Colitis in Horses: Signs, When to Call the Vet and the New Kirkovirus Clue

What is colitis in horses?

Colitis in horses is inflammation of the colon, the big water-absorbing part of the large intestine, and its hallmark is sudden, watery diarrhea. It is a veterinary emergency because a sick horse can lose huge amounts of fluid fast and get sick all over, not just in the gut.

It's back in the news this week. On Oct. 19, 2026, EquiManagement summarized North Carolina State University research that found a newly identified kirkovirus more often in the manure of horses with colitis.

That finding is interesting, but it is an early clue, not an answer. Below we explain what the study really showed, then cover what every owner, rider and barn parent should know: the signs, when to call, and how to keep a diarrhea case from spreading.

The kirkovirus study: what it found and what it didn't

The research, led by PhD student Lillian Haywood and assistant professor of equine medicine Breanna Sheahan, was published in the Equine Veterinary Journal in November 2025. NC State announced it in December 2025.

The team first used genetic sequencing on manure from horses with colitis and spotted a kirkovirus that hadn't been described before.

Then they used PCR tests on fecal samples collected from 2020 to 2025:

Why it's not proof yet

• Positives clustered. Most came from three farms, two of which had farm-wide gut outbreaks. On one, 7 of 8 sampled horses were positive.
• The gap vanishes without those farms. Excluding them, colitis cases were 5.5% positive, compared with 3.6% for colic cases and healthy horses.
• No infection shown. The authors say they have not yet shown the virus infects the equine gut. It might cause disease, or it might simply be shed more when gut bacteria are already disturbed.
• Other germs were present. On the second outbreak farm, 11 of 20 horses tested positive, and 6 of those also had equine coronavirus. That farm also had 2 Salmonella cases, but the researchers couldn't check whether those horses also carried the kirkovirus.

Two details worth watching

• Season. Positive samples were collected from September through April, peaking in January. That hints at a cool-weather pattern, which is why this matters now.
• Unusual impactions. A quarter of positive horses had a small colon impaction, a blockage seen in only about 2.5% of colic cases overall.

The next step, per NC State, is testing whether the virus can infect horse gut cells. Until then, nothing changes for your vet's workup or your barn routine.

Colitis vs colic: what's the difference?

People mix these up all the time. Colic isn't a disease; it's the word for belly pain, and dozens of problems cause it. Colitis is one specific problem: an inflamed colon.

• A horse with colitis may show colic signs, because an inflamed gut hurts.
• Many colic cases have no diarrhea at all, like gas or an impaction.
• Colitis can begin before the diarrhea shows up, with a dull horse, fever and no appetite.

For pain-focused signs like pawing, rolling and flank-watching, see our guide to early signs of colic. Either way, the first move is the same: call your vet.

Warning signs of colitis

These are the most common warning signs:

• Diarrhea: cow-pie to watery manure, often sudden and frequent
• Fever: a temperature higher than your horse's normal
• Depression: dull, head low, not interested in surroundings
• Off feed: leaving hay or grain, sometimes drinking less
• Dehydration: sunken eyes, tacky gums, skin that's slow to flatten when gently pinched
• Belly pain: pawing, lying down more, looking at the sides
• Fast heart rate or breathing compared with your horse's resting numbers

Knowing your horse's normal temperature, pulse and breathing rate makes these much easier to spot. Our first aid kit and vital signs checklist shows how to take them.

Why it's an emergency and when to call the vet

A horse's colon and cecum normally reabsorb enormous amounts of water. When they're inflamed, that water pours out as diarrhea, and dehydration can set in quickly.

An inflamed gut also lets bacteria and their toxins leak into the bloodstream. That can cause sepsis, endotoxemia and laminitis, which is why vets treat colitis aggressively.

Call your vet right away if:

• An adult horse has watery or very loose manure, even once with other signs
• Loose manure comes with fever, dullness, belly pain or poor appetite
• You see any signs of dehydration
• More than one horse in the barn has diarrhea or a fever
• A foal has diarrhea (foals dehydrate even faster)

While you wait

• Move the horse to a stall or pen away from others, if it's safe to do so.
• Write down the temperature, when the diarrhea started and any feed, travel or medication changes.
• Leave clean water available, and ask your vet before giving any food, supplement or medicine.
• Don't try home remedies. Treatment decisions belong to the vet who examines the horse.

Horses with acute diarrhea generally need IV fluids, and severe cases may need intensive hospital care, so it helps to know ahead of time where your nearest equine hospital is and how you'd get there.

Why so many cases never get a cause

This surprises a lot of owners. According to NC State, the cause is never identified in more than half of colitis cases.

Vets test for known culprits like Salmonella, Clostridioides difficile, equine coronavirus, parasites and, in some areas, Potomac horse fever. Antibiotics, anti-inflammatories, sand and sudden feed changes can also play a role.

Shedding can be on and off, so one negative test doesn't always rule a germ out. That's why vets may ask for several samples, and why research like the kirkovirus study matters: every new candidate could eventually shrink the "unknown" pile.

Diarrhea outbreak in the barn: biosecurity steps

Because the cause is often unknown, and some causes can spread to people, treat every diarrhea case as potentially contagious.

The AAEP says contagious or zoonotic germs like Salmonella, coronavirus, rotavirus and C. difficile justify biosecurity steps. Texas A&M advises keeping a horse isolated until it tests negative or has gone at least 14 days without signs.

Barn checklist

• Isolate the sick horse, ideally with no nose-to-nose contact.
• Give it its own buckets, brushes, halter, manure fork and wheelbarrow.
• Care for healthy horses first and the sick one last.
• Wear gloves and boot covers or dedicated boots at the sick stall.
• Wash your hands every time you leave the area, and make sure kids do too.
• Take temperatures on the other horses daily and report any fever.
• Pause horse traffic in and out until your vet says it's okay.
• Ask your vet which disinfectant to use after cleaning out the stall.

The same habits apply to strangles and other contagious diseases, and our safety page covers more barn basics.

Prevention and what to watch next

You can't prevent every case, but you can lower the odds:

• Make feed changes gradually, over about two weeks.
• Keep forage (hay and pasture) the backbone of the diet. See our nutrition guide.
• Feed off the ground in sandy areas.
• Use antibiotics and anti-inflammatories only as directed by your vet.
• Quarantine new arrivals and watch horses coming home from shows.

What to watch next: NC State's follow-up work on whether the kirkovirus actually infects the horse gut. Until there's an answer, there's no kirkovirus test to request or vaccine to give. Keep an eye on fall and winter diarrhea cases in your area, and talk with your vet about your barn's isolation plan before you need it. More health stories live on our trends page.