About one million people worldwide develop bacterial keratitis every year, mostly from a single infectious agent, but polymicrobial keratitis (PMK) has an incidence of 1.9% to 15.8%. PMK is sight-threatening disease and requires immediate treatment, but treatment protocols can vary, making it essential to know the predisposing risk factors and clinical signs to offer the best option for patients. A recent cross-sectional study looked into the clinical characteristics, complications, and the most prevalent microorganisms causing polymicrobial keratitis and their antibiotic sensitivities.

Included in the study was a consecutive analysis of 656 patient’s records with a diagnosis of bacterial keratitis with a positive culture, and an antibiogram report. Patients were grouped into those having monomicrobial and polymicrobial infection. Specific features that were studied include demographic and clinical characteristics, risk factors, Gram stain, cultures and antibiotic sensitivity.

More than one microorganism was found in 31.5% of the patients, 723 gram-positive bacteria were isolated and 46.5% had polymicrobial keratitis. 161 gram-negative bacteria were isolated, and 61.5% were from polymicrobial keratitis. 0.60% of patients presented ring infiltrate, and 71.42% of those patients had polymicrobial keratitis. Multivariate analysis showed that patients with a history of contact lens use, coexistent autoimmune disease, irregular edges of the infiltrate and ring infiltrate have a higher risk for developing polymicrobial infection. In the polymicrobial group, gram-positive and gram-negative organisms showed a high sensitivity to Netilmicin.

A 31% incidence of PMK was found, much higher than what was reported in previous studies— a 1.9 to 15% incidence of PMK. The authors said possible reasonings could be the lower number of patients included or the characteristics in previous studies, compared to this study which includes a large series of patients in a country that has demographic, hygienic and behavior similar to Latin American countries.

Contact lens use was one of the most frequent risk factors. Though not found in univariate analysis, lens wear had an odds ratio (OR) of 1.78 after adjustment of confounding variables. Immunocompromise also showed significant correlation. “Both endogenous and exogenous immunosuppression are associated with more severe infections,” the authors noted in their paper. “In our study, suffering from an autoimmune disease is associated with a PMK with an OR of 4.64; however, in the case of topical steroids, we could not find an association with a PMK (OR 0.63).” This could be related to the association of steroid use with development of fungal keratitis, they suggested.

“We found other clinical findings that can indicate PMK-like indistinct edges of the infiltrate (OR 2.06) and the presence of ring infiltrate (OR 6.04). Ring-shaped infiltrates are rare in fungal keratitis. For this reason, it may not have been reported in previous PMK studies. The cases with ring-shape infiltrate reported include acanthamoeba infection, gram-negative organisms and herpes, and also in animal models after systemic immunizations ring infiltrates have been reported.”
Additionally, the study concludes sensitivities to moxifloxacin are lower than those reported in previous studies, but sensitivity to Netilmicin is higher.