Antimicrobial Resistance

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During the past half century, antimicrobial drugs have dramatically improved health and longevity—in humans and animals alike. Use of these drugs, however, can select for resistant strains of bacteria, and experts are wondering if overuse of these drugs might lead to resistance among bacteria that are harmful to humans. The primary concern at present is with the use of antimicrobials in food animals, but the issue extends to horses.

Mark Papich, DVM, MS, N. C. State University, says veterinarians have the responsibility to administer antibiotics appropriately and prudently to decrease the risk for emergence of resistance. “Development of resistance is inevitable. Bacteria have been known to develop resistance since the discovery and early use of penicillin in the 1940s. Appropriate use of these drugs can hopefully minimize the risk,” says Papich.

There has been a huge increase in use of antimicrobial drugs in horses in recent years due to treatments for diseases like EPM, in which horses may be on certain antimicrobials for many months. And this raises the potential for resistance.

Josie Traub-Dargatz, DVM, MS, Clinical Sciences Department, Colorado State University, was representative to AVMA (American Veterinary Medical Association) from the AAEP (American Association of Equine Practitioners) committee on judicial use of antimicrobials. AVMA’s concern is that inappropriate dosage and subtherapeutic levels of antibiotic (below that needed to destroy pathogens) encourage resistant strains, making them harder to kill in the future. Adverse reactions are another pitfall.

“Antimicrobial resistance is one of several considerations to keep in mind when using these drugs. If there is not a clear indication for using them, we should not use antibiotics,” says Traub-Dargatz.

“The extreme example would be that this antibiotic drug used to work to treat a certain infection, and now it doesn’t,” she says. “There are only so many new antimicrobial drugs being developed—and most are for human use and not for animals. We don’t want to get into a situation where the antibiotics we have don’t work anymore.”

The antimicrobial resistance issue goes beyond the individual animal being treated. “If you don’t use antibiotics properly in one horse, you may have treatment failure in that horse, but, more importantly, in the bigger picture, the pathogens within that horse that become resistant could be shared with other horses, or other kinds of animals,” says Traub-Dargatz.

These are not new concerns. The National Antimicrobial Resistance Monitoring System for Enteric Bacteria (NARMS-ES) was established in 1996 for surveillance of antibiotic resistance among various types of Salmonella and other bacteria in humans and animals, and to try to prolong the lifespan of approved drugs by promoting prudent use of antibiotics.

To that end, says Traub-Dargatz, “Most important is to work with your vet to keep horses healthy so there is no need to treat an infection. These efforts are directed at immunizing or vaccinating to help prevent infection, and biosecurity practices to reduce the risk to your horse if exposed.”

Determining the Proper Treatment

When antibiotics are unavoidable, says Traub-Dargatz, “We recommend that antimicrobials with the narrowest spectrum possible be used at appropriate doses and for as short a period of time as possible for controlling bacterial infection.

“If you are going to use antimicrobials, work with your veterinarian to determine the most appropriate management for that horse.” Remember, antimicrobials are not effective in treatment of viral diseases. “If it’s viral respiratory disease, then we don’t need antibiotics. In a contagious disease outbreak, if we find it is likely bacterial, the veterinarian may suggest collection of samples to culture, to identify specific bacteria, and laboratory testing to determine the most appropriate antibiotic to use,” says Traub-Dargatz.

“Not every horse with a bacterial infection needs a culture and susceptibility testing. But if the horse doesn’t respond to standard treatment, or if we feel it’s a serious infection (and we need to make the right choice as quickly as possible) or if we have a herd situation and want to know the most appropriate drug to use to minimize illness and reduce spread, we need to identify what the organism is, and what to use in treatment,” she says.

Some drugs are chosen for reasons other than concerns about resistance. They might be used because they are easier to give to a certain horse, need fewer administrations per day, have better coverage with a single drug, etc. But resistance is always a consideration as well.

Expert Guidance

Having a veterinarian in the loop in deciding what to give and when to give it is always wise. “We have several years’ worth of data on different kinds of bacteria, which antibiotic they were susceptible to, and how you would dose a horse to achieve a level the lab says would work. There are many things that go into deciding on the most appropriate treatment; we don’t just grab a bottle of penicillin,” Traub-Dargatz says.

“Beyond the issue of antibiotic resistance, we sometimes see complications or adverse reactions to antibiotics in horses. For an example, some horses develop severe reactions to the procaine in procaine penicillin. All related drugs—procaine, lidocaine, etc.—are excitatory to the central nervous system, causing over-stimulation if in high enough levels,” she says.

“We also see horses in which antibiotics have altered gastro-intestinal flora. If normal balance of bacteria is altered, bad bacteria take over and diarrhea occurs. Just about any antibiotic can alter flora in the horse’s intestine. When we use antibiotics in horses this is a risk we take,” she says.

“We’ve occasionally seen problems when horses get a class of drugs called aminoglycosides (like gentamicin), where they’ve received the drug in either inappropriately high doses or when dehydrated, and ended up with kidney compromise. We also see complications at injection sites—local swelling in the muscle, very painful swelling—and sometimes sterile abscesses.”

Traub-Dargatz says a vet can make sure that the horse gets the proper dosage, and at the right times, since there can be some variations depending upon the animal’s size and age.

Resistance Issues

Another concern when we use antimicrobials: other organisms are affected besides those we’re targeting with treatment, she says. For example, “The bacteria that caused the pneumonia are killed [because they were susceptible to the antibiotic], but there may be organisms in the gut that are not as susceptible, or only minimally susceptible, and if these bacteria get exposed we may select for those that are more resistant to the antibiotic we are using,” she says. Even though certain bacteria were not the targets, these resistant intestinal bacteria may become a problem in another horse.

“Also, we’ve learned that certain bacteria can share resistance factors among each other, so there are lots of implications when we use antibiotics,” she concludes. That’s why antibiotics should be used as sparingly and as precisely as possible.