Methicillin-resistant Staphylococcus aureus: An emerging problem in horses?

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Outbreaks of methicillin-resistant Staphylococcus aureus in horses and humans in Canada have researchers concerned that MRSA infections in horses may be emerging as a serious zoonotic and veterinary nosocomial disease. Researchers are recommending that veterinary hospitals launch surveillance programs for the pathogen.

"I really believe that maybe this is something that people are quietly battling or just not noticing," said Barbara Willey, an infection control and methods development technologist at Mount Sinai Hospital in Toronto. Willey presented research results on the outbreaks at the annual Interscience Conference on Antimicrobial Agents and Chemotherapy held in Chicago in September.

Before this research, little was known about the transmission of this pathogen between horses and humans. MRSA is most often seen in high-risk, human patients, including the elderly and those with open wounds, and is believed to be rare in animals. Thus, when Canadian veterinarians recognized clusters of infections in horses, they became worried, and researchers at Mount Sinai Hospital and the University of Guelph's Ontario Veterinary College initiated a human and equine surveillance program.

Between Oct. 1, 2000, and Nov. 16, 2002, scientists isolated MRSA from 82 horses and 29 humans. Although many clinically infected horses were seriously ill and required prolonged hospitalization, MRSA was implicated directly in the death of only one horse. Importantly, 50 percent of the equine cases were from one Thoroughbred farm. When researchers identified horse farms that had outbreaks, they tested people on the farms and commonly found that they, too, were infected.

The scientists found that 95 percent of the MRSA in horses and 93 percent of the MRSA in humans were the same strain, Canadian MRSA-5. This strain accounts for only about 5 percent of MRSA infections in humans in Canada.

Dr. Scott Weese, an assistant professor of clinical studies at the Ontario Veterinary College who works with Willey, believes the strain originally jumped from human to horse and that this strain has an adaptation that allows it to thrive in the horse.

"If there wasn't a special adaptation, you would think that a horse would be more likely to carry other strains that are much more common in people," Dr. Weese explained. Once the pathogen jumps to a horse, it can easily spread to people or other horses. Willey says they even tested twitches on infected farms and found MRSA.

Since November 2002, the Canadian researchers have continued their aggressive surveillance program. Every horse that comes into the veterinary college's large animal clinic is swabbed for MRSA, upon arrival and departure, and periodically during hospitalization. They are continuing to test horses and people at facilities that have infected horses, and, because it is rare, when the infection is diagnosed in individuals at hospitals, physicians ask them if they have contact with horses. "(The patient) will say, 'How did you know?'" says Willey.

Dr. Weese says he is seeing higher incidence rates of the pathogen on breeding farms, which is of concern. The infection is difficult to treat and even more so in young horses.

The investigators note that the equine community needs to recognize the potential for MRSA to become a serious endemic problem in veterinary hospitals. CMRSA-5 causes high rates of colonization and infection on certain farms, not only among horses, but also among humans in close contact with the horses.

It's too soon to tell just how big the problem is going to be. "Whether we are just at the beginning of MRSA in horses or whether it is going to be a low-level peripheral problem is hard to say," Dr. Weese said, noting that the pathogen is most likely widespread in horses at a low level throughout North America.

Dr. Tom Lenz, who has been president of the American Association of Equine Practitioners this past year, says that in talking with colleagues, he has learned that MRSA infection is becoming more common in referral hospitals. Some clinics are also beginning to see secondary MRSA infections in foals in their neonatal care units. "It would be a good research project for someone to look for the organism at the various equine clinics across the country or to do a retrospective survey," Dr. Lenz said.

Dr. Weese said that establishing surveillance programs is key. "The horse industry in North America is a very transient group," Dr. Weese said. "Horses will move around very large distances, very frequently, so basically it is impossible to have a nice, confined disease anymore."

The problem is compounded by the fact that clinical infections are just the tip of the iceberg. Fewer than 10 percent of the infections that Canadian researchers have identified are clinical infections; a large pool of carriers may be silently infecting other horses or humans.

"What we are hoping is that with surveillance, quarantine, and treating human carriers that might be infecting horses, we might be able to keep the prevalence quite low," Dr. Weese said. Willey notes, investigators have their work cut out for them. "It's a delicate situation, looking for the MRSA in breeding farms. Thoroughbred breeding farms are not going to want to be known to have MRSA. These are million-dollar racehorses."