Equine colitis: Causes, consequences and management challenges for veterinarians


Veterinary strategies on fighting this all-too-common and life-threatening condition in horses.

Paynter, a 3-year-old colt, fought down the racetrack's lane, just missing the win by a neck to Union Rags at the Belmont Stakes on June 9. On July 29, he fought again, this time pulling away from the rest of the field at the top of the stretch to commandingly win the $1 million Haskell Invitational.

But sadly those races would not be his biggest fights. After he contracted a fever and pneumonia, Paynter was sent to the Mid-Atlantic Equine Medical Center in New Jersey and administered fluids and antibiotics on July 31. Shortly thereafter, he was shipped to Belmont Park in Elmont, N.Y., to recuperate and then to Saratoga Race Course in Saratoga Springs, N.Y., for a shot at the late summer derby, the Travers Stakes, Aug. 25.

Again showing signs of illness on Aug. 28, he was sent to the Upstate Equine Medical Center, Schuylerville, N.Y., and was placed under the care of Laura Javsicas, VMD, DACVIM. Paynter was treated aggressively with intravenous fluids and plasma on a 24/7 regimen to try to combat a bout of severe colitis. On Sept. 2, Paynter began to show signs of laminitis, a common complication of colon inflammation. As of Sept. 12, Paynter is stable and still being treated.

For the win: Paynter, a thoroughbred shown here winning the Haskell Invitational at Monmouth Park in Oceanport, N.J., has been diagnosed with the debilitating disease equine colitis, and the fight to conquer this condition has begun. (BILL DENVER/EQUI-PHOTO)

Paynter's story is an illustration of the devastating effects equine colitis and its associated complications can have on affected horses.

Equine colitis overview

According to Douglas Thal, DVM, DABVP, Thal Equine LLC, Santa Fe, N.M., the equine digestive tract is a complex and fragile system that is easily disrupted. One sign that the colon is disturbed is the development of diarrhea, from mild to severe—even life-threatening. Once colon health is disrupted, its ability to carry out the normal functions of digestion and absorption are critically affected.

"Serious colitis causes severe diarrhea, which accounts for huge water loss, and can cause rapid loss of fluid from the circulatory system," Thal says. "This leads to a vicious cycle of low blood pressure and reduced blood flow to vital organs and circulatory shock, which can quickly result in death if untreated."

Colitis disrupts the integrity of the mucosa. Once a horse is affected, the ability of its colon to absorb water and nutrients—the colon's normal function—is compromised. In addition, major shifts take place with respect to the bacterial population needed for normal gastrointestinal (GI) function.

GI inflammation can take several forms. It can occur as segmental disease, limited to one section of the GI tract, or as more diffuse, nonsegmental disease, says K. Gary Magdesian, DVM, DACVIM, DACVECC, DACVCP, professor at the University of California-Davis School of Veterinary Medicine. Magdesian notes that the illness is called:

  • Colitis when restricted to the large colon

  • Typhlocolitis if the cecum also is involved

  • Enterocolitis if the small intestine is included

  • Enteritis when it's limited to the small intestine

  • Gastritis if the inflammation affects only the stomach

  • Gastroenterocolitis if the entire GI tract is involved.

In general, the younger the horse, the more likely it is to have primarily small intestinal involvement. But even mature horses can have any of these forms, including diffuse disease, disease limited to the small intestine or disease limited to the large colon.


Making an accurate diagnosis depends on the age of the horse, geography, signalment and history. "In adult horses with acute colitis or enterocolitis, we look for infectious and noninfectious causes," Magdesian says. Infectious causes in adult horses include:

  • Salmonella species infection

  • Clostridium difficile infection

  • Clostridium perfringens infection

  • Potomac horse fever (Neorickettsia risticii infection)

  • Equine coronavirus

  • small strongyles infestation (a potential parasitic cause).

"Less common causes, including isolated reports of other Clostridium species and a bacterium, Aeromonas, have been associated with enterocolitis in horses," Magdesian says.

However, in many cases no specific cause is found. "Sometimes it's a consequence of diagnostic tests being used too late in the course of disease or use of an incorrect test. But there are a lot of cases where we cannot determine a specific cause," Magdesian says.

Javsicas agrees, adding that in about half of colitis cases she sees, a diagnosis is not made even though all the known causes are investigated. "It's pretty common that we're left without a clear-cut answer as to what set things off," she says. "A horse's large colon environment is a very fine balance of normal bacteria, and they can get set off by a number of things, all of which result in fairly similar clinical signs and potential sequela."

When treating horses with antimicrobials for other existing infections, there's a chance the drugs may result in an imbalance in the gut microflora, which then leads to colitis. Infection with Salmonella species, C. perfringens and C. difficile are included in that group, along with nonspecific flora changes.

"Any time you put a horse on antimicrobials, it can have flora shifts within the GI tract," says Magdesian. "It occurs similarly with people, but the risk is greater in horses. When the flora shifts, then particular bacteria may overgrow. In some cases, the most common known shift is to C. difficile, but C. perfringens and Salmonella can do that as well. The most common theory in racehorses is that they're put on antibiotics for respiratory or other infections and they develop a secondary colitis from the [flora] shift. Often in California, we see C. difficile in those horses."

Horses tend to have more sensitive reactions to antimicrobials with resultant flora shift compared with most animals such as dogs and cats—or even people—because they're hindgut fermenters. Experimental models show that other hindgut fermenters such as rabbits and guinea pigs when given a tiny dose of certain antibiotics predictably get colitis.

Concerns, questions

Thal notes several concerns and questions inherent in accurately diagnosing and effectively treating colitis.

1. Protein loss is massive. Huge quantities of plasma and synthetic colloids such as hetastarch are needed to keep pace with protein loss.

2. Massive fluid and electrolyte loss occur. It takes huge volumes of intravenous crystalloid fluids to keep pace with these losses.

3. Laminitis may occur secondary to the GI disease. "Just when you think the colitis has taken a turn for the better, laminitis can start," says Thal. "Cold should be the best prophylaxis in these cases, but is difficult to do well. I know it's controversial and probably refuted by now, but I still use nitroglycerin topically."

4. Thrombosis can occur. Because catheters are necessary to administer intravenous supplementation, a horse's jugular vein and any place that had a catheter may develop thrombosis. The animal may suffer disseminated intravascular coagulation in which anti-clotting factors (e.g., the protein antithrombin III) are lost in great quantity, leading to spontaneous clotting. Another thing to consider is the effect of gram-negative endotoxin on the endothelial cells, which sets up a hyperinflammatory, hypercoagulable state, so the animals end up in thrombosis. "Once a catheter site starts to go, it's usually over for the vein," says Thal. "One horse I sent home lost two major veins—one lateral thoracic and one jugular. A lot of them go home with at least one thrombosis. These are the typical challenges we face in treating these horses."

5. Identifying an agent is extremely difficult in many cases. Even with PCR panels, there are many cases that go undiagnosed.

6. Should you use systemic antibiotics or not? Since the GI flora can be tricky, determining the best regimen is critical.

7. Are probiotics useful? "This would include 'transfaunation' with filtered manure from a healthy horse," says Thal (see below for more).

8. Should you prescribe NSAIDs? They may seem necessary, but studies have shown adverse effects on GI healing that must be considered.

9. Do intestinal protectants and antidiarrheals work? Examples include di-tri-octahedral smectite (Bio-Sponge—Platinum Performance) and bismuth subsalicylate. But how helpful are these treatments it in these cases?

Treatment considerations

Basic treatment for equine colitis includes administering fluid therapy and electrolytes, restoring flora imbalance and providing supplemental protein because the animal's serum protein concentration decreases rapidly.

Javsicas says, "The use of antibiotics depends on the case and whether antibiotic use might have been the cause of the colitis. We reserve antibiotics for cases where the horse has a very low white blood cell count or those that are persistently febrile. In cases in which we're worried about the specific cause, such as those with Potomac horse fever, we'll start them on treatment for that right away. Because if we treat early, we're much more likely to have a positive outcome."

When treating colitis in horses with several complications, secondary disease may occur.

"In the acute period, the concerns are shock from endotoxemia and hypovolemia," Magdesian says. "The consequences of these can include laminitis and disseminated intravascular coagulation, where the horses become hypercoagulable—or at risk for thrombosis in blood vessels, especially at catheter sites. They can develop peritonitis and can experience hypoproteinemia and edema. And they can become immune-compromised and develop secondary infections such as pneumonia or infections at catheter sites."

Thal says, "We try to know what organism we're treating so we can use an antibiotic with a spectrum that actually is doing some good and not doing harm. One of the biggest mistakes I've made when treating these cases is overdoing it with antibiotics."

Thal thinks the horse is one of the most sensitive species regarding antibiotic-associated diarrhea and colitis. "We treated a number of cases with antibiotics for other problems such as musculoskeletal infections and other diseases and ended up with severe antibiotic-associated diarrhea. The key question is whether to treat these horses with antibiotics at all. I've had this conversation with colleagues over the years. We assume that if it's a gram-negative infection, it's probably Salmonella. Of course, Potomac horse fever is a possibility as well, though it is not responsive to our typical course of antibiotics, but it is responsive to tetracycline. We can at least identify that organism, which is truly rare in our area. But if so, we would target it."

When treating a colitis case in which a definitive diagnosis cannot be made, Thal says he treats for gram-negative enterics, (e.g., Salmonella) and uses a narrow-spectrum gram-negative microbe treatment, usually gentamicin. He no longer uses penicillin unless he thinks it's needed, nor does he use metronidazole unless he is confident he is treating a clostridial infection. "And that's really rare in some geographical locations," he says.

New developments

One good development in treating these cases is the use of panel-type PCR tests, whose results can give a good idea of the organism present. "It's better than relying on culture, which is inherently insensitive and sometimes slow," Thal says. "The panels test for the common organisms. I'm still getting probably less than 50 percent return on them, and most of the time they all come back negative. So I'm left with a diagnosis that it's either an antibiotic-associated diarrhea or I've got an organism growing that's causing this condition. What do I do? I just keep the spectrum narrow with the antibiotics."

New treatment methods are under investigation as well. Thal says four to five years ago he started treating these horses with filtered manure from a healthy horse. "The effectiveness of probiotic use in the horse is pretty sketchy. I've tubed a fair number of horses with really bad colitis with large quantities of filtered manure from a healthy horse to try to alleviate the problem. We use three to five pounds of fresh manure from an apparently healthy, well-dewormed horse. We mix with water, and filter off solids to two to three gallons of filtrate, so it can be pumped through a stomach pump and tube. I administered this one to two times per day."

The procedure of tubing horses with fecal filtrate is extrapolated from human medicine, Magdesian says. "The administration of fecal enemas—a fecal liquor from a healthy relative—is a recognized treatment for recurrent forms of C. difficile infection in humans. We've adopted that for horses."

He first treats the horse with omeprazole to increase the stomach pH so the acid of the stomach doesn't kill the microflora (within the manure mixture). He then uses a tube to administer the horse fresh fecal liquor (with large fibers filtered out) from a healthy donor horse, one that has been cultured negative for Salmonella and other dangerous pathogens.

"No studies have proven the procedure definitively works in horses," says Magdesian, "but it's done in some cases. And a lot of clinics use the technique in an attempt to restore a healthy GI microflora in sick individuals. Though there've been no studies to prove it's effective, and since we use so many treatments, it's hard to say whether this makes a difference in outcome without clinical trials. It's one of many in our treatment armamentarium that we use. The idea is good, and there are certainly models in human patients where it helps."


Treatment outcome depends on the severity and the cause of colitis. For example, horses with C. difficile infection have higher mortality and complication rates than those with other forms of colitis, at least in California, says Magdesian. Horses with colitis due to the group B Salmonella infections also have a higher mortality rate. "The severity of Salmonella infection seems to differ with the host response, but also the age of the horse. Group B salmonellae tend to be more virulent than other groups, such as Group E," says Magdesian.

Thal says the nursing care is critical. "The colon takes a number of days to regain function. I understand that in severe cases, the whole mucosa has to be regenerated before these horses start having a functional colon again. You've got to stick it out during that time."

A positive outcome for colitis cases can be a toss-up. "Certainly getting the GI tract back to normal function can be a challenge, but more often than not, it's the secondary complications we see that can be what, unfortunately, lead to a negative or unsuccessful outcome," says Javsicas. "Horses that contract laminitis, but also those with thrombophlebitis and clotting disorders, are at significant risk. Sometimes they'll get hematogenous spread of bacteria or fungus to their lungs, which leads to secondary pneumonia."

Because of the degree of insult, the time course to return the lower gut to good function varies from three to four days to up to two weeks or more before you may see resolution of the diarrhea, says Javsicas. And then the horse can have ongoing inflammation and alteration in the colon for a time after that. In short, it can take quite awhile to get back to a normal GI function.

For the general horse population, this condition can be very expensive to treat because of the time period and the complications that may be encountered along the way. "Sometimes it can be hard to prognosticate for the owners, because you don't know for sure what's going to occur," says Javsicas. "In my opinion, you're always more successful if you treat very aggressively from the beginning. The client needs to be prepared for that."

Ed Kane, PhD, is a researcher and consultant in animal nutrition. He is an author and editor on nutrition, physiology and veterinary medicine with a background in horses, pets and livestock. Kane is based in Seattle.

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