Warm spells in early spring, like the recent warm spell in our area, inevitably lead to (premature) thoughts of summer, and for many people, this includes thoughts of spending time in the swimming pool. I've previously written about the presumably low risk of disease transmission from dogs swimming in pools, and common sense measures that can be taken to reduce any risks. Since more and more people and dogs are carrying resistant bacteria like MRSA, there are increasing questions about the potential for pools to be a source of transmission. A recent paper in the journal Clinical Pediatrics (Gregg & LaCroix, 2010) sheds some light on the issue.
In this study, the researchers obtained different types of swimming pool water (chlorinated water, saltwater, and biguanide (Baquacil) nonchlorinated water) from local pools. No MRSA was identified in any of the samples. They then inoculated the water samples with MRSA. They used a lot of MRSA - roughly similar to the amount that would be released from an MRSA abscess (and much, much greater than the amount that would be released from someone who was just an MRSA carrier). MRSA numbers were greatly diminished after 30 minutes and no MRSA was detected after 1 hour.
This study suggests that pools are likely not a significant source of MRSA exposure. Even with high-level contamination, MRSA died quickly. Presumably, there would be little MRSA shed by a person that was only a carrier, and when you consider the dilutional effect of a small amount of MRSA in a large volume of water, plus the bactericidal effects of treated water, the risks should be extremely low. Common sense would dictate that someone with an MRSA infection shouldn't go in the water because they could shed large numbers of bacteria, but this study suggests that the risks are probably minimal and short-term even then.
Bottom line: Don't worry about pools in terms of MRSA. Pools are a greater risk for certain causes of gastrointestinal disease like norovirus and Cryptosporidium. If you or your dog has an infectious disease, stay out of pools. If not, and you are otherwise healthy, then don't worry. There's always some risk of exposure to infectious disease, but it's very low. Life is full of risks and swimming in a pool is not a big one (at least from an infectious disease standpoint. If you can't swim, that's a different story!).
Image from: http://blog.timesunion.com