The eyes of horses are home to a diverse range of fungi, researchers find

The eyes of horses at pasture had greater fungal species richness and diversity than stabled counterparts.
The eyes of horses at pasture had greater fungal species richness and diversity than stabled counterparts.

The diverse fungi that inhabit the surface of the equine eye have been characterized in an American study.

Researchers from Texas A&M University used molecular-based DNA testing to identify fungi present in the eyes of five pastured mares and a group of seven stallions kept in stables in an open pavilion.

“Fungal community composition and structure were significantly different between pastured mares and stabled stallions,” Mary Walsh and her colleagues reported in the journal PLOS ONE.

The eye surface of the pastured mares had significantly increased fungal species richness and diversity compared to the stabled stallions.

The study team said the eye is host to many bacterial, fungal, and viral organisms that likely influence eye surface physiology in normal and diseased states.

Previous studies indicated there may be a naturally occurring population of various fungi on the equine eye surface, with 13 to 95% of swabs from healthy horse eyes positive for fungal growth. However, the make-up of this population had not yet been described using next-generation sequencing techniques.

For their study, they took swabs from both eyes in each of the 12 horses, none of whom had any eye diseases. All belonged to the teaching herd at the university’s Department of Large Animal Clinical Sciences.

Genomic DNA was extracted and sequenced.

The most abundant genera identified were Leptosphaerulina (22.7%), unclassified Pleosporaceae (17.3%), Cladosporium (16.2%), Alternaria (9.8%), unclassified Pleosporales (4.4%), unclassified Montagnulaceae (2.9%), Fusarium (2.5%), and Pestalotiopsis (1.4%).

The researchers said the study confirms that the eye surface of the healthy horse contains a greater diversity of fungi than suggested by previous culture-based studies.

All 24 eyes sampled contained fungi from at least five genera at an abundance of 1% or more.

They said it had been widely theorized that the environment affects which fungal species colonize the conjunctival sac of horses.

Previous culture-based studies found that stabled horses were more likely to have fungal growth compared to pastured horses as a result of their increased contact with hay and dust. The make-up of the eye fungi may also reflect variations in the stable bedding used.

The authors noted that there was a significantly increased abundance of Alternaria and Aspergillus species in the mares living on pasture.

“This is of clinical interest as these fungal species are often identified in clinical cases of fungal keratitis,” they said.

Fungi, they noted, are common organisms that feed on organic matter.

“Horses exposed to a high density and variety of organic plant material combined with larger fluctuations in humidity and temperature are more likely to harbor a greater diversity of fungal organisms compared to horses living in a more controlled environment,” they noted, offering a possible explanation for the differences between the two groups.

“Living on pasture may therefore increase the risk for fungal infection if trauma occurs to the ocular surface.”

The differences in composition, structure, and richness of fungi inhabiting the equine ocular surface is most likely affected by the housing environment, they concluded.

“Future studies should confirm that the mycobiome is not affected by gender and assess the mycobiome over different time-points to confirm stability.”

Walsh ML, Meason-Smith C, Arnold C, Suchodolski JS, Scott EM (2021) Evaluation of the ocular surface mycobiota in clinically normal horses. PLoS ONE 16(2): e0246537.

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

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