19 March 2008

MRSA and animals - not pets, meat

The International Conference on Emerging Infectious Diseases ended today with more news about a range of infections including MRSA. Most eyebrow-raising: Dr. J. Scott Weese of the Ontario Veterinary College in Guelph, an author of the original MRSA-in-cats paper I described a few posts ago.

Weese's news, tucked in at the end of a comprehensive presentation on MRSA and animals: Analysis of 212 raw pork products sold in four Canadian provinces reveals an average rate of MRSA contamination of more than 9%.

This shouldn't be surprising: Research by Weese and others has been revealing a complex and not well-understood interplay of infection between pigs and nearby humans.
  • A 2005 French paper reported a rate of resistant-staph nasal colonization in pig farmers that was twice as high as among human controls; 57% of the isolates from the farmers were identical to nasal isolates in pigs.
  • Last June, a Dutch study found 39% of pigs in nine major slaughterhouses in the Netherlands carried an identical novel MRSA strain.
  • In December, two studies filled out the picture. A Dutch study reported the prevalence of that novel MRSA strain (dubbed ST 398 and first found in a human in 2003) has risen to more than 21% of all MRSA isolates in the country. A Canadian study (with Weese as senior author), published online ahead-of-print, found MRSA colonization rates of 25% among Ontario pigs and 20% among pig farmers, with most of them sharing the ST 398 strain but some possessing the CA-MRSA strain USA100.
  • A Dutch study in January (first author Engeline van Duijkeren, who did the S. intermedius study from a few posts ago) found that pigs were colonized with MRSA on a variety of types of farms, such as ones that birth pigs and ones that raise them to slaughtering weight.
  • And just to erase any doubts, several Dutch studies have established that the ST 398 strain causes human disease: endocarditis, mastitis, severe hospital-acquired pneumonia and bloodstream infection.
Weese stressed today that his team's finding of MRSA in raw retail meat is very preliminary (though it is backed by a Dutch study from last November that found 34 MSSA strains and two MRSA strains — ST 398 and USA300 — in 79 pork samples). He added, though:

"People don’t tend to handle pork like it is biohazardous, unlike chicken. So there may be a theoretical concern that pork could be a vehicle of methicillin resistance colonization — but it is way too early to say anything about that."
But if you're not already handling raw meat in a careful manner (sterilizing cutting boards, avoiding cross-contamination), it might not be too early to start.


robyn said...

Whoa. Well now. Does this suggest that community-acquired MRSA may have come from our grocery store instead of the hospital? Another question: so all these people walking around with MRSA in their nasal passages. They are carriers but don't get sick themselves? Have they built up an immunity?

Maryn McKenna said...

Thanks, Robyn! Couple of issues:
1. Weese didn't suggest that meat could be a source of infection, but he is raising the possibility that it could cause colonization — acquisition of a strain that lives harmlessly on your skin or in your nostrils but may cause infection in the future.
2. Colonization isn't a sign of immunity — it precedes infection. Staph's ability to live on us without making us sick is a byproduct of its very, very long evolutionary history with humans. Why and when colonization (for S. aureus, ~30% of the population; for MRSA, 1-10% depending on who you study) turns into infection is one of staph's mysteries.
3. Community staph doesn't come from hospitals - CA-MRSA and HA-MRSA are genetically distinct. This was proven pretty conclusively by RS Daum and team from University of Chicago. Mind you, the Daum group's first paper was in 1998, and since then, the landscape has gotten very complicated. There are on the one hand "health care-associated/community onset" cases, in which the person becomes colonized in the hospital but doesn't develop symptoms until they have left, and on the other hand hospital outbreaks caused by the community strain that has been carried in to the hospital. The usefulness of those terms is in many ways outdated — but no one yet has come up with anything better.

Alan Little said...

Do you have any idea why the Vet community has almost no knowledge about MRSA?. My 4 1/2 yr old male bulldog was diagnosed as having MRSA after almost 8 months of being treated with most anti biotics to no avail . He is now being treated with Cholramphenicol which I undertand has a whole lot of other problems. thanks

Maryn McKenna said...

Hi, Alan,

My sense is that an understanding of MRSA as a veterinary pathogen, especially among companion animals, is pretty new, and therefore mostly confined to the research hospitals associated with vet schools. (It's different in the equine world; there MRSA has been a known problem for years.) I don't know how much everyday primary-care vets know about MRSA or think to include it in a differential diagnosis. Are there any vets reading who could comment?

I'm sorry to hear about your poor fella though. Is it possible for you to tell me more of the story of his being diagnosed — what his first symptoms were and why diagnosis took so long?