Maryn McKenna

Journalist and Author

New news on MRSA and animals

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Constant readers, I’ve been behind the Great Firewall of China for two weeks, unable to post. (Apparently Blogger is not always unavailable there, but access has tightened up in advance of the National Day celebrations on Oct. 1.) I left with a file of things to post in my spare time — and so now we’re way behind, with lots to catch up on.

Latest news first, though. A few days ago, an intriguing conference was held in London: Methicillin-resistant Staphylococci in Animals: Veterinary and Public Health Implications. It was co-sponsored by the American Society for Microbiology and the European Society of Clinical Microbiology and Infectious Diseases, and it was the first conference ever convened to examine the behavior in animals of MRSA and other staph species, including our old friend, ST398.

I have the abstracts (which have not otherwise been published), and wow, there was a ton of news.

Here’s the biggest: An investigation by a team at University of Iowa (the same group that first identified ST398 in pigs and pig farmers in the United States) found significant amounts of MRSA in pigs and in human workers on 4 out of 7 conventional farms, but no MRSA on 6 organic farms. MRSA was present — as a colonizing organism, not causing illness — in 23% of the 168 pigs sampled on the conventional farms, and 58% of 45 humans who worked on those farms. “These results suggest a significant number of U.S. swine may be colonized with MRSA, adding to the concern about domestic animal species as a reservoir of this bacterium,” the abstract says. “Furthermore, occupational exposure to these colonized pigs may spread the bacteria from the farm to the community via a high number of colonized swine workers.” (Author: Abby L. Harper, MPH, University of Iowa)

A partial list of the other findings announced:

  • MRSA ST398, which emerged as an animal and human pathogen in the Netherlands, is now causing human colonization and illnesses in other countries. Denmark, which like the Netherlands has a very low background rate of MRSA, has detected 109 cases since 2003, 35 of them with actual infections. Two of the infections were very serious: one pneumonia in a newborn baby, and one septic arthritis in an adult that led to sepsis and multi-organ failure. (J. Larsen, National Centre for Antimicrobials and Infection Control, Denmark)
  • Meanwhile, the Netherlands — which conducts routine screening for MRSA carriage on hospital admission — has seen its annual count of MRSA detections rise from 16 per year between 2002 ad 2006 to 148 per year between 2006 and 2008, with 81% of the current cases due to ST398. (M. Wulf, PAMM Laboratory, the Netherlands) UPDATE: Coilin Nunan of the Soil Association in the UK corrects me (thanks, Coilin!): This study covers only the southeastern pig-farming areas, or about 40% of the MRSA cases in the country.
  • MRSA ST398 spreads from infected to uninfected pigs during transport to slaughterhouses and while being held at slaughterhouses. (E. M. Broens, Wageningen University, the Netherlands)
  • More than 15% of slaughterhouse workers who handle live pigs — but none of those who handled pig carcasses after slaughter — were carrying MRSA 398, and 25% of environmental samples such as dust taken from different parts of slaughterhouses were carrying the organism as well. (B. A. van Cleef, RIVM [National Institute for Public Health and the Environment], the Netherlands)
  • Along with the pig-origin ST398, recognized human strains of MRSA can also colonize pigs, according to a study on one Norwegian farm, but human strains are less successful at persisting in pigs and tend to die out after months. (M. Sunde, National Veterinary Institute, Norway)
  • Animal-origin MRSA is rising in China, the world’s largest producer of pork, but the problematic strain there is ST9, not ST398. That MRSA strain was found on 5 out of 9 farms in Sichuan province in mainland China, and in 33.5% of 260 pigs slaughtered in Hong Kong, where more than 90% of pork comes from the mainland. (J. A. Wagenaar, Central Veterinary Institute, the Netherlands; and M. V. Boost, Hong Kong Polytechnic University)
  • And an intriguing finding for those concerned about humane slaughter methods: Broiler chickens were significantly more likely to carry MRSA, and transmit it to slaughterhouse workers, if they were killed by the traditional method of electrical shock followed by throat-slitting, and less likely to carry or transmit the bug if they were killed by carbon dioxide asphyxiation, which has been held out as a more humane method of killing. (M. N. Mulders, RIVM [National Institute for Public Health and the Environment], the Netherlands)

UPDATE: I’m still a bit jet-lagged and forgot to mention that, of course, we have a long archive of coverage of ST398 and other strains in animals. Find them here.

More MRSA in meat, and not just pork

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In my excitement over the paper by Tara Smith and team on Friday, I failed to sufficiently emphasize an important new finding. (I included it in my story for ScientificAmerican.com, but it was toward the end.) I feel it deserves a post of its own, so here it is:

The Food and Consumer Product Safety Authority of the Netherlands has found MRSA in 12% of 2,217 samples of meat on sale in the country, including not just pork, but beef, lamb, chicken, turkey and game birds, and 85% of the bacterial isolates were the”pig strain” ST 398.

We have talked before (all posts here) about the potential risk of MRSA in meat, especially ST 398 because it seems to have found a preferred host in pigs. In this study, however, the meat most likely to carry ST 398 was not pork, but turkey, followed by chicken and then by veal, and then by pork.

So what does all this mean? It’s still probably too early to tell: Recall that the first isolations of this bug were in 2004, there have still been only a few papers on it, and this finding by Smith and team is the first identification of the strain in the United States. (Though not in North America, as it was identified in Canada in 2007.) It seems likely that ST 398 may have found a niche in other food animals, and that it contaminates the meat when the animals are slaughtered.

The consensus among the Dutch, though, is that this is an effect of the use of antibiotics in food animals. The romantic image of the Netherlands is as a cute little collection of postage-stamp family farms, but the reality, especially in the southeast of the country, is that they have substantial industrial-sized farms housing thousands of animals on relatively small properties. The only way to grow animals efficiently under such conditions is to keep very close tabs on potential illness, and liberally deploy antibiotics when necessary. (NB, I am not talking here about sub-therapeutic, growth-promoting use, but rather prophylactic antibiotics, given to an entire herd when a certain percentage of the herd shows sign of illness.) Evidence for this, according to the current study’s authors: Meat sold as “biologic” — that’s “organic,” in the US — had a much lower rate of contamination with ST 398.

There are still very few reports of human illness from ST 398, though of those reports, some are quite serious, including wound infections and endocarditis. The concern here, as the researchers interested in it have been saying from the start, is that someone will inadvertently colonize themselves with the organism by touching their eyes or nose while handling meat contaminated with ST 398. Colonization does not necessarily lead to disease, but it does lead to a far greater pool of organism potentially spreading unmonitored through human and animal populations, swapping resistance and virulence factors as it goes.

So, you know what I’m going to say: Wash your hands, wash your hands, wash your hands.

Appearing tonight at SciAm.com

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Folks, last summer I told you about the very exciting though disturbing development of ST 398 MRSA — the “untypable” Dutch strain that originated in pigs and spread to humans — being found in pigs in the US for the first time.

But here’s the brand-new second half of that story: It was found in pig handlers as well, on a set of linked farms — a closed production system that takes pigs from birth to just before slaughter — in Iowa and Illinois.

The full study has just been published, in the online Public Library of Science journal PLoS One.

And I have a story describing the research and the background — and the alarming spread of ST 398 in Europe — up tonight at ScientificAmerican.com.

The cite is: Smith, TC, Male, MJ, Harper, AL et al. Methicillin-Resistant Staphylococcus aureus (MRSA) Strain ST398 Is Present in Midwestern U.S. Swine and Swine Workers. PLoS ONE 4(1): e4258 doi:10.1371/journal.pone.0004258

UPDATE: Lead author Tara Smith talks about the paper at her own blog, Aetiology. And for good measure, her Science Blogs sibling (AKA “scibling”) Ed Yong discusses the paper at Not Exactly Rocket Science.

“Pig MRSA” in New York City – via the Dominican Republic?

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Folks: Back in October, I broke the news for you of an intriguing poster presentation at the ICAAC meeting. It revealed the discovery of ST 398, the anomalous staph strain found in pigs, pig farmers and health care workers in Europe, in residents of a Dominican-immigrant neighborhood in northern Manhattan, and also in the Dominican Republic.

Because there is so much traffic back and forth between those neighborhoods, the authors theorized that people are providing an “air bridge” for the bacterium — though they were unable to say whether the bug is moving from the Dominican Republic to the United States, or vice versa.

I was unable to link to that presentation at the time, because it was a meeting poster – yes, literally a poster, the authors stand by it to discuss it with anyone who wanders by. However, now it has been published as a paper, in the CDC journal Emerging Infectious Diseases; and because it is a CDC journal, the full text is available free online here.

Just to underline, despite my headline above, the strain found in NYC was not MRSA: It actually is MSSA, drug-sensitive staph. The ST 398 found in Europe, Canada and the American Midwest is MRSA. The authors hypothesize that the NYC strain is at risk of becoming MRSA also.

To see the multiple posts in this blog about MRSA ST 398 and other strains in the food chain, food animals, and pets, go to the labels under the time stamp on this post, and click “animals” or “food.”

The cite for the paper is: Bhat M, Dumortier C, Taylor B, Miller M, Vasquez G, Yunen J, et al. Staphylococcus aureus ST398, New York City and Dominican Republic. Emerg Infect Dis. 2009 Feb; [Epub ahead of print]