**2.2 Global etiologies**

The principal organisms of infectious keratitis have regional variation [18]. In developed countries such as Europe [21, 26, 27], North America [28, 29], and Australia [14], the highest proportions of infectious keratitis are attributable to bacteria. This incidence correlated with the prevalence of contact lens wear and high gross national income [30]. On the contrary, in developing countries, the proportions of infectious keratitis attributable to bacteria were less [31–33] than or similar to those from fungus [34]. The Asia Cornea Society Infectious Keratitis Study (ACSIKS) group that included data from 13 study centers and 30 subcenters from 6626 eyes of 6563 subjects demonstrated the similar percentage of bacterial and fungal infections. Overall, bacterial keratitis was diagnosed in 2521 eyes (38.0%) and fungal keratitis in 2166 eyes (32.7%) [34]. But in India and China, fungal keratitis was the majority of nonviral microbial keratitis [34]. Similarly, data from the large trial in India by Lalitha et al. that included 17,948 patients demonstrated

**5**

*Infectious Keratitis: The Great Enemy*

and France [5].

**2.3 Burden of diseases**

with less-developed economics [38].

**3. Diagnostic approach**

250,000 hours of clinician time annually [8, 40].

of the definite diagnosis from investigations.

*DOI: http://dx.doi.org/10.5772/intechopen.89798*

infectious keratitis in each country [6, 33].

that 6218 of 10,207 (60.9%) culture-positive patients had a fungal etiology [31]. The recent studies from China by Lin et al. [35] and Xie et al. [33] reported proportions of fungal keratitis to be 44.6% and 77.9%, respectively. Vision 2020: "The Right to Sight" also suggests that we acquire a clearer and more precise understanding of

The frequency of causative organisms of keratitis among different geography may vary by countries [30]. In North America, Australia, the Netherlands, and Singapore, the highest proportion of infectious keratitis was bacterial in origin. The highest proportion of pseudomonas keratitis was reported in a study from Bangkok, whereas the highest proportion of staphylococcal ulcers was reported in a study from Paraguay. On the other hand, the highest proportions of fungal keratitis were reported in studies from India and Nepal [30]. Interestingly, recent data from the Asia Cornea Society Infectious Keratitis Study (ACSIKS) group demonstrated that the commonest etiology of infectious keratitis was viral keratitis, which accounted for 47.7% [34]. A recent population-based study in China estimated that the

prevalence of herpes simplex keratitis was 0.11%, similar to that in the United States

For the worldwide frequency of Acanthamoeba keratitis, around 1% of infectious keratitis was reported [30]. Although the number of infections caused by these organisms is low, Acanthamoeba keratitis is an emerging disease mostly due to the increased use of contact lens worldwide [36]. Cariello et al. reported the higher

Despite the fact that infectious keratitis is a significant cause of corneal blindness [2, 38, 39], there has been relatively little evidences available about the definite global burden of infectious keratitis. Moreover, the actual burden may be higher due to the underreport [8] and a fact that most of the patients reside in countries

Collier et al. reported the burden of this disease in the United States. Infectious keratitis causes an estimated 930,000 doctor's office and outpatient clinic visits and 58,000 emergency department visits per year. Nearly 80% of keratitis visits lead to antimicrobial prescriptions. The direct health care costs for keratitis and contact lens disorders were approximately 175 US dollars. Moreover, it consumed over

Data from the national surveys in China and India reported that infectious keratitis is responsible for the major burden of corneal blindness [5, 23, 41]. However, in other endemic areas of infectious keratitis such as China and India, the actual costs were not available. It may be underreported or underestimated in poor rural and agricultural farm-based populations [8]. The cost of infectious keratitis is magnified as it usually affects the poorest populations during their most productive years [8, 42]. Moreover, the cost may be derived from poor access to health care, corticosteroid abuse, inadequate empiric antibiotics, and difficulty to obtain diagnostic

It is helpful for ophthalmologists to be familiar with the typical signs and symptoms of different organisms that are responsible for infectious keratitis. The provisional diagnosis may help to ensure the prompt management prior to the revelation

cultures that leads to delay presentation of or fulminant infection [8, 42].

rate of 7.4% of infectious keratitis caused by Acanthamoeba in Brazil [37].

#### *Infectious Keratitis: The Great Enemy DOI: http://dx.doi.org/10.5772/intechopen.89798*

*Visual Impairment and Blindness - What We Know and What We Have to Know*

and reduce morbidity and visual loss due to this condition.

**2.1 Global epidemiology**

**2.2 Global etiologies**

**2. Global epidemiology, global etiologies, and burden of diseases**

keratitis leading to corneal blindness occur in developing countries [2].

The principal organisms of infectious keratitis have regional variation [18]. In developed countries such as Europe [21, 26, 27], North America [28, 29], and Australia [14], the highest proportions of infectious keratitis are attributable to bacteria. This incidence correlated with the prevalence of contact lens wear and high gross national income [30]. On the contrary, in developing countries, the proportions of infectious keratitis attributable to bacteria were less [31–33] than or similar to those from fungus [34]. The Asia Cornea Society Infectious Keratitis Study (ACSIKS) group that included data from 13 study centers and 30 subcenters from 6626 eyes of 6563 subjects demonstrated the similar percentage of bacterial and fungal infections. Overall, bacterial keratitis was diagnosed in 2521 eyes (38.0%) and fungal keratitis in 2166 eyes (32.7%) [34]. But in India and China, fungal keratitis was the majority of nonviral microbial keratitis [34]. Similarly, data from the large trial in India by Lalitha et al. that included 17,948 patients demonstrated

The precise prevalence of infectious keratitis is unknown [18]. The actual prevalence may be higher due to the underreporting or reporting under the term of corneal blindness, which also includes traumatic, infectious, inflammatory, and inherited causes [8]. Overall cases may exceed 2 millions cases per year worldwide [8]. The global incidence of infectious keratitis shows wide disparity among regions. The incidence of infectious keratitis was high in south, south-east, and east Asia, but lower in developed world [8]. The estimated general incidence of infectious keratitis per 100,000 population per year varies from 11 to 27.6 in the United States [19, 20], 40.3–52.1 in the United Kingdom [8, 21], 6.3 in Hong Kong [22], 113 in India [23], 339 in Bhutan [24], 710 in Burma [24], and 799 in Nepal [25]. In contact lens wearers, the incidence is about 6 times higher [8, 18, 20]. It has been reported that nearly 90% of the global cases of ocular trauma and infectious

2020 [7] and will inevitably impact the eye care need in the near future. In the developed countries, infectious keratitis is usually associated with contact lens wear, but in developing countries, it is commonly caused by trauma during agricultural work [8–17]. Infectious keratitis is characterized by a corneal epithelial defect with underlying stromal inflammation caused by replicating microorganisms [14]. Acute eye pain and redness are the common presentation [14]. The common causative organisms are bacteria, fungus, Acanthamoeba, and virus [10, 17]. However, corneal infection involving more than one microorganisms such as bacteria and fungus is relatively uncommon [17] Microbial keratitis requires acute ophthalmic care and aggressive treatment to stop the disease progression and limit the extent of corneal scarring, which can cause loss of vision [14]. The precise clinical diagnosis, accurate diagnostic tools, and timely appropriate management are important to reduce the morbidity associated with infectious keratitis. This review addresses the epidemiology, diagnostic approach, clinical manifestations, risk factors, investigations, treatments, and the update of major clinical trials about common pathogens of infectious keratitis. The purpose of this review is to provide a concise and essential knowledge about common etiologic pathogens of infectious keratitis to improve understanding

**4**

that 6218 of 10,207 (60.9%) culture-positive patients had a fungal etiology [31]. The recent studies from China by Lin et al. [35] and Xie et al. [33] reported proportions of fungal keratitis to be 44.6% and 77.9%, respectively. Vision 2020: "The Right to Sight" also suggests that we acquire a clearer and more precise understanding of infectious keratitis in each country [6, 33].

The frequency of causative organisms of keratitis among different geography may vary by countries [30]. In North America, Australia, the Netherlands, and Singapore, the highest proportion of infectious keratitis was bacterial in origin. The highest proportion of pseudomonas keratitis was reported in a study from Bangkok, whereas the highest proportion of staphylococcal ulcers was reported in a study from Paraguay. On the other hand, the highest proportions of fungal keratitis were reported in studies from India and Nepal [30]. Interestingly, recent data from the Asia Cornea Society Infectious Keratitis Study (ACSIKS) group demonstrated that the commonest etiology of infectious keratitis was viral keratitis, which accounted for 47.7% [34]. A recent population-based study in China estimated that the prevalence of herpes simplex keratitis was 0.11%, similar to that in the United States and France [5].

For the worldwide frequency of Acanthamoeba keratitis, around 1% of infectious keratitis was reported [30]. Although the number of infections caused by these organisms is low, Acanthamoeba keratitis is an emerging disease mostly due to the increased use of contact lens worldwide [36]. Cariello et al. reported the higher rate of 7.4% of infectious keratitis caused by Acanthamoeba in Brazil [37].

#### **2.3 Burden of diseases**

Despite the fact that infectious keratitis is a significant cause of corneal blindness [2, 38, 39], there has been relatively little evidences available about the definite global burden of infectious keratitis. Moreover, the actual burden may be higher due to the underreport [8] and a fact that most of the patients reside in countries with less-developed economics [38].

Collier et al. reported the burden of this disease in the United States. Infectious keratitis causes an estimated 930,000 doctor's office and outpatient clinic visits and 58,000 emergency department visits per year. Nearly 80% of keratitis visits lead to antimicrobial prescriptions. The direct health care costs for keratitis and contact lens disorders were approximately 175 US dollars. Moreover, it consumed over 250,000 hours of clinician time annually [8, 40].

Data from the national surveys in China and India reported that infectious keratitis is responsible for the major burden of corneal blindness [5, 23, 41]. However, in other endemic areas of infectious keratitis such as China and India, the actual costs were not available. It may be underreported or underestimated in poor rural and agricultural farm-based populations [8]. The cost of infectious keratitis is magnified as it usually affects the poorest populations during their most productive years [8, 42]. Moreover, the cost may be derived from poor access to health care, corticosteroid abuse, inadequate empiric antibiotics, and difficulty to obtain diagnostic cultures that leads to delay presentation of or fulminant infection [8, 42].

#### **3. Diagnostic approach**

It is helpful for ophthalmologists to be familiar with the typical signs and symptoms of different organisms that are responsible for infectious keratitis. The provisional diagnosis may help to ensure the prompt management prior to the revelation of the definite diagnosis from investigations.

#### **3.1 History**

The common symptoms of infectious keratitis include pain, foreign body sensation, redness of conjunctiva and eyelids, tearing, discharge, photophobia, blurry vision, eyelids swelling, and notice of a white mark on the cornea [24, 43, 44]. In Acanthamoeba keratitis, the patients usually have severe pain out of proportion to the corneal lesion because of perineuritis. Bacterial keratitis usually has an acute onset, and gradual onset of symptoms usually suggests nonbacterial causes such as atypical keratitis or fungal keratitis [43]. Careful history taking about risk factors of infectious keratitis will help confirm the diagnosis. The important risk factors such as history of eye trauma, contact lens wear and hygiene, history of ocular surface disease, previous bacterial or HSV keratitis, history of corticosteroid abuse, hot tub exposure, prior ocular surgery, systemic diseases such as diabetes, immunodeficiency, and vitamin A deficiency will aid in the clinical diagnosis [43–45].

#### **3.2 Physical examination**

If the ophthalmologist is familiar with the characteristics of different etiologic agents of infectious keratitis, this helps confirm the diagnosis and determine the possible etiologic organisms. A thorough physical examination from general appearance and eye examination will aid in the provisional clinical diagnosis [43–45]. Visual acuity will be invariably affected depending on the location of the lesion and degree of intraocular inflammation [43]. Intraocular pressure should be examined unless corneal perforation is suspected such as deep lesion on slit lamp examination, and visible protrusion of intraocular content unless corneal perforation is suspected [43]. Ocular hypotony will be present in perforation cases. On the other hand, a high intraocular pressure can be suggestive of HSV, which is associated with trabeculitis [43]. External examination for the general appearance, lid closure, blinking, and corneal sensation are vital to determine the predisposing factors. For example, the patient that has a lagophthalmos is usually prone to an infectious keratitis at inferior cornea. A slit lamp examination is also crucial to determine the etiology of a suspected lesion. Eyelids may appear swollen, and conjunctiva may reveal injection and tearing and/or mucopurulent discharge. Ciliary flush is common. Cornea examination should document the characteristics of the lesion such as the size, shape, border, density, location, depth, and color. For instances, Pseudomonas ulcer usually shows the classic clinical feature of "ground glass appearance," which is a diffuse ulceration with stromal infiltrate or edema involving adjacent or whole cornea [24]. The classical clinical picture of the filamentous fungal keratitis is dry, raised infiltrate with feathery edges. A brown or dark pigmentation may be presented in dematiaceous fungal keratitis and can be an important diagnostic clue [24, 46]. The size and border of the lesion should be documented because it can suggest a positive response to a treatment if the size is decreased and/ or the border is consolidated [44]. The depth of the lesion should be evaluated. In fungal keratitis, if the lesion is superficial, it may have a better response to the antifungal eye drop treatment than the deeper lesion because of poor ocular penetration and unpredictable bioavailability of antifungal agents [47]. If there is a significant corneal thinning and/or descemetocele, urgent management is required. The patient may have decreased visual acuity if the infiltration is located over the pupil or have a significant intraocular inflammation [24]. Fluorescein staining and cobalt blue light aid in the document of epithelial defect and detect the corneal perforation by Seidel test [24, 44]. The anterior chamber should be documented for a cell, flare, and/or hypopyon. Essential history and physical examination are listed in **Table 1**.

**7**

*Infectious Keratitis: The Great Enemy*

• Contact lens wear (types and hygiene)

• Swimming, using a hot tub, or showering while

○ Previous infection, e.g., bacteria, fungus, herpes

○ Topical nonsteroidal anti-inflammatory drugs

• Duration of symptoms

wearing contact lenses • Past ocular diseases

• Past ocular surgery

(NSAIDs) ○ Topical anesthetics ○ Topical corticosteroids

○ Preservatives

• Past medical history ○ Medical conditions ○ Immunosuppression • Medication allergies

**Table 1.**

○ Glaucoma medications

*References based on references [43, 44].*

simplex virus (HSV)

○ Current treatment

• Previous and current eye drops usage

○ Contaminated ocular medication

• Eye trauma

**3.3 Microbiological investigations**

mal involvement or melting.

rial keratitis are identified.

3.History of corneal surgeries is found.

5.There are multiple locations of cornea infiltrates.

therapy.

before the results of culture are available [26, 44].

*Essential history and examination in infectious keratitis patient.*

The smears and cultures are indicated as follows [44, 48]:

Most of the bacterial keratitis cases resolve with the empiric topical antibiotics

1.A corneal infiltrate is central, large, and/or is associated with significant stro-

4.Atypical clinical features that are suggestive of fungal, amoebic, or mycobacte-

Corneal culture helps to identify the causative organisms and determine antibiotics sensitivity [44]. Moreover, culture can help in modifying therapy in

2.The keratitis is chronic or unresponsive to broad-spectrum antibiotic

*DOI: http://dx.doi.org/10.5772/intechopen.89798*

**Essential history Essential physical examination**

• Visual acuity

corneal perforation) • External examination

corneal sensation • Slit lamp examination ○ Eyelid margins ○ Eye lashes abnormality

> ○ Endothelial plaque ○ Seidel test

○ Pupillary examination ○ Anterior chamber reaction

• Contralateral eye for clues to etiology • Signs of previous corneal or refractive surgery

○ Discharge

○ Sclera ○ Foreign body • Complete eye examinations

• Intraocular pressure (contraindicated in

○ General appearance, lid closure, blinking,

○ Conjunctival injection, ciliary flush ○ Characteristics of lesion, size, shape, border, density, location, depth, color


#### **Table 1.**

*Visual Impairment and Blindness - What We Know and What We Have to Know*

The common symptoms of infectious keratitis include pain, foreign body sensation, redness of conjunctiva and eyelids, tearing, discharge, photophobia, blurry vision, eyelids swelling, and notice of a white mark on the cornea [24, 43, 44]. In Acanthamoeba keratitis, the patients usually have severe pain out of proportion to the corneal lesion because of perineuritis. Bacterial keratitis usually has an acute onset, and gradual onset of symptoms usually suggests nonbacterial causes such as atypical keratitis or fungal keratitis [43]. Careful history taking about risk factors of infectious keratitis will help confirm the diagnosis. The important risk factors such as history of eye trauma, contact lens wear and hygiene, history of ocular surface disease, previous bacterial or HSV keratitis, history of corticosteroid abuse, hot tub exposure, prior ocular surgery, systemic diseases such as diabetes, immunodeficiency, and vitamin A deficiency will aid in the clinical diagnosis [43–45].

If the ophthalmologist is familiar with the characteristics of different etiologic

appearance," which is a diffuse ulceration with stromal infiltrate or edema involving adjacent or whole cornea [24]. The classical clinical picture of the filamentous fungal keratitis is dry, raised infiltrate with feathery edges. A brown or dark pigmentation may be presented in dematiaceous fungal keratitis and can be an important diagnostic clue [24, 46]. The size and border of the lesion should be documented because it can suggest a positive response to a treatment if the size is decreased and/ or the border is consolidated [44]. The depth of the lesion should be evaluated. In fungal keratitis, if the lesion is superficial, it may have a better response to the antifungal eye drop treatment than the deeper lesion because of poor ocular penetration and unpredictable bioavailability of antifungal agents [47]. If there is a significant corneal thinning and/or descemetocele, urgent management is required. The patient may have decreased visual acuity if the infiltration is located over the pupil or have a significant intraocular inflammation [24]. Fluorescein staining and cobalt blue light aid in the document of epithelial defect and detect the corneal perforation by Seidel test [24, 44]. The anterior chamber should be documented for a cell, flare, and/or hypopyon. Essential history and physical examination are listed in **Table 1**.

agents of infectious keratitis, this helps confirm the diagnosis and determine the possible etiologic organisms. A thorough physical examination from general appearance and eye examination will aid in the provisional clinical diagnosis [43–45]. Visual acuity will be invariably affected depending on the location of the lesion and degree of intraocular inflammation [43]. Intraocular pressure should be examined unless corneal perforation is suspected such as deep lesion on slit lamp examination, and visible protrusion of intraocular content unless corneal perforation is suspected [43]. Ocular hypotony will be present in perforation cases. On the other hand, a high intraocular pressure can be suggestive of HSV, which is associated with trabeculitis [43]. External examination for the general appearance, lid closure, blinking, and corneal sensation are vital to determine the predisposing factors. For example, the patient that has a lagophthalmos is usually prone to an infectious keratitis at inferior cornea. A slit lamp examination is also crucial to determine the etiology of a suspected lesion. Eyelids may appear swollen, and conjunctiva may reveal injection and tearing and/or mucopurulent discharge. Ciliary flush is common. Cornea examination should document the characteristics of the lesion such as the size, shape, border, density, location, depth, and color. For instances, Pseudomonas ulcer usually shows the classic clinical feature of "ground glass

**3.1 History**

**3.2 Physical examination**

**6**

*Essential history and examination in infectious keratitis patient.*

### **3.3 Microbiological investigations**

Most of the bacterial keratitis cases resolve with the empiric topical antibiotics before the results of culture are available [26, 44].

The smears and cultures are indicated as follows [44, 48]:


Corneal culture helps to identify the causative organisms and determine antibiotics sensitivity [44]. Moreover, culture can help in modifying therapy in unresponsive cases and eliminating unnecessary medications to decrease ocular surface toxicity [44]. Corneal scraping should be obtained from the advancing borders of infected cornea. The hypopyon in bacterial keratitis cases is usually sterile, and aqueous and vitreous tapping should not be done unless suspected of microbial endophthalmitis. Cultures should be performed before starting antimicrobial therapy, and if cultures are negative, the ophthalmologist may consider stopping antibiotic treatment for 12 to 24 hours and then re-culturing the infectious keratitis [44]. The stain provides provisional diagnosis of pathogenic organisms of infectious keratitis. However, cultures were the gold standard to identify the definitive pathogens and provided antibiotics susceptibility testing [26, 49]. Data from the large trial that enrolled 3300 eyes with infective keratitis in India demonstrated that the sensitivity of KOH was higher in the detection of fungi than that of Gram-stained smears (99.3%; 95% CI 98.6 to 99.6) and (89.2% (95% CI 87.3 to 90.8), respectively [50]. The Gram stain has diagnostic accuracy of 60–75% for bacteria keratitis detection and 35–90% for fungal keratitis [51]. The KOH alone has 62–99% sensitivity and 73–99% specificity for fungal keratitis detection [51]. Giemsa stain has 40–85% sensitivity and 70–96% specificity in detecting fungal keratitis [51].

All media are examined for growth daily and are incubated for 1–2 weeks before considering negative culture. Bacteria such as atypical mycobacteria, Nocardia spp., and Acanthamoeba grow slowly and require prolonged incubation [52]. Nonnutrient agar plate should be observed for at least 10 days [49]. Fungus grows within 24 hours to maximum of 2 weeks, so prolonged incubation at least for two to 3 weeks before the culture is considered negative [49]. The vital stains and cultures that are used for infectious keratitis are concluded in **Table 2**.

Even among the cornea specialists, they can correctly differentiate bacterial keratitis from fungal keratitis in less than 70% of cases [55]. Classic clinical figures may be obscured in large keratitis due to tissue destruction [53]. Acanthamoeba keratitis may be responsible for many cases of clinically presumed herpes simplex keratitis [54]. Cultures and smears are the gold standard to diagnose bacterial and fungal keratitis, but fungal cultures consume several days to weeks to obtain growth and can be falsely negative for deep infiltrates [56]. In these cases, other investigations such as in vivo confocal microscopy (IVCM) and PCR may have a role.

#### *3.3.1 In vivo confocal microscopy (IVCM)*

Bacteria and virus cannot be identified by IVCM but Acanthamoeba and fungal filaments are large enough and can be visualized [56]. Early detection of Acanthamoeba keratitis is associated with favorable outcome. Two most commonly used types of corneal IVCM include slit-scanning (ConfoScan; Nidek Technologies) and laser-scanning (Heidelberg Retina Tomograph with Rostock Corneal Module) [56]. In vivo confocal microscopy provides a rapid, noninvasive diagnostic tool and monitors treatment response, which has 85.7 to 88.3% sensitivity and 81.4 to 91.1% specificity for fungal keratitis detection and 88.2 to 88.3% sensitivity and 91.1 to 98.2% specificity for Acanthamoeba keratitis diagnosis [53–55, 57]. However, the branching angle of Fusarium and Aspergillus spp. was not easily differentiated by IVCM, and culture remains essential to address fungal species [58]. Amoebas are visible as double-walled cysts or as brightly reflective ovoid structures [54, 59]. However, diagnostic performance of IVCM is highly dependent on the operator's experience [60].

**9**

*3.3.2 PCR*

**Table 2.**

of Acanthamoeba keratitis [63].

**4. Etiological agents of infectious keratitis**

Although not yet widely available, the advantages of PCR are rapid specific identification and requiring small clinical sample [51, 61, 62]. PCR is generally a required test for viral diagnosis and detection of organisms that are difficult to culture such as Microsporidia, *Mycobacterium tuberculosis*, fungi, and Acanthamoeba [52, 61, 63, 64]. PCR can be used as adjunct to smear and culture especially in cases where routine diagnostic procedures failure [65]. When compared to standard culture technique, real-time PCR had 100% sensitivity and 96% specificity in the diagnosis

The common causative organisms of infectious keratitis are bacteria, fungus, Acanthamoeba, and virus [5, 14, 18, 34]. This review addresses the epidemiology, clinical manifestations, risk factors, and treatment of each pathogen. The classic clinical features of common causative organisms of infectious keratitis are show in **Figure 1**.

*Infectious Keratitis: The Great Enemy*

*DOI: http://dx.doi.org/10.5772/intechopen.89798*

Special stains

• Ziehl-Neelsen and/ or Kinyoun stain

• Gomori methenamine silver (GMS) stain

Sabouraud dextrose agar Fungi

*Common stains and cultures used to identify common pathogens of infectious keratitis.*

• Immunoflorescence stain • Periodic acid Schiff (PAS) stain

Optional culture media • Lowenstein Jensen medium

• Nonnutrient agar

*References based on [18, 24, 43, 48–54].*

**Type Organism visualized**

Stains Gram stain Most of the pathogenic bacteria, fungi,

Giemsa stain Chlamydia trachomatis

Potassium hydroxide (KOH) Fungi, Acanthamoeba Calcofluor white Fungi, Acanthamoeba

Cultures Blood agar Aerobic and facultatively anaerobic bacteria,

Chocolate agar Aerobic and facultatively anaerobic bacteria

Thioglycolate broth Aerobic and facultatively anaerobic bacteria

MacConkey agar Lactose fermenting (e.g., *Escherichia* 

Acanthamoeba

• Fungi

Herpes simplex virus Varicella zoster virus Acanthamoeba

• Mycobacterium and Nocardia • Chlamydiae and viruses

fungi such as *P. aeruginosa*, *S. aureus*, *S. epidermidis*, and *S. pneumoniae*

including *H. influenzae*, *N. gonorrhea*

Proteus spp., Pseudomonas)

• Acanthamoeba

• *Mycobacterium* and Nocardia spp.

*coli*, Enterobacter, Klebsiella) VS Lactose nonfermenting bacteria (Salmonella, Shigella,

• Fungi, Acanthamoeba


#### *Infectious Keratitis: The Great Enemy DOI: http://dx.doi.org/10.5772/intechopen.89798*

*Visual Impairment and Blindness - What We Know and What We Have to Know*

detecting fungal keratitis [51].

concluded in **Table 2**.

PCR may have a role.

*3.3.1 In vivo confocal microscopy (IVCM)*

unresponsive cases and eliminating unnecessary medications to decrease ocular surface toxicity [44]. Corneal scraping should be obtained from the advancing borders of infected cornea. The hypopyon in bacterial keratitis cases is usually sterile, and aqueous and vitreous tapping should not be done unless suspected of microbial endophthalmitis. Cultures should be performed before starting antimicrobial therapy, and if cultures are negative, the ophthalmologist may consider stopping antibiotic treatment for 12 to 24 hours and then re-culturing the infectious keratitis [44]. The stain provides provisional diagnosis of pathogenic organisms of infectious keratitis. However, cultures were the gold standard to identify the definitive pathogens and provided antibiotics susceptibility testing [26, 49]. Data from the large trial that enrolled 3300 eyes with infective keratitis in India demonstrated that the sensitivity of KOH was higher in the detection of fungi than that of Gram-stained smears (99.3%; 95% CI 98.6 to 99.6) and (89.2% (95% CI 87.3 to 90.8), respectively [50]. The Gram stain has diagnostic accuracy of 60–75% for bacteria keratitis detection and 35–90% for fungal keratitis [51]. The KOH alone has 62–99% sensitivity and 73–99% specificity for fungal keratitis detection [51]. Giemsa stain has 40–85% sensitivity and 70–96% specificity in

All media are examined for growth daily and are incubated for 1–2 weeks before considering negative culture. Bacteria such as atypical mycobacteria, Nocardia spp., and Acanthamoeba grow slowly and require prolonged incubation [52]. Nonnutrient agar plate should be observed for at least 10 days [49]. Fungus grows within 24 hours to maximum of 2 weeks, so prolonged incubation at least for two to 3 weeks before the culture is considered negative [49]. The

Even among the cornea specialists, they can correctly differentiate bacterial keratitis from fungal keratitis in less than 70% of cases [55]. Classic clinical figures may be obscured in large keratitis due to tissue destruction [53]. Acanthamoeba keratitis may be responsible for many cases of clinically presumed herpes simplex keratitis [54]. Cultures and smears are the gold standard to diagnose bacterial and fungal keratitis, but fungal cultures consume several days to weeks to obtain growth and can be falsely negative for deep infiltrates [56]. In these cases, other investigations such as in vivo confocal microscopy (IVCM) and

Bacteria and virus cannot be identified by IVCM but Acanthamoeba and fungal filaments are large enough and can be visualized [56]. Early detection of Acanthamoeba keratitis is associated with favorable outcome. Two most commonly used types of corneal IVCM include slit-scanning (ConfoScan; Nidek Technologies) and laser-scanning (Heidelberg Retina Tomograph with Rostock Corneal Module) [56]. In vivo confocal microscopy provides a rapid, noninvasive diagnostic tool and monitors treatment response, which has 85.7 to 88.3% sensitivity and 81.4 to 91.1% specificity for fungal keratitis detection and 88.2 to 88.3% sensitivity and 91.1 to 98.2% specificity for Acanthamoeba keratitis diagnosis [53–55, 57]. However, the branching angle of Fusarium and Aspergillus spp. was not easily differentiated by IVCM, and culture remains essential to address fungal species [58]. Amoebas are visible as double-walled cysts or as brightly reflective ovoid structures [54, 59]. However, diagnostic performance of IVCM is highly dependent on the operator's

vital stains and cultures that are used for infectious keratitis are

**8**

experience [60].

#### **Table 2.**

*Common stains and cultures used to identify common pathogens of infectious keratitis.*

#### *3.3.2 PCR*

Although not yet widely available, the advantages of PCR are rapid specific identification and requiring small clinical sample [51, 61, 62]. PCR is generally a required test for viral diagnosis and detection of organisms that are difficult to culture such as Microsporidia, *Mycobacterium tuberculosis*, fungi, and Acanthamoeba [52, 61, 63, 64]. PCR can be used as adjunct to smear and culture especially in cases where routine diagnostic procedures failure [65]. When compared to standard culture technique, real-time PCR had 100% sensitivity and 96% specificity in the diagnosis of Acanthamoeba keratitis [63].
