Researchers at North Carolina State University have identified a novel kirkovirus that may be linked to colitis and possibly small colon impactions in horses. The findings, published in the Equine Veterinary Journal, could help pave the way for new treatments for equine colitis cases with unknown causes.
“Horses are uniquely susceptible to colitis, and the structure of their gastrointestinal tracts amplify the negative effects,” said Lilly Haywood, a Ph.D. student at NC State’s College of Veterinary Medicine and first author of the study. “Horses have very large colons and cecums to facilitate water absorption, so when these structures become inflamed the horses dehydrate quickly. And their large intestines contain a lot of bacteria, so inflammation can lead to those bacteria entering the bloodstream and causing sepsis.”
Breanna Sheahan, assistant professor of equine medicine at NC State and corresponding author on the study, noted: “The other issue when dealing with colitis in horses is that in more than 50% of cases we are unable to find the cause. We suspected there might be another viral cause for some of these cases, so we started looking for one.”
The research team conducted metagenomic sequencing on fecal samples from 65 horses diagnosed with enterocolitis. These samples were grouped into 13 pools containing five samples each. The analysis detected a previously unidentified kirkovirus—a type of virus found in various livestock animals—in one pool from five horses at a single farm. Additional evidence of this virus was found in four out of twelve remaining sample pools.
Subsequently, targeted quantitative polymerase chain reaction (qPCR) testing was performed on 218 fecal samples collected between 2020 and 2025. The samples were divided into three groups: colitis (87), colic (56), and clinically normal (75). Kirkovirus was detected in 24% of horses with colitis compared to lower rates among those with colic or no symptoms.
Most affected horses within the colitis group came from two farms that had experienced widespread outbreaks of gastrointestinal disease. Furthermore, researchers observed that one-quarter of kirkovirus-positive cases also involved small colon impactions—an uncommon condition.
“The first step to developing therapeutics is identifying the pathogen,” Sheahan said. “While we can’t definitively say that this novel kirkovirus was responsible for the illness, this work does identify a potential culprit in some cases, particularly because of the association between the virus and small colon impactions. Next steps for this work will be to find out whether kirkovirus infects the cells of the equine gastrointestinal tract.”
The project received support from both an intramural grant by North Carolina State University’s College of Veterinary Medicine Competitive Research Grants Program and funding from North Carolina Horse Council 536126. Former undergraduate Ava Clark and Ben Hause from Cambridge Technologies contributed as co-authors.
According to study results published November 18, 2025 (DOI: 10.1111/evj.70121), next-generation sequencing identified kirkovirus genetic material in five out of thirteen pooled enterocolitis samples—including complete viral genomes—and revealed seasonal patterns among positive cases occurring mainly during autumn through spring months.
Although an association between this novel virus and outbreaks has been established—especially regarding small colon impactions—the authors caution that further investigation is needed before confirming causality or determining if kirkovirus directly infects horse tissues.
