We have found a wide variation in the causative organisms for microbial keratitis in different parts of the world. To some degree this variation is explained by economic factors as well as contact lens wear. A high proportion of bacterial ulcers were reported from centres in developed countries (North America, Australia, and Western Europe). In these countries, patients are far less likely to be agricultural workers, and so have a reduced risk of trauma from organic matter, which is known to be a risk factor for fungal infection.[28
A high percentage of staphylococcus species (79%) was recorded in the study from Paraguay [14
] although the reason for this is not clear. Of note, the authors comment that their patients have to make long journeys to their hospital. Thus, their data may reflect more severe cases of microbial keratitis.
The study from Tamil Nadu [16
] found the highest proportion of streptococcus species (46.8%). The authors noted that this figure was only 18.5% in 1986 and suggest that the trend might represent a genuine change in the bacterial flora due to changes in the climate and environment.
The study from Bangkok [15
] had the highest proportion of pseudomonas infections (55%). Interestingly, this study did not have the highest proportion of contact lens wearers (only 24%). Other studies reported far higher proportions of contact lens wearers, for example 44% in a study from Taiwan [26
] and 50% in the study from Paris [11
]. When we compared the percentage of contact lens wearers with the percentage of pseudomonal infections (), the Spearman correlation coefficient was not statistically significant. Interestingly, Cohen et al. [39
] at Wills Eye Hospital reported a decline in contact lens-related ulcers: during 1998 to 1991, contact lens wear accounted for 44% of all ulcers, but during 1992 to 1995, it accounted for only 30%. The authors speculated that their figures might reflect a reduction in the number of referrals to their unit due to the increased availability of fluoroquinolones in the community.
Trauma was a major risk factor for corneal infection in certain countries. In Paraguay [14
], the percentage of cases with preceding trauma was 48%, in Eastern Nepal[19
], 53%, in Madurai, South India[4
], 65% and 83% in Eastern India[11
] (most commonly from injury by the paddy or its stalk). The authors of this last study noted an increase in keratitis during harvesting season.
The above studies also addressed the frequency of self-medication prior to presentation at a tertiary referral unit. In the Madurai study, 20% of patients had been to a village healer and 87% had been started on topical medication, of whom 8% were on topical corticosteroids. In the study from Eastern India, 18% of patients had used medication before coming to clinic, and in the Paraguay study the proportion was 83%.
Jeng and McLeod[40
] commented on the emerging resistance of bacterial infections to fluoroquinolones. In addition to changes in resistance patterns, studies have also demonstrated changing patterns of causative organisms over time in a given geographical location. Varaprasathan et al.[41
] reported that the proportion of S. pneumoniae and P. aeruginosa ulcers in Northern California had decreased over a 50 year period whilst that of S. marcescens had increased over the same period. Sun et al.[24
] reported a rise in the percentage of gram positive cocci in North China from 25% in 1991 to 70.8% in 1997, as well as a decrease in gram negative bacilli from 69% to 23.4% over a similar period.
Leck et al.[16
] have previously compared corneal ulcers in Ghana and South India, whilst Lam et al.[5
] have discussed differences between Hong Kong, Europe and North America. However, the present study is the first to present a worldwide comparison of corneal infections.
In interpreting this comparison, a number of limitations must be considered. Variations existed in the definition of microbial keratitis between studies. Lam et al, reporting on cases from Hong Kong [5
], included patients with ‘the clinical presentation of a corneal stromal infiltrate >1 mm2
’. This differs from Srinivasan et al [4
] who included patients with ‘loss of the corneal epithelium with underlying stromal infiltration and suppuration associated with signs of inflammation with or without hypopyon’. There were variations in methods of culture. For example, one study [21
] used Sheep’s blood agar, Chocolate, Non-nutrient, Sarbarouds, brain-heart infusion and potato dextrose agar, whilst another [18
] used only Chocolate and Sabourauds media. Some studies did not specify the media used [17
]. All studies included bacterial infections, but not all included fungal, protozoal and yeast organisms. The majority of studies looked at all cases of microbial keratitis whilst some looked only at patients requiring hospital admission (Wong et al. and Cheung et al. [3
]). It is likely that in these studies, particularly virulent organisms will be over-represented. Finally, data are only available from centres that have conducted studies on microbial keratitis, limiting the coverage of certain regions of the world.
Despite these limitations, we have presented to our knowledge, for the first time, a worldwide overview of causative organisms in microbial keratitis demonstrating associations between specific types of microbial keratitis and national income.