Background
Corneal ulcer, an inflammatory or more seriously, infective condition of the cornea involving disruption of its epithelial layer with involvement of the corneal stroma, is one of the major causes of monocular blindness after unoperated cataract in many of the developing nations in Asia, Africa and the Middle East. [
1,
2] It is a sight threatening disorder that affects both males and females across all age groups worldwide. In the United States alone, 930,000 cases seek outdoor medical attention and 58,000 cases visit the emergency department [
3]. The annual financial burden borne in United States in direct health care expenditures due to cases related to corneal ulcer and keratitis is estimated to be $175 million [
3]. In the developing countries, the financial burden related to this diesease is undetermined but speculated to be calamitous [
4].
Herpes Simplex Virus type 1 (HSV-1) is the most common cause of corneal ulcer but other etiological agents frequently associated with corneal ulcer include bacteria (
Staphylococcus aureus,
Staphylococcus epidermidis,
Streptococcus pneumoniae,
Streptococcus pyogenes,
Moraxella species,
Pseudomonas aeruginosa,
Proteus species,
Klebsiella pneumoniae,
Yersinia species and
Escherichia coli), fungus (
Candida albicans,
Aspergillus flavus,
Fusarium solani,
Penicilium species and
Aspergillus fumigates) and parasites (
Acanthamoeba) [
5‐
8]. In addition,
Pseudomonas a Gram negative opportunistic bacteria is also commonly associated with keratitis arising from contact lens wear, which ultimately leads to corneal ulcer [
9]. The etiology of corneal ulcer varies disproportionately in different geographical regions with highest proportion of bacterial corneal ulcers reported from North America, Australia, Netherlands, and Singapore and that of fungal corneal ulcer from India and Nepal [
10].
Corneal ulcer is an ophthalmic condition requiring prompt medical attention. Thus precise knowledge of the causative agents and their susceptibility patterns is important for deciding the proper course of treatment. To the best of our knowledge, the microbial etiology of corneal ulcer and its management in Nepal has remained unclear [
11‐
13]. Thus, the aim of this research is to analyze the etiology of corneal ulcer in Nepal and to determine the antibiotic susceptibility pattern of bacterial isolates thereby reducing antibiotic misuse and the incidence of microbial drug resistance.
Discussion
Proper management and treatment of corneal ulcers, a major cause of blindness worldwide requires precise identification of the etiology so that an appropriate antimicrobial agent targeting the organism responsible can be administered on time. Nonetheless, the inconsistency in prevalence and causes of corneal blindness across geography and ethnic groups make it challenging to administer a standard set of protocols in order to lower the incidence of corneal ulcer [
1]. Given these milieu, the awareness among ophthalmologists of regional epidemiological features, risk factors, and etiological data concerning this ophthalmic condition is necessary. Thus, we explored the etiological agent of corneal ulcer, identified associated risk factors and antibiotic susceptibility of bacterial isolates identified.
Although the culture positivity of 44.6% that we observed in Nepali populations is comparable to previous studies that reported 40–45%, culture positivity in this region [
17,
18], we detected lower positivity than a previous study conducted at the same ophthalmic center [
12]. The reason for such lower prevalence could be due to differences in methods used to ascertain positivity and difference in sample size. Alternatively, improved eye care services at ophthalmological facilities may have resulted in decreased incidence of corneal ulcer cases in Nepal.
The bacterial isolates accounted for 56% (25/45) and fungal isolates for 44% (20/45) of the total corneal ulcer cases which demonstrates the shift from fungi to bacteria as major agent associated with this disease in this region [
10]. This transition from fungi to bacteria as major etiological agent in Kathmandu could be due to rapid urbanization and large reductions in agricultural practices within Kathmandu in the last few years (Table
2). Among the bacterial isolates
S. pneumoniae 31.1% (14/45) showed higher prevalence which is in harmony with the findings of similar studies conducted elsewhere [
12,
19].
S. pneumoniae is the major biological agent causing corneal ulcer in developing as well as industrial nations. The production of virulence factor pneumolysin favors
S. pneumonae to establish infection in corneal epithelium [
20]. Meanwhile,
Fusarium species was the dominant fungi causing corneal ulcer which is in concordance with the finding of previous studies [
18,
21,
22].
The infection ratio of male: female was found to be 0.7:1. This finding is not in conformity with several studies conducted elsewhere which have reported a higher susceptibility of male toward infection compared to female [
7,
17,
18,
23]. The difference in ratio may be due to more exposure of female populaiton in agricultural and household activities in our context compared to those studies. However, the role of gender in corneal ulcer is always contradictory and further rigorous research is required. The highest number of patients, 40% (18/45) from corneal ulcer positive case belonged to age group 51–60. It is due to the fact that people of age between 51 and 60 years have many predisposing factors like CDK (climatic droplet keratopathy), dryness of the eyes, cataract surgery, glaucoma, macular degeneration, previous ocular surgeries and lid deformities due to trachomatous scarring which probably predispose this age group to corneal ulceration more than the other age groups [
24]. However, in our study no statistical significance was established (
p > 0.05) between the age of patient and corneal ulcer.
The higher prevalence of corneal ulcer was seen in the agricultural group (57.8%), which was similar to finding reported by Basak et al. [
23]; but a marked contrast was seen with the study done in Ghana where only 16.1% corneal ulcer cases were associated with agricultural profession. This could be due to the differences in the occupational pattern between the two countires in consideration. However, no statistical significance (
p > 0.05) was seen between the occupation and corneal ulcer in our case.
The age, gender, and education distributions of each cohort correspond to the population distributions of visual impairment as reported by the World Health Organization [
25]. In this study, corneal ulcer was presented with higher prevalence among people receiving less education as has been the pattern reported by other researchers from around the globe [
4,
23]. Individuals with lower education are ignorant and less conscious about their health. However, the culture positivity was not statistically significant (
p > 0.05) with the education status of patients.
Ocular trauma or corneal injury has always been identified as a cause of corneal ulcer [
8,
23]. In our study statistical significant (
p < 0.05) was established between corneal ulcer and trauma (28.9%) as indicated by the culture positivity. Use of contact lenses has become one of the main reasons for microbial keratitis in the developed nations where they are broadly accessible, mainly in young adults [
9,
26,
27]. In contrast to the reports cited above even a single case of corneal ulcer predisposed by contact lens wear was not reported. This may be because of the fact that contact lenses are, as yet not widely used in Nepal due to the extra financial burden borne on patient when opting to lenses instead of glasses/spectacles. Similarly, the less frequent isolation of
Pseudomonas species may also be attributed to infrequent use of contact lens.
In the view of frequent reports of changing pattern of susceptibility among the bacteria, testing of clinical isolates for their susceptibility to antimicrobial drugs is necessary for selection of appropriate antibiotics or for changing an already administered drug. In this study, the isolated bacteria were tested against eight different antibiotics in the laboratory as recommended by CLSI [
16]. Since, there are no susceptibility standards for topical antibiotic therapy in ophthalmology, the resistance determined in this study is based on the systemic susceptibility breakpoints. All the bacterial isolates (Gram positive and negative) were 100% susceptible to fourth generation quinolone antibiotic moxifloxacin, the drug of choice for bacteria incriminated with ophthalmic problems.
All the isolated
S. pneumoniae and viridians group of streptococci were 100% susceptible to the entire panel of antibiotics used. Amikacin, ceftazidime, moxifloxacin and ofloxacin were found to be effective against
S. aureus. Nocardia species were 66.67% resistant to ciprofloxacin, ofloxacin, azithromycin whereas, 100% susceptible to chloramphenicol, moxifloxacin, and amikacin. Similarly,
Bacillus species were 66.67% resistant to chloramphenicol and azithromycin and 100% susceptible to amikacin, ciprofloxacin, moxifloxacin and ofloxacin.
P. aeruginosa was resistant to chloramphenicol and ceftazidime and susceptible to aminoglycosides and quinolones. These results indicate that chloramphenicol should not be used routinely as the topical antibiotic of choice for corneal infection in Nepal, a view supported by studies in Australia, Singapore, and London [
28].
However, failure to perform the susceptibility test of the antifungal agents against the fungal isolates comes under the short coming of this study. Had the resource limitation and financial constrains not restrained us from performing susceptibility test for fungal isolates, the findings generated would have been an updated guideline for Ophthalmologist in this region to choose an appropriate drug among the multiple empirical options available for treatment of corneal ulcer. An extensive microbiological study of corneal ulcer and keratitis with susceptibility testing of broad range of isolates recovered will be our future research preference.
Acknowledgements
We would like to express the gratitude towards the staffs, ophthalmologists and lab workers of Tilganga Institute of Ophthalmology and all the participants without whom this research would not have been possible. In addition, many thanks to Dr. Aasish Jha of Stanford University, USA for his editing assistance and advice on English expression in this manuscript.