Researchers look to the gut in investigating debilitating eye condition in horses

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Scientists have found no evidence of changes in the gut bacteria of horses that may be a factor in equine recurrent uveitis, but say more research in the area is warranted.

Equine recurrent uveitis is an immune-mediated disease characterized by recurrent episodes of inflammation within the eye. The condition has an estimated prevalence of 1 to 2% in the United States.

Despite being a major cause of blindness in horses worldwide, the exact cause remains unknown.

Equine recurrent uveitis shares many characteristics with autoimmune uveitis in humans. In humans, as in horses, there is limited knowledge of the cause.

Recently, changes in the normal balance of the gastrointestinal tract bacteria, known as dysbiosis, have been described in several immune-mediated diseases in humans, including uveitis.

Researchers with the University of Florida and the University of Guelph, in Canada, hypothesized that the composition of the fecal bacterial microbiota would be significantly different in horses with equine recurrent uveitis compared with healthy control horses.

Michelle Martin de Bustamante and her fellow researchers, writing in the journal Animals, noted that the fecal bacterial microbiota of normal horses has been previously characterized using next-generation sequencing technology. However, studies investigating the gut bacteria microbiota in horses with autoimmune diseases, in particular equine recurrent uveitis, were lacking.

In their research, they compared the fecal bacterial microbiota of horses with equine recurrent uveitis with that of healthy horses kept in a similar environment on the same farms.

Samples were collected from 15 horses, of various breeds, with a history of the disease. All horses had a minimum of two previously documented episodes with no identifiable primary cause.

All the horses were either affected by an acute flare-up of the condition or had evidence of chronic uveitis at the time of the fecal sample collection.

Samples were collected from a group of 15 horses with no evidence of the eye disease.

The authors found no significant alterations in the fecal microbiota between the two groups.

Firmicutes, Bacteroidetes, Verrucomicrobia, and Proteobacteria predominated in both groups. No taxa were found to be enriched in either group, nor were any differences observed in alpha and beta diversity between groups.

Equine recurrent uveitis does not appear to be associated with alteration of the gastrointestinal bacterial microbiota when compared with healthy controls, they concluded.

However, they suggested more research in the area was warranted. “There is no standardized treatment for horses with equine recurrent uveitis,” they said.

Client-owned horses may receive a range of medications — for example, topical and systemic non-steroidal anti-inflammatory drugs (NSAIDs) or topical atropine — at differing frequencies based on clinician preference and disease severity.

“This variation in medication may also impact the fecal microbiota. For example, systemic administration of NSAIDs causes transient gastrointestinal dysbiosis in horses with decreased alpha diversity and loss of members of the Firmicutes phylum.

“Additionally, horses were enrolled in the study with fecal samples collected in varying states of disease.

“This may represent a confounding variable, as the fecal microbiota could potentially vary based upon disease state. This could be addressed in future studies with a larger study sample size through further subdividing the equine recurrent uveitis group into active uveitis versus quiescent subgroups during statistical analysis.

“Alternatively, the fecal microbiota could be assessed in several individuals at multiple time-points to include uveitic flare-up, recovery, and quiescent periods.”

“Additionally, metabolomics studies should be performed to characterize the fecal metabolic phenotype of horses with equine recurrent uveitis and compare it with healthy controls.”

The full study team comprised Martin de Bustamante, Diego Gomez, Ralph Hamor and Caryn Plummer, with the University of Florida; and Jennifer MacNicol, with the University of Guelph.

Martin de Bustamante, M.; Gomez, D.; MacNicol, J.; Hamor, R.; Plummer, C. The Fecal Bacterial Microbiota in Horses with Equine Recurrent Uveitis. Animals 2021, 11, 745.

The study, published under a Creative Commons License, can be read here

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