Anti-Ulcer Therapy and Diarrhea
Stomach ulcers are common in horses (especially race horses), and anti-ulcer medications are widely used. The most common of these drugs is omeprazole (sold under the brand name Gastrogard), which is a proton pump inhibitor. There is no doubt that gastric ulcers can cause problems in horses and treatment is necessary in these situations. However, there are a lot of horses being treated for mild ulcers or for prevention of ulcers, and it's unclear whether this is really needed, and whether it could even cause problems.
Picture (right): Endoscopic view of ulcers along the margo plicatus in the stomach of a horse.
In people, proton pump inhibitor treatment has been associated with an increased risk of diarrhea (particularly Clostridium difficile diarrhea). Biologically this makes sense. The acidic environment of the stomach normally helps kill harmful bacteria that a person (or a horse) may ingest. Treatment with anti-ulcer drugs to reduce the acidity of the fluid in the stomach may allow bad bacteria like Clostridium difficile and Salmonella to reach the intestinal tract in larger numbers, and therefore predispose the patient to developing diarrhea. Theoretically the same thing could happen in horses.
Does this actually happen in horses? We don't know. It is certainly a possibility but there is no evidence either way.
Are certain horses at higher risk? We don't know. However, it's reasonable to suspect that this could be a greater concern in horses that are already at higher risk of developing diarrhea for other reasons, such as those being treated with antibiotics.
Should we avoid using anti-ulcer drugs in horses? Not always. These drugs are definitely needed for horses with clinically significant ulcers (i.e. their ulcers are actually painful or making them sick). However, we really need to consider whether treatment is necessary for horses that have very mild ulcers (i.e. that aren't causing clinical signs) or those that don't have ulcers at all and treatment is only "preventative".
Personally, I don't like to use anti-ulcer drugs in hospitalized horses unless they have severe ulcers that are causing problems. In cases where milder ulcers are present or people want to give anti-ulcer drugs for prevention, I prefer them to wait until the horse goes home. The same approach could be taken with horses that are on farms but at higher risk for diarrhea, such as those being treated with antibiotics.
