Oral Antibiotics in Horses
In people, as well as many other species like dogs and cats, oral antibiotics (e.g. liquids or pills that are swallowed) are very commonly used, because it's usually easier (and less uncomfortable) than giving antibiotics by injection with a needle. A wide range of oral antibiotics are available for use in humans, and while antibiotic-associated complications such as diarrhea certainly can occur, oral antiboitics are relatively safe for most people.
So, why don't we use many oral antibiotics in horses?
Horses have a very different intestinal tract than people (and dogs and cats). A horse's intestinal tract is much likely to develop problems from antibiotic use, particularly antibiotic-associated diarrhea or colitis, which can be fatal. The root of the problem in these cases is disruption of the normal bacterial populations that live in the intestine (the microbial "flora"), which can allow harmful bacteria to multiply and spread. This can occur with antibiotics given by any route (even by injection), but using oral antibiotics can result in higher drug levels in the intestinal tract, which creates a greater chance of causing problems. The likelihood of a horse developing complications from any antibiotic is probably a combination of what bacteria the antibiotic kills and how much makes it to the intestinal tract.
- Some oral antibiotics can be used relatively safely in horses. Trimethoprim sulfa (TMS) is very commonly used, and quite safely for the most part, in many areas (including Ontario).
- Chloramphenicol and enrofloxacin (Baytril) are also given orally to horses, but these drugs should be reserved for problems for which they are specifically indicated.
- Some antibiotics can be used more safely in foals than adults, such as erythromycin. While diarrhea can occur in foals, erythromycin (and related drugs in the macrolide class) are commonly used without problems in young stock. Macrolides are higher risk drugs in adult horses. I've used them a few times in adults when I've had no other choice, but it's always a scary prospect.
- Never use an oral antibiotic that is not known to be relatively safe for use in horses. One of the first cases that I saw during my residency was an expensive racehorse owned by a physician. He had cultured a minor foot wound and grown a Staphylococcus that was susceptible to cloxacillin. He got some cloxacillin and gave it to the horse orally - something no veterinarian would do. The horse developed severe diarrhea and died despite intensive treatment.
