Diarrhea’s diverse causes
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 block 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,” explained Dr. Rodney Belgrave, 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 causes of diarrhea, some other factors contributing to the development of diarrhea in horses continue to be overlooked. One of these is proliferative enteropathy, a thickening of the inner lining of the small and large intestines, caused by the bacterium Lawsonia intracellularis. Antibiotics and non-steroidal anti-inflammatory drugs should be used judiciously in horses, as these can also cause diarrhea.

Another potential trigger is infection with various common internal parasites. 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 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,” explained Dr. Martin Krarup Nielsen, 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%.

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

Nielsen said 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.”

Antibiotic bill
A congressional committee recently held a hearing on a bill that proposes severe restrictions on the use of antibiotics in food animals. The Preservation of Antibiotics for Medical Treatment Act of 2009 would ban “non-therapeutic” uses of antibiotics that are also used in humans. It is hoped that this would prevent antibiotic resistance and preserve these drugs to treat human infections.

However, the American Veterinary Medical Association (AVMA) said there is no scientific proof that this ban would have any effect on resistance in human medicine.
The AVMA said the bill currently affects food animals only and will not impact horses.

“As defined within the text of the legislation, elimination of ‘non-therapeutic’ uses of antimicrobials would disallow disease prevention and potentially control uses,” the association noted. “This type of broad-based ban is contrary to the practice of veterinary medicine.”

Excerpted from The Horse: Your Guide to Equine Health Care. Free weekly newsletters at TheHorse.com.