Causes of Diarrhea in Horses (Beyond Salmonella)

Diarrhea can rapidly develop into a life-threatening condition that can cost thousands of dollars to treat. Knowing the potential causes of diarrhea can help ensure a full recovery and minimize the chances of the development of fatal complications.

Despite a horse's apparently sturdy exterior, his various internal body systems are delicate--particularly his gastrointestinal (GI) system. The normal GI tract is populated with various "good" bacteria and protozoa--referred to as a horse's normal flora--that serve one main purpose: to prevent the growth of other microbes, particularly pathogenic (disease-causing) bacteria such as Salmonella spp and Clostridium spp. This is achieved by two mechanisms. One is the physical presence of the "good" microbes that physically blocks the growth of pathogens, and the second is related to the natural flora's production of short-chain fatty acids and other metabolites that inhibit the growth of potential pathogens.

"Any factor that alters the normal population of bacteria that inhabit the GI tract can potentially result in a fatal diarrhea," explains Rodney Belgrave, DVM, MS, Dipl. ACVIM, an internal medicine clinician at Mid-Atlantic Equine Medical Center, located in Ringoes, N.J.

While horse owners are likely aware of some of the more common or well-known causes of diarrhea, other factors contributing to the development of diarrhea in horses continue to be overlooked. This article will look at some of the less commonly considered causes of diarrhea.

Equine Proliferative Enteropathy (EPE), an Emerging Disease

EPE, caused by the bacterium Lawsonia intracellularis, is a disease of the intestines characterized by a thickening of the inner lining of the small and large intestines. L. intracellularis is now widely considered an important pathogen, and veterinarians are diagnosing more and more cases throughout North America, Europe, Australia, and South Africa.

At present, information regarding sources of infection, transmission of the bacterium, and epidemiology of the disease remain poorly investigated. However, veterinarians are getting a better idea of what to look for in order to diagnose and treat affected animals more quickly. For example, affected animals often are:

  • 2-8 months of age (although older animals can also be affected);
  • Presented for treatment between August and January;
  • Have ventral edema (81% of infected horses) and hypoalbuminemia (an abnormally low albumen content in the blood--albumen serves as a transport protein for many necessary body functions; 100% of infected horses);
  • Do not necessarily test positive for L. intracellularis on fecal polymerase chain reaction (PCR) or a blood test (called the serum immunoperoxidase monolayer assay); and
  • Have good survival rates (93% of infected horses lived).

Other signs of EPE include diarrhea, depression, fever, anorexia, weight loss, and intermittent colic.

With appropriate diagnostic testing and antibiotic therapy (e.g., macrolides, tetracyclines), veterinarians are successful in treating most cases.

"In untreated clinical foals, however, EPE is invariably fatal," says Nicola Pusterla, DVM, PhD, Dipl. ACVIM, associate professor in the Department of Veterinary Medicine and Epidemiology at the University of California, Davis, School of Veterinary Medicine. "The key is for owners and veterinarians to be cognizant of this pathogen as a potential cause for diarrhea in weanlings."

To better understand the disease, Pusterla and colleagues are focusing on epidemiology (the sum of the factors controlling the presence or absence of a disease or pathogen), diagnostics, and prevention (including vaccination). As described by Pusterla at the 2008 American Association of Equine Practitioners conference, researchers administered a commercially available modified-live L. intracellularis vaccine to foals and evaluated their responses. The vaccine tested is routinely used in the swine industry for the prevention of porcine proliferative enteropathy.

They divided foals into one of three groups and administered the vaccine orally, orally after pre-medication with omeprazole (a pharmaceutical drug that reduces the acidity of the stomach), and rectally.

"After only one dose, all of the foals vaccinated intra-rectally seroconverted (produced antibodies against L. intracellularis, suggesting protection), whereas only 50% of the pre-medicated orally vaccinated foals and 0% of the orally vaccinated foals that were not pre-medicated seroconverted," relays Pusterla.

No adverse events were noted in any foals after vaccine administration, which means the vaccine or a variation of it could be useful to prevent EPE in foals.

In future studies scientists will focus on determining the efficacy of vaccinating high-risk foals, on refining the vaccine protocol, and on establishing the appropriate dose required to protect foals against L. intracellularis.

Internal Parasites

Infection with various common internal parasites can cause diarrhea. Generally it is a chronic diarrhea, but acute colitis (inflammation of the colon) can occur, particularly in horses infected with small strongyles (cyathostomes).

While a variety of anthelmintics, such as benzimidazoles (e.g., fenbendazole), pyrantel, and ivermectin, are available for deworming horses and foals, these drugs are not generally effective against the encysted larvae of small strongyles. (Small strongyles hibernate, lurking encysted in the walls of the intestine.)

"Cyathostomes are known to cause an acute and potentially fatal colitis when very large numbers of the encysted cyathostome larvae emerge from the wall of the large intestine," explains Martin Krarup Nielsen, DVM, PhD, an assistant professor at the University of Copenhagen in Denmark. "However, this is extremely rare. We see only a few cases of this every year."

While the worms are small, they certainly do have a large impact: death rates due to acute colitis secondary to small strongyles can be as high as 60%.

Diarrhea Causes
Cause Adults Foals (8 months of age)


Antimicrobial-associated diarrhea

Clostridium difficile

C. perfringens

Potomac horse fever (Neorickettsia risticii)

Cyathostomiasis (massive infection of small strongyles)


Equine proliferative enteropathy (Lawsonia intracellularis)

C. difficile

C. perfringens


Rhodococcus equi


Carbohydrate overload

Antimicrobial-associated diarrhea

Non-steroidal anti-inflammatory drugs

Ulcerative colitis

Sand enteropathy

Inflammatory bowel disease

Miscellaneous toxins

Gastrointestinal ulceration

Nutritional (e.g., dietary indiscretion, overfeeding, lactose intolerance)

Interestingly, one of the risk factors for the development of cyathostome- associated diarrhea is the recent administration of an adulticidal anthelmintic.

"We don't understand the exact mechanism, but it appears that killing the small strongyles living in the lumen of the GI tract evokes some sort of activating signal to the encysted larvae," noted Nielsen. "They then leave their cysts synchronously and migrate into the lumen. Horses at risk for developing a severe protein-losing diarrhea are typically mismanaged horses under 5 years of age receiving anthelmintics outside the grazing season."

Nielsen says the best way to avoid cyathostome-associated diarrhea is to develop a parasite control program based on surveillance of parasite burdens and drug efficacy. It has become imperative to screen the efficacy of a farm's deworming program on a yearly basis by fecal analysis, he says. Horses with high fecal egg counts (for strongyles) can be treated more aggressively than horses with low counts.

Nielsen adds, "It is important that owners realize that diarrhea caused by cyathostome larvae is extremely rare and should not be the main worry on a well-managed farm. The main causes of acute diarrhea are Salmonella and Clostridium infections. Owners need to keep this in perspective."

Prescription Drugs

Antibiotics are a well-known noninfectious cause of diarrhea in otherwise healthy horses. While any antibiotic is potentially capable of causing acute diarrhea, oral lincomycin and parenteral (intravenous or intramuscular) tetracycline are extensively cited in the veterinary literature as inducing severe diarrhea. Others are:

  • Orally administered penicillin, erythromycin, trimethoprim sulfa, metronidazole, and doxycycline.
  • Parenteral ceftiofur, enrofloxacin, and gentamicin.

Belgrave explains, "Anecdotal information suggests that there is a geographical association with certain antibiotics and their ability to disrupt the gastrointestinal tract of horses in that region of the country. It is also believed that the more potent, effective antibiotics put the patient at a greater risk of developing diarrhea."

Antibiotics are thought to cause diarrhea by killing the normal, protective bacteria that reside in the GI system.

Belgrave explains, "The death of this resident flora allows other bacteria, such as C. perfringens, C. difficile, and Salmonella spp, to go unchecked within the GI system, proliferate, and cause disease. Thus, while not initially an infectious problem, a drug-induced diarrhea can develop into an infectious process."

Phenylbutazone, or "Bute," is another drug that can potentially cause diarrhea. Bute is a non-steroidal anti-inflammatory drug (NSAID) widely administered long-term for the management of painful musculoskeletal injuries. Bute inhibits the synthesis of a group of hormonelike substances called prostaglandins that are involved in a variety of body functions, such as the contraction and relaxation of smooth muscle, the dilation and constriction of blood vessels, control of blood pressure, and modulation of inflammation.

Bute's inhibition of prostaglandin production can decrease blood flow to the lining of the horse's GI system, resulting in ulceration (erosion) of the lining of the gut and diarrhea. Affected horses lose large amounts of protein into their GI systems, develop endotoxemia (a life-threatening condition associated with laminitis), and are slow to respond to therapy.

"Long periods of intensive care are frequently required, and prognosis is often guarded, as complications can arise while the mucosal lining of the right dorsal colon recovers," adds Belgrave.

This less-common adverse event associated with even recommended dosages of Bute was recently described by Rebecca McConnico, DVM, PhD, Dipl. ACVIM, associate professor of veterinary medicine, and her co-workers from Louisiana State University in the article, "Pathophysiologic effects of phenylbutazone on the right dorsal colon in horses" (published in the November 2008 edition of the American Journal of Veterinary Research).

McConnico and colleagues administered Bute (8.8 mg/kg orally once daily for two weeks) or a placebo to assess the systemic effects of routine doses of Bute on healthy horses. Two horses treated with Bute developed colitis during the 21-day study period, which suggests there's an extreme variability in how horses tolerate Bute.

McConnico explains, "Some horses cannot even tolerate short-term administration of the drug."

Based on their results, the researchers strongly recommend running routine blood tests as early as three to five days after initiating Bute treatment. Further, they emphasized the importance of decreasing or even discontinuing Bute administration--if these tests reveal even mild adverse events--to avoid life-threatening problems such as ulcerative colitis.

McConnico suggests using clinical proven alternatives to Bute such as firocoxib (a NSAID with a slightly different mechanism of action than Bute), as she says it is safer and equally effective.

Take-Home Message

"In approximately 60% of diarrhea cases, an underlying cause is never identified," says Belgrave. "Treatment is largely supportive and can sometimes aid in circumventing secondary complications and, hence, minimize the costs incurred in treating these cases. However, in general, treating these intense medical cases with large volumes of fluids, thawed plasma, anti-inflammatory drugs, and gastroprotectant therapy can be costly."

Antibiotics and NSAIDs should be used judiciously in horses. "While the percentage of horses that develop diarrhea while on antibiotics is small, the consequences of disruption of the normal flora can be dire," concludes Belgrave.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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