Methicillin-resistant infections worry researchers

Canadian researchers have identified 16 animals with methicillin-resistant Staphylococcus aureus and proved, with a little detective work, that many of these infections were caught from humans, sometimes the pet's owner. Methicillin-resistant S aureus is an anthroponosis, a disease that is communicable from humans to animals. According to Donald Low, M.D., chief microbiologist at Mount Sinai Hospital in Toronto, who led the research, MRSA outbreaks have previously been documented at veterinary hospitals, but this is the first time they have been reported in the community.

"We may have a new problem emerging in veterinary medicine," Dr. Low commented. "If this eventually becomes a bigger problem, several antimicrobials may become useless."

Methicillin-resistant Staphylococcus aureus is most often seen in high-risk, human patients, including the elderly and those with open wounds, but it is rarely seen in animals. Thus, when veterinarians brought several infections to the attention of Dr. Low, he set out to investigate.

Reporting at a recent meeting of the American Society for Microbiology, researchers say they identified the infection in eight horses, six dogs, and two cats. Using genetic tests, they found a connection between human and animal infection in five of the horses, two of the dogs, and two of the cats.

"We may have two problems emerging here: the spread of MRSA from the hospital setting to the community, both humans and animals, and the increasing use of a class of antibiotics that don't work against these agents and, often, other gram-positive bacteria that cause wound infections," Dr. Low mused.

In one case, an Irish Thoroughbred was admitted to a large veterinary hospital for removal of a melanoma and developed an MRSA-related abscess. After discovering that the horse's owner had also suffered from this infection after an operation, the investigators isolated the bacterium from the woman's nose and determined it was identical to the horse's strain. When two other horses contracted the same infection at the hospital a few months later, the researchers investigated and found that a veterinary surgical technician and a veterinary student were carrying the same bacterium. This, they say, suggests the workers may have caught it from the first horse and passed it on.

A second cluster began with a previously healthy foal living on a small farm. The foal developed a gluteal abscess, which was drained and treated by a veterinarian. After a second horse that was cared for by the same veterinarian was determined to have MRSA in its secretions, the veterinarian was screened and found to carry an identical strain.

The first dog case to appear involved an elderly Bichon Frise that underwent an operation to remove an eyelid cyst. Despite the use of antimicrobials, the dog developed a lingering infection that turned out to be MRSA. The doctors discovered that the dog's owner had contracted the same pesky bacterial infection following a surgical procedure. Genetic tests proved the strains were identical.

Two other cats and one dog were found to have the same infections, and all had been treated at the same Quebec clinic.

"The fluoroquinolones have become the de facto first-line therapy for the treatment of skin, soft tissue, and respiratory infections in animals because of the ease of administration, the lack of side effects, and the convenient dosing," Dr. Low said. "These types of bacteria, however, don't respond to the quinolones, as they are typically resistant, and if not resistant, can develop resistance on treatment. We have seen this in humans."

Dr. Low strongly recommends that veterinarians determine the susceptibility of an organism before choosing an antimicrobial for treatment.