CHICAGO -- Scientists who worry about the spread of nasty germs from animals to people have found the opposite can also happen: Cats and dogs catch bad things from their owners.
Canadian researchers documented 16 cases of dangerous, hard to treat staph infections in horses, cats and dogs. They believe that all of them probably began with owners or veterinarians infecting the animals.
The germ is methicillin-resistant Staphylococcus aureus -- called MRSA for short -- a microbe that until recently was seen only in hospitals, where it often spreads to elderly or especially ill people who have open wounds or tubes. Healthy people may carry it on their skin without getting sick.
''We've got some pretty strong evidence that owners were responsible for their companion animals developing MRSA soft tissue infections,'' said Dr. Donald E. Low, chief microbiologist at Mount Sinai Hospital in Toronto.
Low and colleagues presented the data Sunday at a meeting in Chicago of the American Society for Microbiology.
They found that the animals had resistant staph infections that were genetically similar to the ones that occur in people. In some cases, they showed that the animals got sick months after their owners caught identical germs.
Such transmission is often difficult to prove, but Dr. Shelley Rankin, a microbiologist at the University of Pennsylvania, believes vets are seeing it more often in domestic animals.
''People think it only goes one way, from animals to humans,'' she said. ''This shows the other side of the story.''
The germ passes through close person-to-person -- or person-to-pet -- contact, which is one reason why hospitals constantly urge doctors and nurses to wash their hands. In people, staph can cause pimples and boils as well as much more serious conditions, such as pneumonia and lethal bloodstream infections.
The first Canadian case researchers examined was a 9-year-old bishon frise operated on in January 2000 to remove an eyelid cyst. Despite antibiotics, the dog developed a lingering infection that turned out to be MRSA.
The dog's owner had undergone surgery in late 1999 for testicular cancer. While in the hospital, he, too, had caught MRSA. The researchers did genetic tests to compare the germs from man and dog. They were identical.
The researchers believe veterinary clinics can also serve as a source of the bug -- just as hospitals spread the germ to sick people. Two cats and one dog with identical infections had all been treated at the same Quebec clinic.
The team documented two separate outbreaks of the infection involving eight horses. One of them began when an Irish thoroughbred was admitted to a large veterinary hospital for removal of melanoma cancer. Two days after returning home, the horse developed an abscess caused by MRSA.
The horse's owner had undergone a hysterectomy nine months earlier. The researchers isolated MRSA from her nose, and the bacteria turned out to be identical to the horse's. Two other horses got the same infection at the hospital a few months later. The researchers found that a surgical technician and a veterinary student carried the same germ, suggesting they may have caught it from the first horse and passed it on.
Before the introduction of antibiotics, staph was such a big problem that sometimes wards and even entire hospitals had to be shut down. The advent of penicillin at first stopped the bug, but it quickly grew resistant. In 1960, methicillin became the standard treatment.
The Centers for Disease Control and Prevention estimates about half of all hospital-acquired staph infections in the United States -- 80,000 annually -- are now resistant to methicillin, and they often are impervious to other antibiotics, as well. Some can be treated only with vancomycin, an injected drug, or Zyvox, a pill introduced last year.
For pets as well as people, the infection can be life-threatening. One of the dogs died, and another required amputation of its infected leg.
For several years, experts have warned that overuse of antibiotics on farms can promote the transmission of drug-resistant microbes from animals to people, what's known as zoonotic spread. However, the Canadian researchers say they believe they have found evidence of a different hazard -- humanotic spread.
Medical Editor Daniel Q. Haney is a special correspondent for The Associated Press.
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