MRSA and pets: should they get tested?

Over at the Worms and Germs blog, Scott Weese has a great post on MRSA testing. He notes the he's frequently asked by human MRSA patients whether their pet should be tested as well, since several studies have documented transmission of MRSA between humans and their companion animals. His first response is always, "why?"

One big question I ask is 'why do you want to know and what would you do with the results?' Sometimes people want to know their pet's status to see if the pet was the source of their infection. However, MRSA in pets is typically human-associated, and if a pet is carrying MRSA, it probably got it from the owner. Finding MRSA in a pet after someone was diagnosed with an MRSA infection doesn't mean the pet was the source. More likely, the person got MRSA somewhere else and passed it on to their pet.

Sometimes, people want to know if their pet is at risk of an infection. Carrying MRSA presumably increases the risk of an MRSA infection, but likely only in animals already at risk of an infection because of underlying disease or other risk factors such as surgery. The risk to the average pet from short-term MRSA colonization is probably limited. Also, if the pet was identified as a carrier, we wouldn't be doing anything to eliminate carriage, since we have no idea if we can do it and it doesn't seem to be needed (since dogs and cats almost always get rid of it on their own). Therefore, it's hard to justify screening for this reason. If the animal was getting ready to undergo surgery, then that might change my answer.

I think Scott makes great points. As he notes later in his post, his advice would be the same whether an animal was found to be colonized or not (since even if the animal is negative, that only means they were *negative at the time of the testing,* and they could become colonized later: practice good hygiene.

I'd add one addendum to his response, though. As he notes, his answer may change if the animal may be facing surgery. I'd add also from the human side that the carriage status of pet animals may be something important to know if there are recurring infections in the family. If this is an issue, then the entire family will need to be looked at as a unit, to see if one of them is a reservoir (potentially re-infecting others in the family)--and the family unit should include pets at this point. As Scott notes, typically pets become colonized from their owners, but once colonized, they do have the potential to pass it back to the other human family members like the world's worst game of hot potato. As such, they may keep the MRSA transmission chain going, even if their humans are subjected to decolonization measures (as was described in this NEJM paper).

So, in special cases, it may be helpful to have your animals tested for MRSA--but for the vast majority of people, having that knowledge won't do much one way or the other.

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Is there a point in humans being tested, absent some clinical infection? What fraction of healthy people are carrying SA in their noses? And what fraction of that is MR? It's not as if SA is some exotic microbe, right?

Outside of surveillance or an incident infection (in that person or a family member as mentioned above), not really. About 1/3 of us carry some strain of S. aureus, and about 1-2% of us in the U.S. carry MRSA.

Many years ago, I had an MRSA infection (which nearly killed me three times!)

A couple of years back, someone suggested I could be a carrier, something that had never crossed my mind. So I got tested.

Negative. I'm not sure how I might have dealt with things if I was positive.

According to The Textbook of Bacteriology, nearly all of us carry SA in the gut.