**5. Viral pneumonia**

infections. Dromedary camels are considered as the intermediate host, with closely related virus sequences in bats. Human-to-human transmission has been noted in households and

The World Health Organization (WHO) established the Child Health Epidemiology Reference Group (CHERG) to monitor the incidence of childhood [23]. In 2000, CHERG compiled pneumonia statistics in children under age 5. It was found that there were 150 million new cases of pneumonia in children under 5. Of these, approximately 4 million occurred in developed countries, while the majority occurred in developing nations [24]. Hospitalization rates for pneumonia were approximately 9%. More than half of all worldwide cases of childhood pneumonia occurred in just five countries: India, China, Pakistan, Indonesia, Nigeria and Bangladesh. Europe had the lowest rate (0.06 episodes per child-year), while Southeast Asia and Africa had the highest overall incidence rates (0.36 and 0.33 episodes per child-year, respectively). Factors that increased the risk of developing childhood pneumonia included low birth weight, malnutrition, crowded living quarters, indoor air pollution, insufficient breast feeding, and lack of vaccinations. Other possible contributing factors included pre-existing medical conditions such as asthma, and annual rainfall levels. Also, parents who smoked and

Pneumonia is a serious concern in adults and viruses cause 15–30% of cases in immunocompetent adults hospitalized with pneumonia [26]. Increased rates of pneumonia-associated hospitalizations have been reported in the United States, Denmark, United Kingdom, and the Netherlands. In 2010, approximately 1.1 million patients in the USA were hospitalized for pneumonia with an average length of hospital stay 5.2 days [27]. In the UK, the number of hospitalizations due to pneumonia increased by 34% during the period from 1997 to 2005. This was particularly notable in older adults. Approximately 26,000 people died from pneumonia and influenza in England and Wales in 2013 according to national statistics. Contributing factors may be due to other chronic diseases such as heart disease, diabetes, and

Atypical pneumonia refers to pneumonia caused by atypical bacteria, including *Legionella* species, *Chlamydia pneumoniae* and *Mycoplasma pneumoniae* [29]. It is called "atypical" because the symptoms and signs differ from those of pneumonia due to other common organisms. It is generally regarded that *M. pneumoniae*, *Legionella* spp., *C. pneumoniae, Chlamydophila psittaci and Coxiella burnetii* are the key CAP pathogens not readily identified by standard culture methods [30]. Other atypical pathogens include viruses, atypical mycobacteria, *Francisella* 

health care setting. But community-wide transmission has not been observed [22].

**3.1. Pneumonia in children**

194 Contemporary Topics of Pneumonia

lack of parent education [25].

immunocompromised patients [28].

*tularensis* and an extensive list of agents of bioterrorism.

**4. Atypical pneumonia**

**3.2. Pneumonia in adults**

The advent of molecular diagnostics has greatly improved the identification of viruses in patients with CAP. Over the last decade, several studies have used PCR to establish the importance of viruses in the etiology of CAP. Globally, it is estimated that 200 million cases of viral pneumonia occur annually. Most commonly are influenza viruses (A and B), parainfluenza viruses 1, 2 and 3, rhinoviruses, and coronaviruses [34].

Viral pneumonia prevails mostly in young children and older adults. Etiologies include influenza, adenovirus, parainfluenza, H1N1 and respiratory syncytial virus (RSV). Influenza A and B occurs in the winter and spring. Symptoms include, headache, fever, and muscle aches. Respiratory syncytial virus (RSV) is most common in the spring and infects young children. Adenovirus and parainfluenza viral pneumonias exhibit cold symptoms (runny nose and conjunctivitis). Post-influenza pneumonia is often accompanied by secondary bacterial infection due to *Staphylococcus pneumoniae* and *Staphylococcus aureus.* Pneumonia in immunocompromised patients is attributed to measles, HSV, CMV, HHV-6 and Influenza viruses. There is also an increased risk of secondary bacterial lower respiratory tract infection (LRTI). The known complication following influenza infection is *Staphylococcus aureus* pneumonia [35].

Respiratory syncytial virus (RSV) has been identified as an important cause of pneumonia in adults, especially in the elderly. The rate of RSV, overall is between 2 and 5% throughout the year and between 5 and 14% during winter. Adults with severe immunodeficiency are at particular risk of severe RSV infection [36].

Viral pneumonia infections include both DNA and RNA viruses. Some are well-known lung pathogens that produce common clinical and radiologic manifestations. Others are rarely involved as lung pathogens. Many viruses can cause pneumonia, either directly or indirectly. They include:

#### **5.1. Adenoviruses**

Adenoviruses are enveloped DNA viruses and a diverse group that cause a wide spectrum of clinical illnesses. At least 52 serotypes exist, classified into 7 subgroups or species (A-G). Adenovirus pneumonia typically is limited to newborns, immunocompromised hosts, and school or military camp populations. Severe adenovirus pneumonia has been more commonly described in immunocompromised patients. Respiratory infection in immunocompetent patients is usually self-limited and mild [37]. However, with advances in molecular techniques, adenovirus has been increasingly discovered to be involved in sporadic cases and in severe CAP in healthy adults. Pulmonary disease is predominantly caused by serotypes 1, 2, 3, 4, 5, 7, 14, and 21. Adenovirus infection accounts for up to 20% of childhood pneumonias, primarily in those children younger than 5 years of age, but such pneumonias occur infrequently in the non-military adult population. Types 4 and 7 viruses can cause outbreaks of respiratory disease in military recruits, whereas Type 7 viruses can cause bronchiolitis and pneumonia in infants. A virulent strain of Adenovirus, serotype 14 (subgroup B) has been reported to cause greater symptoms of respiratory illness and pneumonia. It was first observed in 2005 among civilian and military populations. Outbreaks occurred subsequently at military academies throughout the United States and in the Pacific Northwest [38].

pneumonia may occur and is often fatal in immunocompromised individuals, primarily hematopoietic stem cell transplant (HSCT) and solid organ transplant (SOT) recipients. The severity of pneumonia is related to the extent of immunosuppression [41]. CMV pneumonia has a prevalence of 15% and a mortality rate of approximately 85% in cancer patients receiving allogeneic bone marrow transplants. The major risk factor for CMV pneumonia in these

Pneumonia of Viral Etiologies

197

http://dx.doi.org/10.5772/intechopen.71608

Pneumonia is the most common presentation of CMV disease following lung transplantation CMV infection in lung transplant recipients is approximately 50%. Lung is a CMV latency site and viral reactivation has been associated with direct systemic infection and indirect effects such as acute rejection. The risk of CMV infections in these cases is dependent on the immu-

Moreover CMV pneumonia is identified to be a predictive factor for the later development of chronic rejection in lung transplant. Several studies have been carried to find the optimal preventive strategy to avoid CMV infection after lung transplantation. An effective treatment strategy has been valganciclovir prophylaxis for at least 180 days following combined pro-

Epstein-Barr virus (EBV) is well-known to be transmitted through infected saliva. The virus can cause pneumonia without mononucleosis. Lung involvement secondary to EBV infections usually only occurs in immunocompromised people. However, in 25% of pediatric patients with HIV infection, the virus can cause lesions due to lymphocytic interstitial pneumonia or pulmonary

Infectious mononucleosis occurs in young adults aged 15–30 years and usually resolves without *sequelae*. This disorder may cause chronic tiredness and fevers but can also be complicated by life-threatening problems. Pulmonary involvement associated with Epstein-Barr virus infection is uncommon but can occur as a complication of infectious mononucleosis. Pathologically, mononuclear inflammatory cells are evident along interlobular septa in inter-

Herpes simplex virus (HSV) is primarily implicated in severely immunocompromised patients primarily, e.g., solid organ transplant recipients, patients who are undergoing chemotherapy or are neutropenic, or those who have congenital immunodeficiency. Herpes simplex virus is spread by viral shedding from asymptomatic excreters or from active lesions. It is a rare cause of lower respiratory tract infections. HSV pneumonia can occur as a secondary infection to upper airway infection or following viremia due to genital or oral lesions [44]. Herpes simplex

Herpes simplex virus type 1 pneumonia is relatively uncommon that generally affects patients who are immunocompromised. It often occurs as a polymicrobial infection and is frequently

virus can also cause pneumonia in compromised hosts, with a mortality rate of 80%.

nosuppressive therapy and serologic status of the donor and recipient.

phylaxis together with ganciclovir and CMV-immune globulin (CMV-IG) [42].

patients is acute graft-versus-host disease.

**5.4. Epstein-Barr virus**

lymphoid hyperplasia.

stitial pulmonary infiltrates [43].

**5.5. Herpes simplex virus**

#### **5.2. Coronavirus**

Coronaviruses are from the family Coronaviridae and are single-stranded RNA viruses. As the name indicates the surface is covered by crown like projections. This virus is spread via droplet and fomite exposure. Coronaviruses were not thought to significantly cause pneumonia until recently. However, the severe acute respiratory syndrome (SARS) pandemic in 2003 brought the ability of this virus to cause life-threatening pneumonia to worldwide attention.

There are six human coronaviruses (HCoVs) that are established human pathogens with worldwide distribution, causing upper and lower respiratory tract infections: HCoV-229E, OC43, HKU1, NL63, MERS-COV (Middle East respiratory syndrome) and SARS-COV (severe acute respiratory syndrome) [39].

MERS-COV was first identified in Saudi Arabia in September 2012, approximately 2000 MERS-CoV cases have been detected in over 20 countries. The newly reported cases lift Saudi Arabia's MERS-CoV total since the virus was first detected in humans in 2012 to 1598 cases, 661 of them fatal and the majority of MERS-CoV cases continue to be reported from the Middle East. The source of the virus has remained a mystery but transmission and virological studies point toward dromedary camels in the Middle East by which humans may become infected through zoonotic transmission. Human-to-human transmission is then exacerbated through close household contacts and in healthcare settings [40].

#### **5.3. Cytomegalovirus**

Cytomegalovirus (CMV) is a herpesvirus that is a common cause of infections. In hosts who are immunocompetent, acute CMV infection causes a mononucleosis-like syndrome. CMV pneumonia may occur and is often fatal in immunocompromised individuals, primarily hematopoietic stem cell transplant (HSCT) and solid organ transplant (SOT) recipients. The severity of pneumonia is related to the extent of immunosuppression [41]. CMV pneumonia has a prevalence of 15% and a mortality rate of approximately 85% in cancer patients receiving allogeneic bone marrow transplants. The major risk factor for CMV pneumonia in these patients is acute graft-versus-host disease.

Pneumonia is the most common presentation of CMV disease following lung transplantation CMV infection in lung transplant recipients is approximately 50%. Lung is a CMV latency site and viral reactivation has been associated with direct systemic infection and indirect effects such as acute rejection. The risk of CMV infections in these cases is dependent on the immunosuppressive therapy and serologic status of the donor and recipient.

Moreover CMV pneumonia is identified to be a predictive factor for the later development of chronic rejection in lung transplant. Several studies have been carried to find the optimal preventive strategy to avoid CMV infection after lung transplantation. An effective treatment strategy has been valganciclovir prophylaxis for at least 180 days following combined prophylaxis together with ganciclovir and CMV-immune globulin (CMV-IG) [42].
