Is This the Missing Virus Behind Mystery Horse Colitis? NC State Researchers May Have Found a Clue
Colitis remains one of the most frustrating and high stakes diagnoses in equine practice. It can escalate fast, dehydrate patients rapidly, and lead to sepsis before clinicians have time to pinpoint a cause. Even more challenging, more than half of equine colitis cases never receive a definitive diagnosis. A new pilot study from North Carolina State University may bring the profession one step closer to solving that mystery. Researchers have identified a previously unknown kirkovirus that appears to be associated with colitis and possibly small colon impactions in horses. While causation has not yet been proven, the findings open the door to new diagnostic and therapeutic pathways for cases that currently fall into the idiopathic category.
Equine gastrointestinal anatomy sets horses up for particularly severe outcomes when inflammation strikes. Their large cecum and colon are designed for water absorption, but inflammation in these structures can quickly lead to dehydration. At the same time, the bacterial load in the large intestine increases the risk of bacterial translocation and sepsis during episodes of colitis. This unique vulnerability makes identifying infectious contributors especially important, particularly when standard bacterial and parasitic testing comes back negative.
The NC State team began by using metagenomic sequencing on fecal samples from 65 horses diagnosed with enterocolitis. Samples were grouped into pools of five horses each. In one pool, all five horses came from the same farm and sequencing revealed a novel kirkovirus. Additional evidence of the virus appeared in four other sample pools. Kirkoviruses have been detected in several livestock species and have been linked to gastrointestinal disease in other animals. Until now, their role in equine disease had not been explored in depth.
To dig deeper, researchers used targeted quantitative polymerase chain reaction testing on 218 fecal samples collected between 2020 and 2025. Horses were divided into three groups: colitis, colic, and clinically normal. The results were notable. Kirkovirus was detected in nearly one quarter of horses with colitis. In contrast, the virus appeared in just over five percent of colic cases and under seven percent of clinically normal horses. Most of the positive colitis cases came from two farms that had experienced farm wide gastrointestinal outbreaks. Even more intriguing, one in four kirkovirus positive horses also had small colon impactions, a condition that is typically uncommon. This association raises important questions about whether the virus may contribute to altered motility or localized inflammation in the small colon.
At this stage, the findings do not establish the virus as a definitive cause of colitis. However, identifying a potential pathogen is a critical first step toward developing diagnostics and targeted therapies. The association with small colon impactions adds another layer of clinical relevance, especially for cases that do not follow typical patterns. Next steps for the research team include determining whether the virus actively infects cells within the equine gastrointestinal tract and how it may interact with the microbiome and immune system. For clinicians, this study reinforces the importance of staying alert to emerging pathogens and recognizing that some cases of colitis may have viral contributors that current diagnostic panels do not detect.
As equine medicine continues to embrace advanced molecular tools like metagenomic sequencing, the black box of idiopathic colitis may start to shrink. This study highlights how collaborative research can translate into practical insights for the field, offering hope for better outcomes in one of the most challenging equine emergencies.
The study was published in Equine Veterinary Journal and supported by the North Carolina Horse Council and NC State University College of Veterinary Medicine Competitive Research Grants Program.
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