Antimicrobial resistance

Doing your part
Published on September 01, 2005
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In a hospital in the United States, physicians do their best to contain an outbreak of methicillin-resistant Staphylococcus aureus. They test, they screen, they clean, and finally manage to get their wards MRSA-free, but weeks later, it comes back. Who's to blame? It turns out a nurse whose dog had an infected eye had unknowingly been tracking the pathogen into the hospital.

Such a situation is not too surprising, said Dr. Colleen Murphy, who is with the Ontario Veterinary College, University of Guelph. "There are a lot of little cases out there in the literature. MRSA can move quite nicely between humans and animals."

For some time now, physicians, veterinarians, and others have been playing the blame game as to who is responsible for the growing problem of antimicrobial resistance, but, really, what's the point?

"They are really everybody's problem. We all contribute to it somehow," Dr. Murphy said. Her talk July 16 at the AVMA Annual Convention focused on antimicrobial resistance among companion animals and what veterinarians can do to improve the problem.

Dogs can be infected with and be carriers of many important multiple-drug-resistant bacteria, including extended-spectrum beta-lactamase-resistant (ESBL) Escherichia coli, cephamycinase-2-resistant (CMY-2) E coli, methicillin-resistant S aureus, multiple-drug-resistant Salmonella spp, and vancomycin-resistant Enterococcus.

The ESBL E coli and CMY-2 E coli, which cause disease in human and veterinary patients, are multiple-drug-resistant pathogens expressing resistance to higher (second, third, and fourth) generation cephalosporins. Dr. Murphy said that infections with these two organisms have been blamed on community and nosocomial sources.

Important, Dr. Murphy noted, is that healthy dogs without any recent exposure to antimicrobials have been identified as fecal carriers of CMY-2 E coli. "CMY-2 is going to be a bigger problem than ESBL," she said. "We should really keep an eye out for it."

She also reported, on the other hand, that in a recent study of dogs and cats conducted at the University of Guelph, no animals were identified as carriers of Salmonella spp, ESBL E coli, MRSA, or methicillin-resistant S intermedius. Thus, the actual percentage of companion animals in clinical practice harboring such pathogens may be lower than the percentages reported by veterinary diagnostic laboratories. Data from these laboratories usually reflect animals that have undergone antimicrobial therapy or have complicated infections.

But still, companion animal veterinarians need to be aware that the patients they treat can harbor pathogens and that some of these pathogens, particularly the drug-resistant ones, may pose a risk to the human population.

So what can be done? Dr. Murphy suggests that veterinarians reexamine the practices in their clinics. A recent investigation into a sick dog at one clinic, for example, led researchers to discover the dog had been infected at another veterinary clinic. A clinician treating an infected dog with an open wound had not worn gloves and passed the drug-resistant pathogen on to other clients. Using gloves and other barrier methods in certain situations could minimize situations such as this.

Human hospitals take two strategies in their attempts to minimize antimicrobial resistance. The first involves restricting the use of antimicrobials by prohibiting the use of specific agents within the hospital, limiting prophylactic antimicrobial administration, rotating agents, and minimizing the use of pharmaceuticals with high resistance potential. Some argue that although this strategy is somewhat effective, resistance may simply shift to other drugs.

The second strategy revolves around infection-control procedures such as regular hand washing, screening and isolating high-risk patients, and barrier precautions. Although this, too, has been shown to be effective, opponents argue that this approach ignores the generation of resistant bacteria. Neither strategy has been adequately assessed in veterinary hospitals, but it is likely that a combination of approaches will be necessary to control resistance.

Veterinarians can also warn owners about the potential dangers of feeding their pets pig ear treats, raw meat, and commercial raw food diets. Antimicrobial resistance to trimethoprim-sulfamethoxazole has been linked to consumption of pig ear treats. Consumption of raw beef products has been linked to antimicrobial resistance to ampicillin and cephalothin. And raw food diets pose a risk of causing a variety of foodborne illnesses.

Other measures that veterinarians can take are limiting the use of antimicrobials in animals with documented infections, which may require submitting more patient specimens for bacterial culture; using agents with a narrow spectrum of activity, which may necessitate additional susceptibility testing; and ensuring adequate drug concentrations at infection sites by adopting appropriate dosage intervals. Finally, clients must be educated about the importance of treatment duration.

Dr. Murphy emphasized that all veterinarians need to do their part. If wearing gloves makes some clients uncomfortable, practitioners need to sit down and explain why they are wearing them. When looking at the bigger picture, it's not such a hard thing to do, and everyone needs to chip in, she said.

While 50 million pounds of antimicrobials are produced each year in the United States, at least one form of microbial resistance has been described for every antimicrobial that's on the market.