Long hailed as the greatest advance in medicine, antibiotics have become the go-to treatment for nearly every scratch, cough, or fever, whether or not they were caused by bacteria. Many doctors and veterinarians began indiscriminately prescribing antibiotics simply because their patients expected — even demanded — them. This overuse of antibiotics eventually created today's crisis, in both human and animal medicine: antibiotic resistance.
Dr. Ken Marcella, founder of KLM Equine in Canton, Ga., has been beating the drum for years to alert horsemen to the danger of overuse of antibiotics. Last year, he treated his first case of methicillin-resistant staphylococcus aureus (MRSA) in a horse. In humans, the disease is widespread, with government agencies requiring hospitals to screen incoming patients for MRSA to curtail its spread.
In equine medicine, researchers and practitioners are doubly concerned about antibiotic resistance because there aren't that many types of antibiotics in their arsenal. If a disease becomes resistant to the antibiotic of choice for treating it, horsemen — and the horse — are in trouble.
“First-choice drugs out in the field are going to be things like the sulfas — Trimethoprim [SMZ] or Uniprim are still the #1 deal,” Marcella said. “Because [sulfa] is generally broad-spectrum, and it is real simple for the client to administer, I think it's probably overused. So we try to hold off on that.”
Gimme some SMZs, Doc
Horsemen have become accustomed to stopping at the veterinary clinic to pick up a supply of antibiotics, either for a special case or just to have on hand. Marcella said his clinic has had a long-standing policy not to dispense antibiotics unless he has examined the horse. Even then, he may choose to allow the horse's immune system to do the work.
“Say a horse presents with just a runny nose,” Marcella said. “A lot of times, if the horse is eating and drinking and its temperature and physical exam are normal, we'll hold off on any sort of treatment.”
He'll follow up with the owners to assure the horse is recovering and if it isn't, he'll do lab work to determine which antibiotic is effective against that particular bacteria.
“It really doesn't help in terms of resistance if you go ahead and treat with the wrong antibiotics for 10 days and then find out it's not working,” he said.
Marcella may turn to stronger antibiotics — penicillin, gentamicin, enrofloxacin, or the tetracyclines — but only after blood work or cultures indicate their need.
What is the risk?
Risk management Is the key factor in deciding if and when to treat with antibiotics. Some horses may be healthy enough to fight the bacteria on their own, while others may be more at risk because of their age and overall health.
Another consideration is the disease itself.
“If I think it's a disease that if I wait and I'm wrong, the horse is going to have more of a problem, then I'm more likely to go ahead and start antibiotics if I think they are needed,” Marcella said. “Or if I go ahead and do [blood work] and there's a good chance of an infection somewhere, I'm going to put the horse on antibiotics, even if I can't find the source to culture it.”
Rhodococcal pneumonia in foals is a deadly disease that begins as small focal points of infection in the lungs that eventually become abscesses. This insidious pneumonia infects the foal in the first days of life but does not show symptoms until the advanced stage months later, a point where little can be done to save its life. In the early stages, foals continue to nurse normally, and they even may appear bright and healthy.
Researchers have developed a way to identify and monitor lesions in the foal's lungs using ultrasound. The lesions are graded on a scale of zero to 10. A companion study found that smaller lesions (less than Grade 2) are able to resolve without medical intervention. But with such a serious disease, are farms willing to take that risk?
Three Chimneys Farm routinely ultrasounds its foals' lungs for R. equi, but the size of a lesion is not the only factor farm manager Chris Baker said they use to decide if a foal should be treated.
“Clinical signs, ultrasound, and blood work — those three criteria are what we rely on in determining our choice whether to treat,” Baker said. “We'll follow those along, but we only treat when we feel we have an active infection that needs to be treated, not an inflammatory [condition] or a low-grade infection process that the foal is dealing with on its own.”
For serious infections, ranging from wounds to respiratory disease, Marcella prefers to use Excede, an antibiotic with effects lasting four days.
“If you get a puncture wound or a respiratory issue, four days of Excede and then re-evaluation is probably a better way to go,” Marcella said.
Researchers also are looking to natural cures as a way to fight antibiotic resistance. Natural honey has shown promising results. A Swedish study found the beneficial lactic-acid bacteria in a bee's stomach is effective in treating MRSA, non-healing wounds, and other antibiotic-resistant diseases. Commercial honey does not contain the lactic-acid bacteria; the source must be fresh raw honey.
Researchers in the United States also are looking into the antibiotic properties of the metal gallium maltolate to treat wounds and Rhodococcal pneumonia.
“The biggest thing from us out in the field is not to jump on antibiotics for no particular reason,” Marcella said.
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