Diarrhea in Adult Horses: Better Safe Than Sorry

You've seen the television ads for Pepto-Bismol and Kaopectate. They're geared to make you grin through your grimaces about that little problem everyone's embarrassed to talk about--diarrhea. But in horses, diarrhea--particularly persistent diarrhea in adult horses--is no laughing matter, and it's certainly not something you should keep to yourself. Because of direct consequences such as dehydration and malnutrition, as well as underlying causes such as parasitism and bacterial infections, diarrhea in adult equids can turn deadly.

"Due to the large hindgut (large colon and cecum) in the horse, if they develop an inflammatory condition or a malabsorption syndrome, they can lose tremendous amounts of fluid from the body into the lumen of the intestine and thus out into the feces and out of the body," says Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, a professor of equine medicine at Colorado State University.

It's the water content that makes feces loose and soft. By some estimates, a horse with diarrhea can lose as much as 10 gallons (40 liters) of water and salts per day. The resulting dehydration can be fatal, or it can lead to other serious problems, including electrolyte imbalances, weight loss, laminitis, and kidney failure.

In addition, continues Traub-Dargatz, "With certain causes of diarrhea, the causal agent may release or secrete toxins into the intestine, which may be absorbed into the blood, causing toxemia." (Toxemia is blood poisoning caused by toxins--poisons produced by organisms such as bacteria.)

Not every case of diarrhea is cause for high alarm. "It's important to recognize that there are two main situations," advises Traub-Dargatz. In the less worrisome scenario, the horse's feces might be looser than normal, but he doesn't lose more fluid than he can compensate for and does not defecate more frequently than usual. It's the second case that raises red flags: Feces are loose to very loose, the horse experiences whole-body fluid loss, and he defecates more frequently than normal.

If the diarrhea appears suddenly and dramatically, there's definitely cause for concern, says Steve Reed, DVM, Dipl. ACVIM, head of the Equine Medical and Surgical Division at The Ohio State University's College of Veterinary Medicine.

Reed and Traub-Dargatz note that the following clinical signs accompanying a case of diarrhea are signals that you should seek a veterinarian's help without delay:

  • Blood, mucus, or foul odor to the feces;
  • Diarrhea persisting more than eight to 12 hours;
  • Loss of appetite, depression, or other deviations from normal behavior;
  • Fever;
  • Signs of colic;
  • Increased pulse rate; and
  • Signs of laminitis.

Regardless of the signs, Traub-Dargatz encourages you to call a veterinarian with any concerns about a mild to severe case of diarrhea. "It's better to have the horse examined and find it's not serious than to wait until the horse is seriously ill and treatment may be less likely to correct or minimize the impact of the diarrhea," she says.

Pinpointing the Cause

"Once a veterinarian arrives, he or she will most likely collect fecal samples for culturing and blood samples for testing. The goal, of course, is to discover what's causing the diarrhea--not as simple a task as you might think. The ailment has numerous potential causes, but Traub-Dargatz says they can be grouped into the following categories:

  • Infectious disease agents that can be parasitic (particularly small strongyle infestation) or bacterial (including Salmonella and Potomac horse fever);
  • Altered intestinal flora (microbes in the intestine), such as occurs in some horses with the use of antimicrobials;
  • Diet (including excessive carbohydrates, lush pastures, and sudden changes in feed);
  • Inflammatory bowel disease syndromes;
  • Neoplasia (tumor) in the gastrointestinal tract;
  • Altered organ function, such as heart failure that causes the bowel to become edematous (swollen with fluid);
  • Sand in the large intestine; and
  • Idiopathic (of unknown origin).

Veterinarians have an impressive arsenal of diagnostic tools at their disposal, from basic blood counts and fecal cultures to serum biochemistry panels, serum protein electrophoresis (separation of ionic molecules, such as proteins), peritoneal (within the body cavity) fluid analysis, rectal mucosal biopsy, d-xylose absorption tests (which measures whether nutrients are being absorbed properly), abdominal radiography, and even exploratory surgery. Yet, in as many as 40% of cases of serious equine diarrhea, a definitive diagnosis is not made, according to Terry C. Gerros, DVM, MS, Dipl. ACVIM, assistant professor of large animal internal medicine at Oregon State University.

Traub-Dargatz notes that proper collection and handling of fecal and blood samples is critical to achieve a diagnosis, but she, too, notes that in some cases, a cause simply cannot be found even when using all of the tests currently available.

Vital Measures

"Regardless of the cause--or whether a cause is ever definitively pinpointed--your veterinarian will most likely prescribe supportive treatment to stabilize the horse until the diarrhea is resolved," says Traub-Dargatz. Such treatment typically centers on restoring and maintaining hydration and controlling any gastrointestinal inflammation, helping the intestine heal itself or absorb toxins, controlling systemic signs of toxemia, and controlling discomfort.

Depending on the severity of the horse's situation, he might require painkillers (often non-steroidal anti-inflammatories such as Banamine), intravenous fluids, and intestinal protectants (such as activated charcoal, which absorbs toxins; bismuth subsalicylate, the active ingredient in Pepto-Bismol; or di-tri-octahedral smectite, the active ingredient in Bio-Sponge). Some veterinarians also prescribe probiotics to help rebuild the gut's population of beneficial bacteria.

Traub-Dargatz and Reed also recommend isolating any horse with diarrhea away from other animals, since some causes of equine diarrhea are contagious--not only to other horses, but also to other kinds of animals, including humans. For this reason, it's important to take special sanitary precautions to prevent contamination of the stable area, paddocks, and pastures with diarrheal feces. You should also pay extra attention to personal hygiene after handling an affected animal.

For instance, you can help control contamination by wearing disposable boot covers when working with an affected horse and using a foot dip with disinfectant at the stall door. You can also wear latex gloves and should always wash your hands after working on a horse with diarrhea. Reserving a set of equipment (buckets, grooming utensils, rakes, etc.) just for use on that horse can also help control contamination.

Beyond that, treating a case of equine diarrhea focuses on battling the known or suspected cause. In the rest of this article, we'll look at what Reed considers the three most worrisome triggers: Salmonella, Potomac horse fever, and clostridial infections.


Salmonella is a rod-shaped bacterium that exists throughout the environment, including the soil and air. It typically enters a horse's digestive system through contaminated feed, contaminated pastures, or exposure to feces on surfaces, surface water sources, or handlers' hands. Essentially, it causes food poisoning.

"We worry about it for a number of reasons," says Reed. "First, it can be found in some horses as inapparent carriers--the horse carries the bacteria, but shows no evidence of disease or may have only moderate, intermittent soft stools that could turn into dramatic diarrhea in that horse or affect other horses in the environment."

Furthermore, says Reed, Salmonella invades the colon and cecum walls, causing colitis (inflammation of the colon) and typhlocolitis (inflammation of the junction of the cecum and colon). Edema (fluid swelling) and hemorrhaging of small arteries can cause the intestines to thicken by as much as two to three inches, resulting in an inability to digest and absorb nutrients in a normal fashion. The damaged intestinal wall also allows absorption of toxins and possibly even bacteria from the gastrointestinal tract into the bloodstream, causing toxemia (toxins in the bloodstream) and septicemia (systemic infection). The biggest worry, says Reed, is that Salmonella can lead to the production of endotoxins--poisons released when the bacterium breaks down in the horse's body. The result can be a dramatic drop in white blood cell count, leaving horses with often neglible circulating white blood cells, and that is bad because it leaves them very suseceptible to secondary infections such as pneumonia, thrombophlebitis, etc.

In rare cases, a horse will die from Salmonella infection before diarrhea and other signs even show up. Reed says the bacteria do not multiply within the lumen of the bowel for a long time, which is why doing fecal cultures for five consecutive days is so important for a proper diagnosis. Unfortunately, he continues, because the bacteria invade the wall of the bowel, the toxemia and septicemia might lead to death before a positive culture can be obtained.

One reason that veterinarians often opt to not give antibiotics is that these drugs can kill not only the bad bacteria, but also the good ones. In cases where the damaging bacteria are resistant to the antibiotic--as are certain strains of Salmonella--giving antibiotics can actually do more harm than good, allowing the bad guys to gain a stronger foothold as the horse's defenses weaken. The antibiotics can actually exacerbate the diarrhea.

However, adds Reed, if the horse is septic (has an infection in the blood) or there is a concern that the toxemia is overwhelming, then the veterinarian might feel warranted in using antibiotics, often in a combination of drugs such as gentamicin and penicillin. Once the infection is controlled, says Reed, the horse usually recovers quite well.

It's also important to note that Salmonella is highly contagious, as the bacteria are shed through the manure of affected horses, even if the animal is a carrier who shows no signs of infection. Thus, any area where the manure lands can be contaminated, and any person or animal coming into contact with the manure can become a carrier. If you suspect your horse has the bacteria, take immediate quarantine and sanitizing measures. Some veterinarians recommend maintaining the quarantine as long as 30 days--even if your horse's outward signs disappear sooner--and taking regular fecal exams until three to five exams show up "clean."

Potomac Horse Fever

Behind Salmonella, Reed sees Potomac horse fever (PHF)--also known as equine monocytic ehrlichiosis and equine ehrlichial colitis--as the next biggest diarrhea catalyst in adult horses. Caused by the organism now known as Neorickettsia risticii (formerly known as Ehrlichia risticii), the disease is most prominent in the eastern United States, but can occur in any state. A vaccine exists, but it is not foolproof, and you should not rule out the possibility of PHF causing your horse's diarrhea simply because he has been vaccinated against the disease.

While parts of the N. risticii life cycle remain unknown, much has been learned in recent years about how the disease is transmitted, giving the horse industry a better handle on controlling the disease. For instance, researchers have learned that an insect vector is required to spread the disease from its source (flukes, or small flatworms, from water snails) to the horse, which is believed to be a dead-end host (not part of the natural life cycle). Therefore, we now know that the disease can't be spread directly from horse to horse, so animals positively diagnosed as having PHF do not need to be quarantined.

Veterinarians treat PHF-infected horses with oxytetracycline, an antibiotic that typically generates an improvement in the horse's clinical signs within 24 hours. It's the treatment of choice when a horse has been definitively diagnosed using serologic blood tests. But it's not recommended for long-term use if the diagnosis is questionable, because, says Traub-Dargatz, "the drug is excreted via the liver into the digestive tract and can alter the normal bacterial flora in an adverse manner--it can kill off the good bacteria and leave the bad bacteria."

Reed notes that if you can't get a PHF test done and you're in an area that's known for PHF outbreaks, then your veterinarian might give the horse one or two doses of oxytetracycline just in case the diarrhea is due to PHF, then see if the horse starts to look better. If the horse doesn't respond to the drug, then its use should be discontinued, he advises.

Even if the problem is PHF, says Gerros, you might not have to use oxytetracycline. "You can let the disease run its course, which takes five to 10 days," he writes. But he cautions, "5-30% of untreated horses die."

Clostridial Organisms

Reed identifies clostridial organisms--particularly Clostridium difficile and Clostridium perfringens--as the third-most-threatening cause of diarrhea in adult horses. Clostridia are rod-shaped, Gram-positive, anaerobic bacteria that live in the soil and in the intestinal tract. So, a horse normally carries clostridial organisms in his intestine. They aren't likely to cause problems until the horse becomes susceptible to overgrowth of these clostridial bacteria through changes in the microbial balance inside his intestines, or when the horse faces increased exposure to the bacteria during a hospital stay, according to Traub-Dargatz and an article by J. Scott Weese, DVM, DVSc; Josepha DeLay, DVM, DVSc; and Marie Archambault, DVM, MSc, PhD, in the September 2000 newsletter of the University of Guelph's Animal Health Laboratory.

Aside from causing diarrhea, the toxins and enzymes released by clostridial bacteria can cause serious side effects such as necrosis (death of sections of tissue), colic, and inflammatory intestinal disease. To battle C. difficile and C. perfringens, veterinarians typically prescribe metronidazole, an antimicrobial agent that generally brings about signs of improvement within two days of the first treatment, says Reed.

An experimental treatment being investigated by Weese and fellow University of Guelph researchers Nathalie Cote, DVM, DVSc, and Rolph De Gannes, GV, DVM, might provide additional help in the future. Di-tri-octahedral (DTO) smectite, tested in the brand-name product Bio-Sponge, appears to have the ability to absorb toxins such as those produced by C. difficile and C. perfringens, while not binding to the antimicrobial metronidazole. This is important, because DTO smectite does not seem to inhibit growth of the clostridial organisms. Thus, it would absorb toxins from the animal's digestive tract so they could be eliminated in the feces without interfering with treatments designed to eliminate the bacteria. In testing on affected humans, the binding properties of DTO smectite led to a decreased duration of diarrhea.

As with Salmonella, C. difficile infections are contagious. Thus, affected horses and those suspected to be infected should be placed in isolation and strict hygiene protocols should be followed. This will not only inhibit spread and exposure, but will also prevent the establishment of clostridial spores, note Weese, DeLay, and Archambault in their article.

Take-Home Message: Don't Wait

While severe, chronic diarrhea in adult horses is not exceedingly common, but when it occurs, it can have serious consequences. If you suspect that your horse's runny manure is more than the result of short-lived stress or a sudden shift in diet, your smartest move is to call in your veterinarian for an assessment and diagnostic testing. As Traub-Dargatz says, "Better safe than sorry."

About the Author

Sushil Dulai Wenholz

Sushil Dulai Wenholz is a free-lance writer based in Lakewood, Colo. Her work appears in a number of leading equine publications, and she has earned awards from the American Horse Publications and the Western Fairs Association.

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