Pets may be reservoir for MRSA infection

Key Points

A letter to the editor in the New England Journal of Medicine, published on March 13, told of an otherwise healthy German woman with multiple deep abscesses that cultured positive for MRSA.

Despite treatment, her infections recurred. Her husband and children were found to be colonized and were decontaminated, but still, the woman's lesions returned.

Ciprofloxacin and rifampin were administered to the cat. (Note: Some veterinarians consider this an inappropriate regimen, because safety and efficacy studies are lacking in cats.) The family declined follow-up screening of the animal, but the bacteria presumably cleared.

Follow-up screening of the humans a month later found no MRSA colonization. More importantly, the index patient's abscesses resolved completely with treatment.

Previous case

This was not the first report of MRSA in animals.

A 1972 case report noted the bacteria had been found in the milk of a cow with mastitis, says Jeff Bender, D.V.M., a professor of veterinary public health at the University of Minnesota, St. Paul, Minn.

He says, "There has been an explosion of literature in the last five years of MRSA in animals, both in Europe and North America."

"Typically, pets clear this rather rapidly, in a couple of weeks, as long as there is not reinfection," says J. Scott Weese, D.V.M., a professor at the Ontario Veterinary Collage at the University of Guelph, Ontario.

"If you see long-term colonization, usually it is because it is passing between different individuals in the household - humans or animals," he tells Dermatology Times.

The rapid clearance of MRSA by dogs and cats makes epidemiologic study difficult. It is possible that the infection can move from the animal to the owner, but the pet may have cleared it by the time the human develops symptoms or the animal is cultured.

It is also difficult to evaluate passage between animals within a household.

Transmission modes

Hands likely are the most common mode of transmission, from human nose to animal fur and vice versa, though playing "kissy-face" with a pet may be a more direct route of exposure to nasal colonization, experts say.

The animal's licking and grooming can spread the bacteria to other sites. Colonization in dogs and cats is most commonly found in the nose, mouth and anus.

Dr. Bender has characterized MRSA isolates from household pets and found that the majority of them are healthcare-associated strains.

"When we interviewed the owners, we found that a fair number of them worked in healthcare, had recently been hospitalized or were caring for someone who had been hospitalized," he says.

"So, we found that a lot of our pets were picking it up from their caretakers."

Researchers conducting one study of therapy dogs, used in hospitals and long-term care facilities, swabbed the fur before and after those visits, and found the animals were acquiring the bacteria during those visits.

Other studies have shown variability between animals, perhaps due to differences in interactions with patients.