MRSA in pigs and pig farmers

Methicillin-resistant Staphylococcus aureus (MRSA) is very difficult to kill. This notorious "superbug" can withstand a broad and growing range of antibiotics, and is the leading cause of hospital infections in many countries. But it's not restricted to hospitals. According to studies coming in from all over the world, MRSA has found a new route into our bodies -piggyback.

Pig farms throughout the world have become breeding grounds for strains of MRSA that can jump from swine to humans. These strains have already been isolated in the Netherlands, Denmark and Canada, and now, the latest study adds the USA to that list. The research was led by Tara Smith from the University of Iowa, who I know as a Scibling and who many of you will recognize as the author of the excellent Aetiology blog.

Smith found widespread traces of MRSA in two different production systems in the states of Iowa and Illinois. Within the nostrils of 49% of pigs and 45% of pig farmers, her team detected traces of the "superbug" (although it's worth noting that none of the farmers had experienced any actual infections). Piglets had the highest rates of infection and in fact, every single pig under the age of 12 weeks harboured MRSA colonies.

The high levels of the bacterium in both man and pig suggest that it can spread readily between the two species. To MRSA, both four leg and two legs are good...

The link between MRSA and pigs was first discovered a few years ago in the Netherlands. Dutch hospitals have little to fear from MRSA - aggressive "search-and-destroy" policies and restricted antibiotic use have controlled the bug to such an extent that hospitals list "treatment in foreign hospitals" as a risk factor for infection!

But in 2003, Dutch researchers started to find unexpected cases of MRSA in pig farmers and eventually identified the culprit as a new strain that became known as ST398, or non-typeable MRSA (NT-MRSA). It's this same strain that Smith found in the Iowan pigs.

In 2007, Albert de Neeling and Xander Huijsdens found ST398 in 39% of pigs and 81% of local pig farms, suggesting again that the bacteria was jumping from pigs to humans. In the same year, Huijsdens, together with Inge van Loo, found further proof for this theory. By comparing 35 people with ST398 to 76 people carrying other strains, they found that the ST398 carriers were 12 times more likely to have come into close contact with pigs and 20 times more likely to have come into contact with cattle. On a map, they saw that the distribution of ST398 uncannily matched the spread of pig and cattle farms.

ST398 is a newcomer. By reviewing a national MRSA database, Dr Huijsdens and Dr van Loo found that the strain was non-existent in 2002 but now accounts for over one in five human infections. Its origin is unclear. It almost certainly jumped from pigs to humans, but the researchers think that this was just the return part of a round-trip. They think the bacteria may have originally jumped from humans to pigs.

In their new porky hosts, the bacteria developed new antibiotic resistances and returned, stronger than ever. Unlike other strains, ST398 strongly withstands tetracyclines, a group of antibiotics that is heavily used to medicate livestock. Unnecessary antibiotic use has been blamed for the evolution of MRSA strains in humans and the same could apply to pigs, especially since the farming industry uses more antibiotics than hospitals. Under such heavy assault, it is almost inevitable that bacteria, which reproduce quickly and swap beneficial mutations, will develop resistance.

So far, it is not clear if ST398 is spreading beyond pig farms or if it causes so-called community-associated MRSA, which occurs outside the hospital setting. For the moment, people who frequently come into contact with pigs or cattle, including vets, have the highest risk of infection. The bacteria could also spread to their friends and family by hitching a ride on clothes or skin.

Even the food chain is not entirely safe. Van Loo detected ST398 in 2 out of 79 meat samples from Dutch supermarkets and butchers, but only at low levels that are unlikely to cause disease if food is properly prepared. For the general population, there's no need to panic over ham and bacon but the risks could be higher for people who handle meat directly, or those with weakened immune systems. At least one hospital outbreak began when an immunocompromised patient ate contaminated meat. Even though this was a different MRSA strain, it set a dangerous precedent.

The problem is not confined to the Netherlands either. Pigs are heavily exported around the world and ST398 may have stowed away inside them. It has already been detected in France, Singapore and Denmark. Scott Weese from the University of Guelph in Ontario found the Dutch strain in a quarter of local pigs and more worryingly, in a fifth of pig farmers. Human cases of ST398 are extremely rare in Canada but that could change as it spreads among the pig population.

And as we've seen, the US is affected too. As the largest importer of Canadian pork, their farms could have become contaminated by swine brought in from their northern neighbour, but only further studies can confirm that. It's also unclear how widespread ST398 is in the US, but Iowa alone accounts for a quarter of all the swine raised within its borders.

It is clear that we need more research and better monitoring to fully understand the scale of the MRSA epidemic in pigs and the role that agricultural antibiotics have played in it. Only then we recommend the right control measures to protect farmers and the wider population.

Reference: Tara C. Smith, Michael J. Male, Abby L. Harper, Jennifer S. Kroeger, Gregory P. Tinkler, Erin D. Moritz, Ana W. Capuano, Loreen A. Herwaldt, Daniel J. Diekema (2008). Methicillin-Resistant Staphylococcus aureus (MRSA) Strain ST398 Is Present in Midwestern U.S. Swine and Swine Workers PLoS ONE, 4 (1) DOI: 10.1371/journal.pone.0004258

More on drug resistance:

• Drugs that work against each other could fight resistant bacteria
• The secret of drug-resistant bubonic plague
• Super-bacteria eat antibiotics for breakfast

Image: My MRSA GiantMicrobe, with enormous thanks to Rebecca Skloot

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