04 March 2010

"Pig MRSA" causing human infections

Hi, everyone. Apologies for dropping out of sight! As SUPERBUG's publication draws closer (and it's very close now), I keep finding new tasks that I have do to. Last week's was to go to New York to shoot a video for the Simon & Schuster website — and while there, I got caught in Snowpocalypse, got delayed coming home, and picked up a nasty cold. So I'm a bit behind.

But there's exciting news tonight to start us up again: "pig MRSA," ST398, causing human infections in Canada and Denmark.

"Infections" is important, because up til now, most evidence for  the spread of MRSA ST398 in humans has been through detection of colonization, the symptomless carriage of MRSA on the skin and in the nostrils. The first finding of ST398 in the Netherlands was via colonization; so was its first identification in humans in Canada, and also in the United States just about a year ago.

But comes now a team of public and university scientists from Canada to say that ST398 is also causing infections in Canada. They analyzed 3,687 MRSA isolates that had been collected from patients seen for infections in Manitoba and Saskatchewan. Five were ST398. That is an exceedingly low percentage, of course. But it is striking, and odd, that the infections were present at all:
The earliest identified LA-MRSA isolate (08 BA 2176) associated with an infection was obtained from a postoperative surgical site. ... This patient is unlikely to have had any recent direct contact with livestock because she had been confined to her home with limited mobility for several years before her hospitalization. Additional nasal swabs from this patient remained positive for this strain for at least 7 months. ...
The isolate submitted to the NML by Sunnybrook Health Sciences Centre... was from a 59-year-old man from Ontario. He had been hospitalized in December 2007 for treatment of metastatic squamous cell carcinoma of the larynx. In the previous year, he had undergone a total laryngectomy, neck node dissection, and tracheostomy. .... He was unaware of any animal contact and had no history of exposure to pigs or pig farms. A review of the medical records and standard epidemiologic investigations determined that this was not a nosocomial or healthcare-associated isolate.
Just to underline, we have here a MRSA strain that is strongly associated with close contact with pigs, or with pig meat, and that has spread far enough from farms to be present in people who had no connection with pigs. You can argue that its very low prevalence means that it is not so much a threat as a curiosity. But I'd counter-argue that this is significant: because it establishes that this strain is spreading; because it demonstrates that the strain is causing infections, not just colonization; and because it inserts, into the swarm of isolates that make up MRSA, additional resistance factors that can be traded and exchanged unpredictably among the bacteria — and are likely not to be detected because our surveillance in animals is so sparse.

The authors say:
...additional surveillance efforts are required to monitor the emergence and clinical relevance of this MRSA strain in Canada, including communities, the environment, livestock, farmers, and production facility workers. Whether these strains pose a major threat to human health in light of the low livestock density and continued spread of epidemic hospital and community strains of MRSA in Canada remains unknown.
There's also a new and tantalizing report from Denmark that appears to describe not only human infections, but human to human transmission, resulting in a very serious pneumonia in a baby. I can't access the full-text even through my university account, but the abstract says:
Carriage of pig-associated methicillin-resistant Staphylococcus aureus (MRSA) is known to occur in pig farmers. Zoonotic lineages of MRSA have been considered of low virulence and with limited capacity for inter-human spread. We present a case of family transmission of pig-associated MRSA ST398, which resulted in a severe infection in a newborn.
Not good.

The cites for these are:
Golding GR, Bryden L, Levett PN, McDonald RR, Wong A, Wylie J, et al. Livestock-associated methicillin-resistant Staphylococcus aureus sequence type 398 in humans, Canada. Emerg Infect Dis; [Epub ahead of print] DOI: 10.3201/eid1604.091435
Hartmeyer GN, Gahrn-Hansen B, Skov RL, Kolmos HJ. Pig-associated methicillin-resistant Staphylococcus aureus: Family transmission and severe pneumonia in a newborn. Scand J Inf Dis. Epub Feb. 3, 2010 ahead of print.

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