Pigs, antibiotics, and staph where it shouldn't be

The “third epidemic” of MRSA (drug-resistant staph) — the one that started in farm animals given antibiotics, and subsequently spread to humans — has been contentious since its emergence. This week there are several pieces of new news about it. They’re not likely to solve any of the disagreements, but they’re certainly interesting.

Very quick recap for those coming in late: MRSA, short for methicillin-resistant Staphylococcus aureus, describes strains of staph that have become resistant to most common antibiotics. It’s been gaining ground on us for about 50 years, first in hospitals, then in the everyday world and now in farm animals and farm-workers. Surveillance for it is not excellent, but in various studies, it kills 19,000 Americans, puts about 370,000 in the hospital, and sends possibly 7 million to a primary care visit or ER, and causes billions of additional dollars in health care spending — all in a year. For the most serious infections, there are only a few drugs that still work. It’s the leading organism in the under-appreciated international epidemic of antibiotic resistance. (If you’d like to know more, I wrote a book about it.)

Livestock-associated MRSA — many researchers just call it “pig MRSA,” which makes swine agriculture very unhappy, but the more technical term is MRSA ST398 — was first noted in 2004 in a Dutch toddler being prepped for surgery; then identified in her family and their social circle, all of whom were pig farmers; and then was found in their pigs. Along with the standard suite of MRSA resistance factors — all the beta-lactam antibiotics, which means anything ending in “-illin,” most cephalosporins, the monobactams and carbepenems, and also erythromycin — this new strain was resistant to tetracycline. That was odd, because the Netherlands’ rate of MRSA was so low that they were not bothering to give humans tetracycline for MRSA; but tetracycline was the most common drug given to pigs in large-scale agriculture there. It was proof the organism had been resident in pigs, become resistant as a result of ag antibiotic use, and then crossed to humans. (Yeah, this is all told in the book. OK, no more shilling.)

Since that first finding, ST398 has spread throughout the European Union, into Canada, and in one state in the US, Iowa. (Veterinarians assume it has spread more widely than that, but Iowa happens to be the only state where researchers have looked for it.) It is less common than other strains of MRSA, but it has been identified as the cause of mild skin infections among farm workers, serious hospital infections such as ventilator-associated pneumonia, and life-threatening community infections such as flesh-eating disease. It’s also been found in retail meat in several different countries. (We haven’t yet managed to move my archives over, but there’s a 2-year history of ST398 coverage at my old blog.)

Despite the low number of known cases, MRSA ST398 is important, for several reasons: First, because as those death and illness numbers demonstrate, any additional MRSA is bad news. Second, because MRSA ST398 bridges the human and animal worlds, demonstrating how easily an organism that is resident in animals can cause illness to humans, and also can move across the world with agricultural trade. And third, because its occurrence underlines the consequences of antibiotic overuse in agriculture: If they hadn’t been giving tetracycline to pigs in the Netherlands — a country that, within about a decade, went from small family farms to the largest user of ag antibiotics in the EU — “pig MRSA” might not exist.

So, this week’s update, courtesy again of the Interscience Conference on Antimicrobial Agents and Chemotherapy, or ICAAC: two findings that are somewhat contradictory.

First, a team in the Netherlands wanted to know how much of an infection risk ST398 truly poses. That’s an especially important question in the country where the strain got going. In the Netherlands, which exerts very close control over MRSA overall, certain categories of hospital patients are checked to see whether they are carrying the resistant bug, and if so, put into isolation and treated until they are clear. But some people are considered to be such high infection risks that they are put into isolation presumptively — and since 2007, those categories have included farm workers and veterinarians.

That’s a lot of tests, which means a lot of expense. So the team who presented this week tried to assess whether that isolation and testing are necessary, by measuring how often carriage of MRSA ST398 converts to an active infection. They looked at every MRSA isolate identified in 2009 at two hospitals in the southeast, where pig-raising is most dense, and identified a huge reservoir of carriage of ST398: 61% of the MRSA-positive patients, or 298 out of 486. But of those 298, only 7 developed an infection, and none of them passed the infection to a second patient. The 188 patients carrying hospital MRSA, on the other hand, caused 83 secondary cases — enough to force closure of a department in one of the hospitals. (van de Sande et al., ICAAC 2010)

So while pig MRSA’s easy to acquire, given the right exposure, it may not often cause illness. Problem is, according to a second piece of research discussed this week, when it does, it can be unexpected and devastating.

A coalition of 21 hospitals in Spain wanted to know how many of the MRSA bloodstream infections occurring in their institutions were caused by the community strain of MRSA instead of the hospital strain — something that would signal a change in the epidemiology of what can be a devastating illness. They checked every MRSA bacteremia case from June 2008 through December 2009. They found 324, overall, with 10 of them caused by community strains. But they also found something they didn’t expect: Almost as many, eight of 324, were caused by ST398. (Camoez et al., ICAAC 2010)

This isn’t the first time that MRSA ST398 — an organism linked to agriculture and to spread through farm workers and veterinarians — has been found to cause hospital infections or serious infection. It has caused ventilator-associated pneumonia in Germany and post-surgical infections in Canada.

One of the ways that epidemiology measures the seriousness of an outbreak is to compare it to the expected background occurrence of a disease. In the case of MRSA ST398, that background rate is zero. The strain’s an artifact of the overuse of antibiotics in agriculture. It’s really worth thinking about how many more such organisms we want to produce.

Image: photographer unknown.


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