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
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,"
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.