Antibiotic Resistance in Horses?

Heading Off the Development of Antibiotic Resistant Pathogens

By: Heather Smith Thomas

There are a number of serious diseases that can affect horses and humans. A big concern today is that some of these pathogens are becoming resistant to the antibiotics we use to treat them. Veterinarians are stressing the importance of responsible use of antimicrobials. This responsibility involves not just the choice of drugs, but also the duration and concentration of the drug being used.

If we need to use antibiotics, they should be used at sufficient concentration for the appropriate amount of time, to do the job. Sometimes, people use an improper dose (generally too low), or use the drug for too short of a duration. If the antibiotic therapy has not completely eliminated the bacterial infection by the time you stop using the drug, the bacteria that are left are the ones that are most hardy. They are the ones that survive to reproduce in future, and this can lead to development of antibiotic resistance.

Dr. Josie Traub-Dargatz, Professor of Equine Medicine at Colorado State University, says we should use antimicrobial drugs judiciously to avoid encouraging the increase of resistant organisms. “We still don’t know how much of a problem resistant bacteria pose to the equine industry. At this point, there is no national compilation of data on this topic. Logically, however, we should use antimicrobial drugs wisely to avoid resistance problems,” she says.

“We don’t have very many drugs in the pipeline if bacteria become resistant to the ones we now have available. I tell owners that they should work with a veterinarian in deciding what the animals need, and which antimicrobials to begin treatment with, and when a culture to determine the bacteria involved might be indicated in the decision on which drug would be best,” she says.

“I believe it is appropriate to select a drug that’s worked before in certain cases, such as a bacterial respiratory infection, without having to do a culture on every animal. But, if the horse fails to respond or has severe disease, then you need to more critically assess the treatment plan. A culture can help guide you to make a more informed decision rather than playing antimicrobial drug roulette, going from one drug to another,” says Traub-Dargatz.

“If the animal has viral respiratory disease, we can’t expect a response to antibiotics. I think sometimes people move from one type of antibiotic to the next type because the horse’s fever hasn’t gone down, but if it’s a virally induced fever it’s not going to respond. Two or three different antibiotics have been used to treat the horse even though he wasn’t going to respond to antibiotics,” she says.

“Horse owners also need to realize that there’s no drug that doesn’t have some risk for the horse itself. You always have to weigh the benefits of using a drug against the limitations and possible risks. We shouldn’t be afraid to use drugs when they are indicated, but there can be side effects—not only the potential for development of bacterial resistance, but also the other side effects that can be substantial in some situations, such as procaine reactions, diarrhea (due to change in gut flora) or anaphylactic reactions,” she says.