Microbial keratitis is a potentially vision-threatening condition and it leads to significant public health problem.1 Although nonsurgical trauma to the eye accounted for 48.6-65.4% of all corneal ulcers in the developing countries of Nepal2 and India,3 in the United States, it accounted for only 27% of corneal ulcers.4 In the developed nations it is contact lens (CL) wear that has emerged as a major risk factor for microbial keratitis.1 The purpose of this study was to determine the current trends in the incidence and the microbiological profile of CL-induced microbial keratitis.
Materials and Methods
All culture-positive cases of CL-associated microbial keratitis presenting between September 1999 to September 2002 were included in this retrospective study. Each patient was examined at the slit-lamp; clinical features were noted and drawing was made for patient's records. A corneal scrape was performed using flame-sterilized Kimura spatula or Bard-Parker blade (# 15) following instillation of 0.5% proparacaine hydrochloride. The material obtained was subjected to direct microscopic examinations (10% potassium hydroxide wet mount and Gram-stain) and culture (on 7% sheep blood agar, chocolate agar, Sabouraud dextrose agar, nonnutrient agar, thioglycollate medium and brain heart infusion broth media). In addition to corneal scrapes, CL storage cases along with lenses and lens care solution bottles were collected at the time of presentation and were subjected to microbiological evaluation for determining the microbial contamination of lens care product. A standardized protocol was followed for each patient with corneal ulceration for the evaluation of microbiological and clinical features.5 Pearson's Chi-square test was used to carry out the statistical analysis wherever required and P value < 0.05 considered to be statistically significant.
A total of 3295 consecutive patients with corneal ulceration were evaluated, of which, corneal ulcers associated with the use of CLs were identified in 35 patients (1.06% of 3295). All 35 patients wore soft CLs for their refractive error corrections under the daily wearing schedule. The patients ranged in age from 14 to 28 years (mean; 20.8 years, SD 3.65) [Table 1]. Females (74.29%) predominated males and the majority of them were students (82.86%). Of 35 patients, single eye was infected in 32(91.43%) patients and both eyes were affected in three (8.57%) patients. Total of 38 eyes were studied [Table 1].
A total of 42 bacterial pathogens were recovered from 38 ulcerative cornea, of which 30(71.43%) were Pseudomonas aeruginosa (28 were isolated as single species and the remaining two were mixed with Enterobacter sp.). The culture of CL storage cases (total of 70 wells) and the lens care solution of 35 patients yielded positive bacterial growth in all 70 wells and six of the 35 bottles of lens care solution respectively. Bacterial pathogens recovered from CL storage cases were identical with the bacterial species recovered from the corneal scrapes of the respective infected eyes [Tables 1 and 2].
With growth of soft CL wear, the incidence of CL-associated microbial keratitis has increased to up to 30% of all keratitis in developed countries,16 whereas in this present study, it was found to be 1%. This low incidence in the present study may be attributed to the limited number of people wearing CL in our region due to economic factor. The microbes responsible for CL-associated keratitis include gram-negative bacteria and rarely, gram-positive bacteria and fungi, whereas Acanthamoeba predominated in the developed countries.1 Several CL-related and non-CL-related factors were attributed to the higher incidence of Acanthamoeba keratitis among CL wearers in developed nations.7 In contrast, bacteria was found to be the only pathogen for all CL-associated keratitis in this study. Pseudomonas aeruginosa was reported to be the most common organism isolated from CL wearers in the developing world8 and similarly P. aeruginosa (P < 0.001, odds ratio: 11.54, 95% CI: 5.81-22.92) is more frequent than other gram-negative bacilli in this study.
All CL storage cases and six lens care solution bottles were found to be contaminated with potential gram-negative bacilli in this study and the bacterial pathogens recovered from the corneal scrapes were identical with the bacterial pathogens isolated from the CL storage case wells of corresponding infected eyes. It can be assumed that the source of contamination was the CL storage cases. In developing countries like India, commonly used water is contaminated by gut commensals, especially Pseudomonas. Contact of CLs and CL storage cases with water can cause contamination by Pseudomonas, which survives well in the moist environment offered by CLs, CL storage cases and lens care solutions. Contaminated CLs which were used by the patients, acted as a vector for transmitting the microbes from the CL storage cases to the patients’ conjunctiva and cornea by forming polysaccharide-containing bio-film on the posterior surface of soft CLs by bacterial adherence. Bacterial adherence to artificial surface is also thought to be mediated by hydrophobic bonding and relatively hydrophobic strains adhere very readily to CLs. In addition, more acidic environment due to raised level of lactic acid and carbonic acid in the tear film could reduce the pH and increase bacterial adhesion.9 Although Pseudomonas adhere poorly to intact corneal epithelium, corneal surface damage due to trauma by poorly maintained CLs during the insertion and removal of CLs provide entry into the cornea leading to keratitis by Pseudomonas.
The type of microorganisms recovered from the corneal scrapes and CL-care products- P. aeruginosa, Enterobacter sp, Klebsiella sp, Alcaligens sp. and Serratia sp, are not part of the resident ocular flora and are widely distributed in soil, water, sewage, gastrointestinal tract of humans and their presence indicates that the source of contamination is external in nature.10 In conclusion, the incidence of CL-induced microbial keratitis is lower (1%) than the incidence due to other risk factors in our region. Undoubtedly the microbial contamination of CL storage cases was a great risk for gram-negative corneal infection among soft CL-wearers. Pseudomonas spp. were the dominant causative agents. Increased awareness of adequate lens care and disinfection practices, continuous supervision of all CL-wearers and frequent replacement of CL storage cases would greatly help to reduce this risk of infection on cornea.
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