(MCP-5), CCL5(RANTES), IL-8, CXCL10(IP-10), and CXCL2(MIP-2) [126]. During *C. neoformans* infection, IL-10-deficient mice display reduced expression of IL-4, IL-5, and IL-13, but enhanced TNF-α and IL-12 expression [127]. Studies have further shown that IL-10 signaling blockade can promote fungal control even if administered after persistent infection has been established [128]. IL-10 expression also occurs and dampens fungal control in response to other fungal pathogens such as *C. albicans*, *H. capsulatum* and *A. fumigatus* [129–131]. These studies suggest that IL-10 production plays an essential role in the development of persistent fungal infections. Deficiency or blockade of IL-10 may result in better fungal control, however, it comes at the cost of excessive inflammation that may cause greater tissue damage [127, 132, 133].

 The roles of Tregs in the CNS during fungal infection, however, remain less studied. One report shows an increase in the abundance of Treg cells within cIRIS patients [134]. Further clinical and animal studies are needed to investigate the functions of Tregs during fungal CNS infections.

#### **6. Conclusions and future directions**

 A tightly-regulated balance between inflammatory and regulatory mechanisms is required to control fungal infection, maintain host homeostasis, and ultimately develop protective immunity (**Figure 3**). Recent studies have demonstrated that disease and mortality in cryptococcal infection can result from either uncontrolled fungal growth due to defective host immunity, or excessive host inflammation. As the spectrum of hosts with cryptococcal disease expands, it is critical to understand and distinguish pathology caused by the pathogen or host responses. For example, additional suppression of weak immunity by steroid therapy in patients with uncontrolled fungal growth may lead to enhanced fungus-mediated damage and mortality in HIV-associated cryptococcal patients [135]. Instead, adjunctive IFN-γ therapy to bolstering immunity in these patients has the potential to ameliorate fungus-mediated damage and mortality [136]. However, in cIRIS patients, who experience inflammation-mediated tissue damage and mortality, corticosteroids can be effective to control disease-related deterioration [30]. Furthermore, mounting evidence implies that the top priority for cIRIS and PIIRS is to control the devastating immunopathology. Thus, comprehensive therapeutic strategies that take fungus- and host mediated damage into account could have the potential to significantly improve therapeutic outcomes.

Recent studies have identified the involvement of a number of immunopathogenic mechanisms including CD4+ T cells. However, the function of CD4 T cells overlaps with the mechanisms required for fungal clearance. Little is known about whether it is possible to uncouple the anti-fungal host defense mechanisms from the host immune responses that mediate deleterious immunopathology. One of

*Cryptococcus neoformans-Host Interactions Determine Disease Outcomes DOI: http://dx.doi.org/10.5772/intechopen.83750* 

#### **Figure 3.**

*C. neoformans and host interactions determine disease outcomes. In the center, innate immune cells such as DCs recognize C. neoformans using multiple pattern recognition receptors. A weak immune response due to HIV, cancer, or organ transplant can result in fungal-mediated tissue damage. Macrophages recruited into the infected tissue without T cell stimulation fail to control fungal growth. In healthy individuals, the DC-initiated Th1 response, which completely activated macrophages to efficiently control C. neoformans infection. Cytokines such as TNF-α and IFN-γ, as well as iNOS, are critical for fungal control. Tregs play important roles in limiting inflammation and maintaining homeostasis of the host. During IRIS and PIIRS, however, excessive inflammation can cause tissue damage despite fungal control. Whether Tregs are functional under this state is not clear. Macrophages may not be activated in PIIRS patients even with strong Th1 and IFN-γ production. Overproduction of cytokines and iNOS may promote tissue damage and cause disease. For certain host genetic backgrounds or highly virulent strains, an allergic Th2 response is developed and control of fungal growth fails. Host tissue damage in this circumstance may stem from both the pathogen and the detrimental Th2 responses.* 

 the future directions in this research field is to identify mechanisms that are not required for fungal clearance but are major culprits in immunopathology which could be promising targets for future immunotherapies.

#### **Acknowledgements**

 The authors thank Mr. Mack Reynolds for thoughtful edit of the manuscript. This work was supported in part by the Intramural Research Program of the NIH, NIAID, AI001123-01 and AI001124-01 to PRW and Veterans Administration Merit Review Awards to M.A.O. (1I01BX000656) and VA RCS Award M.A.O. (1IK6BX003615).

#### **Conflict of interest**

The authors declare that there is no conflict of interest regarding the publication of this article.

#### **Author details**

Jintao Xu1,2\*, Peter R. Wiliamson3 and Michal A. Olszewski1,2

1 Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, University of Michigan Health System, Ann Arbor, Michigan, USA

2 Research Service, Ann Arbor VA Health System, Department of Veterans Affairs Health System, Ann Arbor, Michigan, USA

3 Laboratory of Clinical Immunology and Microbiology, National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health (NIH), Bethesda, Maryland, USA

\*Address all correspondence to: xxjjtt07@gmail.com

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Cryptococcus neoformans-Host Interactions Determine Disease Outcomes DOI: http://dx.doi.org/10.5772/intechopen.83750* 

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#### Chapter 3

## Invasive Candidiasis: Epidemiology and Risk Factors

Jorge Alberto Cortés and Ivohne Fernanda Corrales

#### Abstract

Invasive candidiasis is a severe infection caused by the yeast of the genus Candida. This highly lethal infection can affect any organs, but it is usually identified by the growth of the yeast in bloodstream samples. Although C. albicans was the most frequently found species, there has been a global trend to the non-albicans isolates. The appearance of C. auris, a newly identified species around the world, is a cause of concern because of resistance to antifungals. In this chapter, the epidemiology and risk factors for the acquisition of candidemia and other forms of invasive candidiasis are reviewed, while showing the current knowledge of worldwide epidemiology.

Keywords: Candida, Candida albicans, invasive, candidiasis, fungemia, candidemia, intensive care units, surgery, immunosuppression, microbiota

#### 1. Introduction

Candidiasis is the common name for diseases produced by the yeast of the genus Candida. This is the most frequently found yeast in human microbiome and is capable of causing disease at different sites of the human anatomy and with diverse severity [1]. Invasive candidiasis refers to severe fungal infections in which the yeast might be found in deep organs or blood [2]. Due to the difficulty of identifying Candida yeasts in tissues, since it requires a biopsy of the tissue compromised, invasive candidiasis in the literature has been primarily found as bloodstream infections, alone or with accompanying tissue compromise.

#### 2. Microbiology and environment

Candida species are yeasts (i.e., they mainly have a unicellular form). They are small, with a size of 4–6 μm, with a thin wall and an ovoid aspect, named blastospores [3]. They reproduce by budding. Using the microscope, these yeasts can be seen in the form of pseudohyphae, budding cells that do not separate, or truly hyphae (multicellular organisms). Candida organisms belong to the class Ascomycetes, order Saccharomycetales, and family Saccharomycetes [4]. There are around 200 species of Candida; however, a limited number has a pathogenic effect on humans [4]. Table 1 shows the most frequently found species. Due to their previous prevalence and pathogenic significance, they were usually classified as albicans


#### Table 1.

Most frequently found Candida species in human disease.

versus non-albicans Candida species. However, due to changes in epidemiology, this overall classification might not be useful any more.

They grow in agar as colonies with a smooth, creamy, white appearance. The formal identification can be made by use of biochemical physiological reactions, which can differentiate an important number of isolates. The metabolic reactions include carbohydrate fermentation, nitrate use, and urease production.

Candida yeasts might be seen with direct stains like KOH with 10–20% concentrations, but also with others like Gram, Giemsa, Wright para amino-salicylic (PAS) acid, and Papanicolaou. In direct stains, Candida might be seen as big aggregates of blastoconidiae, with short and large pseudohyphae. Usual growth media include Sabouraud agar, brain infusion, heart, and yeast extract. While C. albicans and C. dubliniensis grow in usual Sabouraud agar with antibiotics, some species might be inhibited by cycloheximide [4]. Usual growth time is 2–3 days at 28–37°C. Chromogenic agars were developed more than 20 years ago and are capable of identifying the most commonly found species, and speciation is desirable due to pathogenic and susceptibility differences among them. There are several commercial methods using chromogenic agars. The sensitivity for detection of Candida yeast is over 95%, usually with a low number or no false positive results [5]. The finding of a positive culture does not imply an invasive infection, and a special consideration has to be made for isolates from sterile sites.

Candida species differ in their susceptibility to different antifungals available in different countries. Most frequently found isolates of C. albicans and C. parapsilosis are susceptible to all antifungals available. C. tropicalis might have some resistance to fluconazole, while maintaining susceptibility to equinocandins and amphotericin B. C. glabrata tends to have higher minimal inhibitory concentrations (MICs) to azoles, while remaining susceptible to equinocandins and amphotericin B. C. lusitaniae isolates can be found to be resistant to amphotericin B. The recently found that C. auris is frequently found multidrug resistant.

Susceptibility testing can be performed by different methods, including broth microdilution (recommended in the USA and Europe), but there are other different commercial methods available in hospitals. Two slightly different standards for susceptibility testing are currently available. One is suggested by the Clinical Laboratory Standards Institute (CLSI, in USA), while the other is proposed by the European Committee on Antimicrobial Susceptibility Testing (EUCAST), sponsored by the European Society of Clinical Microbiology and Infectious Diseases

#### Invasive Candidiasis: Epidemiology and Risk Factors DOI: http://dx.doi.org/10.5772/intechopen.81813

(ESCMID). Basic differences between both methodologies include time and instrument to read the results. Different clinical breakpoints have been established for the most commonly found species, with the intention of differentiating the risk of clinical failure after treatment. The experience with fluconazole has allowed to develop better prediction models, in comparison with newer antifungals [6]. In summary, an isolate of Candida is exposed to different concentrations of the antifungal and the in vitro growth is observed. If there is no important growth, determined optically or by a spectrophotometer, a minimal inhibitory concentration (MIC) is established. As mentioned, data from clinical trials and observation cohorts with common species such as C. albicans and C. glabrata have allowed to identify clinically relevant breakpoints to differentiate isolates with low MICs (susceptible); intermediate MICs (also called susceptible dose dependent—SDD), in which an increase in the administered antifungal can control the infection; and high MICs, (resistant), for which a lower probability of success is expected. For some other uncommon species, only epidemiological breakpoints are available. These breakpoints are also MICs, but there is no clinical evidence of correlation with the clinical outcome after treatment. However, since MICs are higher than those in usual isolates, a worse outcome might be expected. These breakpoints are expected to identify isolates with natural or acquired mechanisms of antifungal resistance. The epidemiological breakpoints are based on the statistical distribution of MICs of the wild-type isolates (i.e., isolates without any previous resistant pressure). Commercial methods are modifications of the standard methods that use dyes to identify the growth (e.g., Alamar Blue) of the microorganisms. Examples include Sensititre™ and YeastOne™. Other methods are based on agar, in which a gradient of the antifungal is diffused in the solid growth media, which allows to directly read the MIC (e.g., Etest ™) [7].

Candida species are part of the human microbiota and they live in human mucosae and skin. Candida species can be found in the ground, animals, fruits and vegetables, and in the hospital environment. It is not considered a laboratory contaminant. It is considered an endogenous pathogen since around 60–75% of the people might have it in the mucosal epithelium, especially in the gastrointestinal and genital tracts [8]. In the hospital area, they have been found over inanimate surfaces, including percutaneous catheters and tubes. They might even be found in the hands of healthcare workers. Among patients in healthcare centers, the colonization of the mucosae has been related to antibiotic use and hospitalization time [9]. In patients in the intensive care unit (ICU), colonization might be found in different anatomical sites with ample variations [10, 11]. Pharyngeal colonization rate has been found to be between 34 and 65%, gastric colonization between 42 and 67%, rectal colonization between 21 and 40%, and colonization in other sites between 11 and 40% [10, 11]. These data show the possibility of colonization that has this microorganism in patients under stress conditions (in this case, severe disease). In the normal host, the colonization rate might reach over 50% in the mouth, 40% in the vaginal tissue in women, and 73% in any mucosa of the gastrointestinal or genital tracts [8].

#### 3. Pathogenesis

Candida species have some characteristics that permit them to adapt to different environments and act as an opportunistic pathogen. These factors include adaptation to pH changes, permitting to survive in blood or some alkaline environments, as well as in the acidic environment of the vaginal tissue; these species have adhesins, mannoproteins with capacity to adhere to different cells and cell products. These adherence proteins allow the isolates to survive in tissues, but also over inanimate surfaces that have been exposed to plasma or inflammatory host proteins like urinary or endotracheal catheters. Candida species have also important enzymes as virulence factors, since some of them have keratinolytic, peptidase, hemolysin, and other effects. One of the most frequently mentioned virulence factors include the possibility of a morphologic transition, which has been extensively studied. It refers to the possibility of morphologic changes of blastoconidia to pseudohyphae to real hyphae. These changes are stimulated by environmental conditions. The filamentous forms are related to active infection in the host, except for C. glabrata. Other factors related to pathogenicity or virulence also include a phenotyping change, the possibility of adopting different phenotypes in the cultures (color or aspect of the colonies), and biofilm formation. A biofilm is a large community of symbiotic microorganisms adhered to a surface. This conformation allows the microorganisms to have a highly defensive capacity, persistence, and a highly antimicrobial resistance.

As mentioned before, Candida might be part of the human flora. The majority of infections are due to the interplay between the risk factors, that pose a risk to the individual, the interaction with other microorganisms present in the skin or mucosa and the total quantity of microorganisms present. This was demonstrated some years ago in an experiment [12]. An individual ingested directly from a C. albicans culture. After some hours, this immunocompetent individual began to have fever. After 12 hours, Candida isolates were found in the bloodstream and, after 16 hours, they were found in the urine. After 24 hours, Candida isolates were cleared from the body and the individual returned to the normal state. This experiment proved the importance of colonization. With posterior evidence, it has been demonstrated that the first step to have an infection is colonization by Candida especially in the gastrointestinal tract, but otherwise in contact with indwelling catheters, the skin, or wounds that may permit the entry of the yeast into the bloodstream. In another critical observation, patients in the ICU were followed with cultures. The colonization index (it is the proportion of positive cultures for the same Candida species taken from different anatomical places) increased over time and was correlated to the probability of developing an invasive candidiasis [9]. These studies suggest that in individuals with Candida colonization, those factors that promote the grow of the yeast, by eliminating the bacteria that can compete for the environment, that alter or facilitate the penetration of the yeast to the bloodstream (lesions in the gastrointestinal mucosa, indwelling catheters) will promote the entry of Candida yeast to the blood, while the net state of compromise of the immune system will affect the probability of fungal clearance and the possibility of seeding on specific organs.

#### 4. Epidemiology

#### 4.1 Risk factors

#### 4.1.1 Candida infection in the intensive care unit

Patients in the ICU have the highest rate of Candida infections in the hospital. In comparison with patients in other wards, patients in the ICU have more frequent abdominal surgery, stay longer in the hospital, and are more severely ill [13]. They also have a worse prognosis in the long term, with increased mortality after one year of the event.

#### 4.1.1.1 Vascular devices

Patients in the ICU have higher rates of Candida infection in comparison with patients in other wards. Critically ill patients often require multiple vascular and other indwelling devices for their management and candidemia has been related to catheter colonization in 20–80% of the cases [14, 15]. One study in Japan identified the presence of a solid tumor, the use of total parenteral nutrition, and the administration of anti-anaerobic agents as the main risk factors for the development of Candida infections [16]. As mentioned, Candida colonization of the catheter might provide a route for entering into the bloodstream without a heavy gastrointestinal colonization. Studies have shown that Candida catheter-related bloodstream infections have a shorter time to grow in comparison with those from other sources [14]. With a breakpoint of 30 hours, the time to grow in patients with Candida bloodstream infection might identify 100% of those catheter-related infection. Probably, patients with catheter-related infection have a higher inoculum, which would explain the faster time to grow and the fact that observational studies have shown a lower mortality when catheter is removed [17, 18]. On the other hand, patients with non-catheter-related candidemia were more seriously ill, had a higher mortality, and the removal of the catheter did not affect the outcome [17].

#### 4.1.1.2 Parenteral nutrition

Another commonly identified risk factor is the use of parenteral nutrition or the length of its use [15, 19]. This group of patients shares several risk factors, but parenteral nutrition has been identified in multivariate analysis [20]. Usually, they have an abdominal procedure (see below) and they require parenteral nutrition for several days. Lack of appropriate measures to handle the nutrition, colonization of the catheter or the ports used to infuse it, and probably the availability of optimal growing conditions are conditions related to its use. But clearly, the use of parenteral nutrition leads to the development of mucosal atrophy and a loss of mucosal epithelial barrier function [21], which might affect the relationship between microorganisms in the gut and the possibility of gaining access to blood vessels. Total parenteral nutrition has also a profound effect in the gastrointestinal microbiome [22].

#### 4.1.1.3 Surgical procedures

Several studies have shown the relationship between candidemia and a previous surgical procedure [19, 23], specially an abdominal surgery. There are several explanations to this observation, but gut manipulation, and the effect of resected sections over the gut microbiology, microbiota abundance, and epithelial function might contribute to the possibility of candidemia. Studies have shown that patients with high anastomotic leak, as well as those with recurrent gastrointestinal perforation, or acute necrotizing pancreatitis, have a higher risk of candidemia [15].

#### 4.1.1.4 Antibiotic use

Almost all studies of candidemia have shown an extremely high use of antibiotics previous to the identification of bloodstream or tissue infection. The proportion of patients with antibiotic use is over 80% [24]. The number and spectrum of the antibiotics used might affect the risk of candidemia. Antimicrobials also have an effect over gut microbiota, and some studies have shown some impact from antibiotics with anti-anaerobic effect, and those with higher gastrointestinal concentration [25]. They contribute to the observed increased colonization over time observed in patients in the ICU. With more antibiotic effect, there is a net decrease in the number of species in the gastrointestinal tract, an increase in the number of patients colonized, and the proportion of them being heavily colonized [26].

#### 4.1.1.5 Other risk factors

Studies have identified several risk factors that alone, or in combination, might increase the probability of having candidemia. The presence of renal failure, the use of antihistaminic blockers, the severity of illness, and the length of stay in the ICU contribute to colonization and development of candidemia [24, 27]. All these factors contribute to the acquisition of Candida, its colonization, or failure in the gastrointestinal epithelial function, favoring the entry of the yeast to the bloodstream.

#### 4.1.1.6 Scores based on risk factors

The identification of risk factors lead to the use of some scores based in the presence of such factors to identify patients with higher risk of Candida infection. The first and most simple of those scores was introduced in mid-1990s. Pittet et al. in a surgical ICU followed prospectively patients admitted in the ward with cultures of several anatomical sites [9]. They defined the colonization index as previously stated, establishing that with an index of 0.5 or more (50% of the sites with the same species), there was an increase in the risk of candidemia. With a lower colonization index, the risk in the original study was 0%. They defined a second index based on the density of colonization, in which patients overpassing some thresholds in the number of colonies isolated per site, being able to improve the identification of the patients at risk.

A second score to identify risk factors in patients was developed in Spain by León and his collaborators [20]. They identified colonization (with a different definition from that used by Pittet et al.), surgery at ICU admission, and use of total parenteral nutrition as risk factors independently associated with candidemia. They also identified sepsis as independently related, but this is clearly more a clinical syndrome than a risk factor. A third score was developed by a multicenter collaboration group, in which they again identified the same risk factors [28]: antibiotic use, having an intravascular catheter, in conjunction with at least two additional risk factors such as any surgery, immunosuppressive use, pancreatitis, total parenteral nutrition, dialysis, or steroid use.

Common to these scores has been the presence of the aforementioned risk factors. The problem, however, is that such scores identify a huge number of patients at risk with a final intermediate risk of developing candidemia, in a range from 7 to 30% [29, 30]. The great advantage of the diagnostic scores relies in their high negative predictive value. Patients with a negative score have a low probability of candidemia, below a 1% probability.

#### 4.1.2 Hematological malignancy, solid organ transplantation, and other immunosuppressive states

These disorders share a common factor: immunosuppression. However, different types of immunocompromise entail different risks for the patients. The incidence of candidemia among patients with cancer is higher in comparison with other patients in the hospital. In a multicenter study in Greece, patients with

Invasive Candidiasis: Epidemiology and Risk Factors DOI: http://dx.doi.org/10.5772/intechopen.81813

hematological disease had an incidence of candidemia of 1.4 cases per 1000 admissions, while other patients hospitalized had an incidence of 0.83 cases per 1000 admissions [31]. A multicenter European study found an incidence of 1.2% cases of candidemia among patients with bone marrow transplantation (BMT) and leukemia [32]. An Italian multicenter study from a surveillance network showed a diminishing trend for candidemia among patients with cancer, especially among those with acute myeloid leukemia [33]. Whether this trend can be inferred to other European countries or not is not known, and the most likely explanation for this decrease in the number of cases could be related to the use of prophylaxis among those patients with acute leukemia with posaconazole. In general, non-albicans Candida species are more frequently found among these groups of patients [31].

#### 4.1.2.1 Neutropenia

Neutropenia, a count of leukocytes in peripheral blood below 500 cells per μl, is the common risk factor among patients with hematological disorders (i.e., leukemia, lymphoma, multiple myeloma among others) as well as those with bone marrow transplantation (BMT). Neutropenia might be a consequence of the activity of the hematological disease, an effect of chemotherapeutic strategies or side effect of multiple medications including antimicrobials. It also is a marker of the intensity of chemotherapy. Patients with chemotherapy-induced neutropenia accumulate various risk factors: they usually receive wide spectrum antibiotics for several days, they have serious gastrointestinal epithelial tissue dysfunction, usually with diarrhea and signs of mucosal damage, and the use of vascular catheters for the infusion of chemotherapeutic drugs and antibiotics [34]. Several studies have shown that isolates of C. tropicalis are more frequently found among patients with cancer [35]. A study that looked for risk factors identified underlying leukemia as one of the major risk factors, together with chronic lung disease [36].

In patients with prolonged neutropenia, a condition called hepatosplenic candidiasis might be seen. In it, seeding of yeasts occurs during the neutropenic phase which might be not clinically evident until neutropenia recovery. In these patients, fever persists and lesions can be seen in the liver, usually known as bull-eye lesions [37] (Figure 1).

#### Figure 1.

Tomographic image of liver and spleen showing abscesses (bull's eye, arrows) and hypodense lesions in a patient with chronic disseminated candidiasis. Reproduced with permission from Cortés et al. [37].

#### 4.1.2.2 Concurrent conditions in patients with cancer

In patients with cancer and candidemia, several factors were identified in comparison with those with cancer and bacterial infections [38]. Total parenteral nutrition over 5 days, urinary catheter for more than 2 days, distant metastasis of cancer, and gastrointestinal cancer were independent risk factors. Patients with solid tumors might accumulate factors as patients in critical care, since they have abdominal surgery (gastrointestinal neoplasm), require vascular catheters for extended periods of time (for chemotherapy or antibiotics), total parenteral nutrition and received antibiotics frequently [39]. A study to identify factors predicting catheter-related infections with Candida identified solid tumors and the use of antianaerobic antibiotics as risk factors [16].

Among patients with leukemia and BMT, the risk factors for occurrence of candidemia included bone marrow or cord blood stem cell source, T-cell depletion, use of total body irradiation, and acute graft versus host disease [32]. These data were derived from a huge multicenter registry of patients with cancer and transplantation, which allowed to identify more precisely the risk factors.

#### 4.1.3 Neonates

Newborns have no gastrointestinal flora at birth and have to be colonized by enterobacteria and other microorganisms, which is made via maternal breast feeding. Any alteration in the normal process can lead to colonization by pathogenic microorganisms, including yeasts [40]. Neonates in the intensive care unit usually have limited breastfeeding, indwelling vascular catheters, total parenteral nutrition, and antibiotics [41]. Such combination of risk factors put. this group of patients at a higer risk of infection, reaching over 10% of patients in units with extreme prematures and low weight at birth (the group that requires more invasive interventions) [42]. Some studies have illustrated this relationship with proportion of candidemia between 3 and 10% among those with a weight of less than 1000 g while showing an incidence of less than 1% for those weighting over 2500 g [43]. In this scenario, disseminated candidemia can be found and near 10% of those with invasive disease can compromise the central nervous system. Another important risk factor includes the time that the patient has been in the unit [44]; clearly, patients with low weight, lower gestational age, and more comorbidity tend to spend more time in the neonatal ICU and to accumulate other risk factors (surgery, indwelling catheter, antibiotics, etc.) [45]. There are some high-risk units, in which the incidence of candidemia traditionally has been high, usually over 10% of the admitted cases. In this scenario, prophylaxis has been suggested for the prevention of infection [46].

#### 4.1.4 Outbreaks

Candida yeast can survive in inanimate surfaces and in the hands of healthcare personnel, which confers the risk of outbreak and cross dissemination among highrisk units such as neonatal, intensive care, and surgical intensive care units [44, 47]. An interesting study in Iceland over a long period of time allowed to confirm the presence of clonal isolates of different Candida species among patients in the ICU and other wards [48]. The proportion of patients involved at one time with an outbreak of all patients with Candida isolates might be as high as 38%. Other study in Spain showed that clusters (of patients with candidemia) were possible with C. albicans and C. parapsilosis, and reached in a period as high as 40% of the isolates [49]. Besides, the use of chlorhexidine has been shown to diminish the number of

Invasive Candidiasis: Epidemiology and Risk Factors DOI: http://dx.doi.org/10.5772/intechopen.81813

candidemia events in patients in the ICU, showing the importance of colonization and cross infection among high-risk patients and establishing this recommendation in the guidelines for the prevention of candidemia [50].

As shown, colonization is the preliminary step to infection. Besides, a number of interventions are common to immunosuppressed and critically ill patients including indwelling catheters (urinary and vascular), severity of illness, total parenteral nutrition, etc. These conditions predispose the patients to cross contamination. An outbreak among newborns was demonstrated to be due to poor practices of catheter ports disinfection [51].

A study in China in a cancer institute showed that 21 out of 36 episodes of candidemia were caused by two endemic genotypes [52]. In this study, gastrointestinal cancer and insertion of a nasogastric tube were related to infection. As mentioned before, cancer patients with solid and hematological tumors share several of the risk factors of colonization and infection.

#### 4.2 Global epidemiology

Since 2013, the Leading International Fungal Education (LIFE) portal has facilitated an important effort to know the epidemiology and burden of fungal infections around the world and allowed a better understanding of their epidemiology in different countries [53]. The real incidence of candidemia is difficult to calculate due to differences in the approach. While studies based on hospitals might overestimate the importance of some groups of high-risk patients, they are difficult to compare. Data from population studies might reflect better the real situation, but this kind of information is scarce. Studies have shown ample differences in the incidence in some regions and at specific times [54].

#### 4.2.1 Changing trend for non-albicans Candida

Traditionally, C. albicans had been the most frequently isolated species. However, a trend toward non-albicans species has been observed around the world in the last 15 years. In United States, C. glabrata has been identified as second in frequency, while C. parapsilosis or C. tropicalis dispute this place in other regions. Table 2 shows the proportion of isolates in some studies around the world in the last 10 years [55–59].

Two studies deserve a detailed description. The first one is a multicenter study from the Southeast Asia region, including 25 hospitals from 6 countries: China, Hong Kong, India, Singapore, Taiwan, and Thailand [60]. They found differences between the countries that include the frequency of C. tropicalis isolation, being


#### Table 2.

Proportion of Candida species in selected studies of candidemia around the world.

#### Fungal Infection

more commonly found in hematology-oncology wards and in tropical areas. This study confirmed the observed trend for a lower frequency of C. albicans isolates. The other study is the Latin-American surveillance study [55]. It involved patients from 20 centers in 7 seven countries: Argentina, Brazil, Colombia, Chile, Honduras, Mexico, and Venezuela. Important differences were seen among institutions, reflecting difference in healthcare systems, access, population types, and risk factors. However, in these two studies, the incidence of candidemia is higher than in developed countries in Europe and North America. In Latin America, C. parapsilosis frequency is over 30% of the isolates while this place is occupied by C. tropicalis in the Asian countries.

#### 4.2.2 Epidemiology in Europe and North America

There are data from some population surveillance surveys in Europe and United States. In general, the incidence might be lower than in some other areas of the world. Table 3 shows the incidence from data from North America and European countries [61–77]. In Europe, the highest incidence has been observed in Hungary, while in North America the highest incidence has been seen in some cities in United States.

#### 4.2.3 Epidemiology in Central and South America and the Caribbean

This region has profound differences in healthcare systems, access to care, and medical technology development. With a transition toward a higher income, a growing number of institutions with capacity to attend cancer patients, and more


#### Table 3.

Estimated incidence of invasive candidiasis or candidemia in countries of the European or North American regions.

#### Invasive Candidiasis: Epidemiology and Risk Factors DOI: http://dx.doi.org/10.5772/intechopen.81813


#### Table 4.

Estimated incidence of invasive candidiasis or candidemia in countries of Central and South America and the Caribbean.

complex medical needs, the number of candidemia cases seems to be higher than in developed countries.

Ample information exists about the problem in Brazil, where a number of studies have been carried out in high-complexity hospitals in the main cities of the country [78, 79]. These studies show a higher frequency of invasive candidiasis in comparison with developed countries, an increased isolation of C. glabrata for the last period and an important exposition to fluconazole (which might have increased the selection for non-albicans species) [79]. Country-wise estimates for incidence are shown in Table 4 [80–89].

#### 4.2.4 Epidemiology in Africa and Asia

A multicenter in Asia gathered information from various countries, including nine hospitals from China [60]. The incidence rate among patients hospitalized was 0.38 per 1000 admissions, which is lower than that observed in the Latin-American region with 1.08 cases per 1000 admissions [55]. The estimated incidence of candidemia in countries in Asia is shown in Table 5 [90–100]. In Asia, the highest incidence has been observed in Pakistan, followed by Qatar and Israel. In China, geographic variations in the causative species and susceptibilities were noted, with increasing isolates resistant to fluconazole [101]. The numbers for the African countries are lacking and for some countries like Algeria, Burkina Faso, Cameroon, Egypt, Malawi, Mozambique, and Tanzania, the estimated incidence is 5.8 cases per 100,000 inhabitants, a standard calculation based on previously reported incidence in other countries [102–108].

#### 4.2.5 Azole resistance epidemiology

Azole-resistant Candida isolates have had an increased frequency over the years. Susceptibility changes with the species, and fluconazole use has been related to an increase in the frequency of C. glabrata and C. krusei, and a low increase in the number of resistant C. albicans or C. tropicalis. A large multicenter study in French ICUs identified the age and the exposure to antifungals as independent risk factors for resistance [109]. Patients with isolates resistant to fluconazole tended to be older than 15 years and to have been exposed to this drug, while those with


#### Table 5.

Estimated incidence of invasive candidiasis or candidemia in countries of Africa and Asia.

equinocandin-resistant isolates were younger and found to have been exposed to equinocandin. In general, risk factors for resistance remain the same as in resistant bacteria: immunosuppression, previous use of antifungals [110, 111]. Other identified risk factors include chronic renal failure and anti-tuberculous treatment. This last one might be due to a medication interaction.

Among patients with cancer, not only are non-albicans Candida species more frequently found, but also resistance to azoles has increased. In a study in Greece, resistance to fluconazole among patients with cancer reached 27% [31]. Since azoles have been widely used in the prophylaxis against fungal infections among cancer patients [112, 113], this seems to be a natural consequence of their use. Among patients with cancer, isolates of C. tropicalis, C. glabrata, and C. krusei have increased resistant proportions [35].

#### 4.2.6 Candida auris global outbreak

Up to 2009, there was no report on C. auris. In that year, a clinical case from Japan was published, and 2 years later three cases of candidemia were identified [114, 115]. During the following years, isolates of C. auris were responsible of outbreaks around the world, affecting hospitals in India, Pakistan, South Africa, England, and Venezuela [116–119]. It was detected in the USA in 2013 with growing frequency [120]. A worldwide alarm was raised in 2016 because of two problems related to this species. The first one was the difficulty in proper identification [121]. C. auris is most commonly identified as C. haemulonii and Rhodotorula glutinis by the commercial systems and sometimes as C. famata, C. guilliermondii, and C. parapsilosis [121]. The other problem is the higher frequency of resistance to multiple antifungals, including azoles and amphotericin [122]. Currently, C. auris has been isolated in several areas in the USA, continental Europe, and the Caribbean coast of South America, including the islands [123–125], and continue to extend to other areas, where reports are being published. A search for virulence factors in the isolates of C. auris has shown some different properties, specially the capacity for biofilm formation [126]. Molecular observations have diverse geographic dissemination caused by unique clades in each geographic region [127].

Invasive Candidiasis: Epidemiology and Risk Factors DOI: http://dx.doi.org/10.5772/intechopen.81813

#### 5. Outcomes

Patients with candidemia and cancer are considered to have higher mortality, but this issue has not been clearly assessed. Older studies showed an attributable mortality around 40%. Although mortality among patients with candidemia or invasive candidiasis is reported usually around 40–50%, they occur in patients with important comorbidity. A recent multicenter analysis showed a crude mortality for patients with candidemia of 53%, while those without candidemia had a mortality of 26% [128]. After adjusting in a propensity score analysis, the crude mortality was 51% for the candidemic patients and 37% for the others and the difference was not statistically significant. The study shows that an increase in mortality might exist for those patients with candidemia, but it is clear that patients with candidemia also have severe comorbidity and some of them can die with candidemia instead of because of it.

Risk factors for mortality among patients with candidemia include ascites, presence of septic shock, ICU admission, concomitant bacterial infection and catheterrelated infections [129]. Studies with diverse population have shown that elderly patients have higher mortality [130]. In these patients, a combination of comorbidity, poor clinical situation, and more pathogenic species might contribute to their mortality [131]. A pooled analysis from patients included in randomized clinical trials comparing micafungin and amphotericin B showed differences among geographic regions, severity of disease (measured with Apache score for patients critically ill), and catheter removal [132]. In those with abdominal candidiasis, the lack of control of the source of infection has been related to increased risk of death [133]. Among patients with cancer, risk factor for mortality includes infection by a C. tropicalis isolate, a high Charlson index score, neutropenia, and septic shock [35, 134]. One multicenter study identified tachypnea as a risk factor for mortality [135], while others identified respiratory failure and use of non-antifungal medications [39]. Besides, antifungal prophylaxis and remission of the underlying cancer had a protective effect over mortality [135].

The impact of the antifungal treatment in the mortality of patients with candidemia is not entirely clear. There are several constrains to identify the benefits of the antifungal treatment: An important proportion of patients did not receive antifungal treatment despite the identification of a bloodstream infection; of those that receive the treatment, some of them can receive it as empirical treatment, based on the risk factors, clinical condition, while others have an antifungal started upon detection of candidemia. Besides, some of them are infected with a resistant isolate and some do poorly, and an additional antifungal must be started. Although meta-analysis with patient-level data has showed the benefit of equinocandin use (in contrast to azole treatment) [136], neither the cohort data [137], nor the randomized trials have confirmed this finding [138]. There is an additional complication in understanding this relationship; the laboratory breakpoints for identification of susceptible versus resistant isolates have changed over the time, especially for azoles [130]. Among those patients with septic shock, the delay in the administration of the antifungal treatment has been associated with increased mortality.

#### 6. Conclusion

Candidemia is the most frequently found form of invasive candidiasis. The Candida species might be found as part of the flora and patients with previous colonization are at risk of developing an infection. They share some common factors like antibiotic exposure, use of indwelling catheters, parenteral nutrition, and surgery. These factors affect the normal physiology of the gastrointestinal tract or provide access to the bloodstream to yeast in patients with some comorbidities, in critical care or with immunosuppressive states.

### Conflict of interest

There is no conflict of interest to declare.

### Author details

Jorge Alberto Cortés<sup>1</sup> \* and Ivohne Fernanda Corrales<sup>2</sup>

1 Department of Internal Medicine, School of Medicine, Universidad Nacional de Colombia, Bogotá, Colombia

2 Neonatal Intensive Care Unit, Fundación CardioInfantil, Bogotá, Colombia

\*Address all correspondence to: jacortesl@unal.edu.co

© 2018 The Author(s). Licensee IntechOpen. This chapteris distributed underthe terms oftheCreative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Invasive Candidiasis: Epidemiology and Risk Factors DOI: http://dx.doi.org/10.5772/intechopen.81813

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Section 2
