The more we look, the more we find when it comes to MRSA (methicillin-resistant Staphylococcus aureus). As people start looking for it in different animal species, it’s often found. We’ve found it in many species already, including dogs, cats, rabbits, pigs, walruses, dolphins and alpacas, so it’s not a big surprise to see a recent paper in the Journal of Clinical Microbiology (Ferreira et al 2011) about suspected MRSA transmission between a human and a hamster.

The case report describes a person with advanced cystic fibrosis who had undergone a lung transplant and had various other medical problems. Prior to another surgical procedure, MRSA was identified through routine pre-operative screening. Nasal and rectal swabs were then collected from the person’s three pet hamsters, one of which was positive. The MRSA isolates from the human and hamster were the same, supporting transmission from one to the other. Given the person’s underlying health problems, frequent contact with the healthcare system, the typical human origin of the strain that was found, and limited contact of hamsters with other animals or people, it is most likely that MRSA was transmitted from human to hamster in this case.

The paper concludes with: "Should testing of the pets of MRSA-positive patients be recommended? At this point, we recommend that MRSA-positive patients be informed that their companion animals can be potential sources of infection or reinfection. In the presence of a MRSA-positive human or animal, heightened hygiene practices should be instituted and unnecessary close contact should be avoided. Screening of household pets might be indicated in situations of recurrent MRSA infections despite adequate treatment or when immunocompromised patients live in the household."

That’s consistent with our standard recommendations and hits most of the key points:

• Awareness is critical. People need to know what the risks might be and what they can do about them so that they can make informed decisions and realize why recommendations are being made.
• Testing of pets is rarely useful, particularly in the absence of a recurrent MRSA problem.
• Good hygiene practices are critical.

I don’t really agree with the comment that testing of pets might be indicated when immunocompromised people are in the household (although the comment is properly hedged by saying "might be indicated"). A large percentage of the population has some degree of immunocompromise, and there’s a huge spectrum from minimal risk to tremendous risk. Even in high-risk patients, screening is questionably useful to me because it doesn’t really change what I’d do.

• If I screened a hamster from a high-risk person and found MRSA, I’d say that it probably came from the person, that it’s possible it could be transmitted back to the person, that good hygiene practices should be followed and close contact should be restricted. There’s no indication (or ability, in reality) to treat the hamster.
• If the hamster was negative, I’d say it was possibly negative because screening is not 100%, that the hamster could be exposed to MRSA from the owner at any time, and so to manage unknown colonization and reduce human-hamster transmission, I’d recommend good hygiene practices and restriction of close contact.
• If I’m going to do the same thing with a positive and negative result, I don’t do a test.

This paper should be yet another reminder that we live in a complex relationship with our pets, including microbiologically. While we need to consider the role of pets in human infection (and the role of humans in pet infection), and we need to balance that with the positive aspects of pet ownership in order to maximize the benefits while minimizing the costs.