Swine and MRSA

by revere, cross-posted at Effect Measure

The peanut butter/peanut paste ingredient based salmonella outbreak has been in the news lately and we've discussed it here (and here, here, here, here, here). There are now about 500 reported cases and six deaths. That's a case fatality ratio of just over 1%. So what if there were a disease outbreak of 100,000 cases with a case fatality ratio of 20%? I think we'd be pretty alarmed. But it happened in 2005. And it happened in 2006 and 2007 and last year, 2008 And it's happening, now, too. It isn't salmonella or or even HIV/AIDS, although it is estimated to kill more people in the US than both put together. It is methicillin-resistant Staphylococcus aureus (MRSA), the difficult to treat antibiotic resistant bacterial infection sometimes called a killer superbug. Originally associated with hospitals, MRSA has now moved into the community. It is a major medical and public health problem and there is still much we don't know about it, like where it hangs out. Now, thanks to our Scibling blogger (Aetiology) Tara Smith and her colleagues, we know a good deal more. And what they found out is disconcerting.

I'll let Tara give you the gist:

A little over a year ago I put a post up documenting research out of Canada which found methicillin-resistant Staphylococcus aureus (MRSA) in Canadian pigs. This had also been seen in Europe (with a lot of research coming out of the Netherlands). What I didn't note at the time was that we were gearing up to start some sampling of our own on area swine farms.[snip]

For this research, we swabbed pigs and humans from 2 large swine farming companies in the area. The first ("production system A", PSA) had about 60,000 pigs at any one time; the second (PSB) was smaller, with about 27,000 pigs. These were distributed over several different farms in the area (with several thousand animals on each farm) and are typically age-segregated.

We didn't find any MRSA on PSB, in either the pigs or the people. However, we found quite a lot on PSA, in roughly similar percentages of people and pigs (70% of PSA's pigs, 64% of PSA's people). We carried out molecular typing on all of the human isolates and a subset of the swine, and all that we tested were found to be ST398, the so-called "piggy" MRSA. To our knowledge, this is the first publication of this strain in the US. (Tara Smithy, Aetiology; more info and links to the paper [open access] at Tara's post)

 

Tara's paper has been well covered by Ed (Not Exactly Rocket Science) and Mike (Mike the Mad Biologist) here on Scienceblogs.com and by Maryn McKenna at SciAm (Maryn also writes a superb blog devoted to MRSA, Superbug). The ST398 MRSA strain is spreading fast but so far hasn't been shown to cause serious human disease. The deadly form of MRSA infections comes from hospital strains and presumably they differ from ST398 in virulence factors, genetic elements that make infection with the strain into a serious illness rather than asymptomatic carriage. But virulence factors can move between strains and strains can also evolve to become more virulent.

The subject is well covered in Tara's paper and the links from my blogging colleagues, so I'll just add a few observations about things not covered in those places (and if you have any interest at all, by all means visit those links; they are very informative). ST398 was discovered in The Netherlands, then Canada, now the US. It is unlikely it arose de novo in each place. It moved. MRSA has generally been thought of as a problem involving human to human transmission, possibly through fomites (inanimate objects) or directly. Now we need to add the food supply to the equation, both as a mode of transmission and as a reservoir. One of the producers was ST398 free while the other was heavily infected. We need to find out why and incorporate that knowledge into our food safety regulations.

Second, the likely fact that this bug moved internationally means that the US food safety system can't solve this and similar problems by itself. Unfortunately the current international system is poorly equipped to deal with global problems because it is built on the notion of national sovereignty, which is quickly becoming obsolete in public health (see our posts on this here, here, here, here and here).

It's time to get the problem of antibiotic resistant organisms under control. That will mean taking on agribusiness, the biggest user of antibiotics and likely one of the main causes of the prevalence of antibiotic resistance. Yes we can.