The Equine Ocular Microbiome 
A study used sequencing of DNA to evaluate ocular surface microbiota in both eyes of horses with unilateral ulcerative keratitis.
Equine eye, equine ocular microbiome
The ocular microbiome might be implicated in the pathophysiology of ulcerative keratitis. | Getty Images

Ulcerative keratitis is painful for the horse and has the potential to create protracted disease from bacterial or fungal contamination. A study at Texas A&M used state-of-the-art, next-generation sequencing of DNA to evaluate ocular surface microbiota in both eyes of horses with unilateral ulcerative keratitis [Julien, ME.; Shih, JB.; Correa, Lopes B.; Vallone, LV.; Suchodolski, JS.; Pilla. R. et al. Alterations of the bacterial ocular surface microbiome are found in both eyes of horses with unilateral ulcerative keratitis. PLoS ONE 2023, 18(9); doi: 10.1371/journal pone.0292018].

In the study, 15 horses with unilateral corneal ulcers had conjunctiva of both eyes—infected and normal—were swabbed for cytological and DNA analysis. For controls, 15 horses free of ocular disease were used; one eye was swabbed for analysis, and another unused swab was evaluated as a negative sample to look for the presence of environmental contamination.

Of the 15 infected eyes, 15 bacterial isolates were identified, with the most prevalent being Staphylococcus spp. in 4/15 eyes, and Streptococcus spp. in 4/15 eyes. Eight fungal isolates were identified with Aspergillus spp. found in 3/8 eyes. Bacterial or fungal cultures did not grow in 4/15 horses. However, next generation sequencing detected organisms in all samples despite negative cultures.

Both the ulcerated and unaffected eyes showed evidence of microbial dysbiosis/immune compromise. It is notable that increased numbers of Bacilli occurred in ulcerated eyes. Bacterial numbers increased in both the affected and unaffected eyes, pointing to a possibility that the ocular microbiome might be implicated in the pathophysiology of ulcerative keratitis. The ocular surface resident microbiome of the healthy eyes (controls) was able to maintain stability when not affected by disease. The authors suggested that shifts in the homeostatic microbiome may be linked to infectious pathologies.

The results demonstrate that “the bacterial community structure was altered in both the ulcerated and unaffected fellow eyes of horses with unilateral ulcerative keratitis compared to control eyes of healthy horses.” Further, eyes affected by ulcerative keratitis had decreased species richness and diversity when compared to the horses’ unaffected fellow eyes. 

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