More MRSA, in milk: A new strain in cows and humans

Despite the massive AIDS anniversary this week, I was expecting to write about the EU E. coli outbreak next. But there’s striking new news on MRSA that makes it worth putting off E. coli one more day. I’m traveling many time zones away from home, though, so this will be quick.

Researchers in England and Denmark have announced they have found a never-before recorded variant of MRSA in cow’s milk in England that has already caused human infections in England, Scotland and Denmark, and researchers in Ireland have simultaneously announced that they have found the same strain in hospitalized patients there as well.

Here’s how this unfolded:

Researchers at the University of Cambridge’s Veterinary School were looking at S. aureus for its known potential to cause bovine mastitis. In samples of bulk milk, they found two staph isolates that returned perplexing results: They reproduced in the lab in the presence of antibiotics, suggesting they were resistant. But when the researchers performed the standard test for MRSA, the samples did not appear to possess the mecA gene that confers methicillin (the “M” in MRSA) resistance.

Except, as it turns out, they did. A second round of genomic analysis revealed that the isolates did harbour a mecA gene in the staphylococcal chromosomal cassette (SCCmec) where it usually resides– but the gene was 37 percent different from previously identified versions, and so returned a “false negative” on the standard tests that look for it.

The researchers then looked for the same signature in preserved human isolates — some from known MRSA patients and others from surveillance programs that were looking for MRSA — and found it. It was present in 12 patients from Scotland, 15 from England, and 24 from Denmark. On a second pass, they also found it in 15 samples from live cattle from different parts of England.

Tellingly, when they subtyped the MRSA strains, they found that there were several different groupings in different parts of England — but that the geographic variants, in each area, appeared in both humans and cows. And they found that the incidence of the new strain had been increasing: There was one matching sample in 2002, nine in 2009 and 12 in 2010. (The milk samples dated back to 2006 and 2007.)

Meanwhile, researchers in Ireland were investigating the puzzling phenomenon of a MRSA strain in two hospital patients — a 64-year old woman treated in Dublin in February 2010 and an 85-year old man who was cared for down in the southeast in May 2010 — that also did not appear to be MRSA based on the standard PCR-based test. Their analysis found the same results as the Cambridge team independently: a novel form of the mecA gene that was not detected using the usual identification methods.

The two teams became aware of each other’s work when their articles were being finalized by the two journals that published them Thursday, Lancet Infectious Diseases and Antimicrobial Agents and Chemotherapy.

So what does this mean? The British authors suggest that cattle are the reservoir for this strain. A story in the Guardian says the British dairy industry is cooperating with the Cambridge researchers to do wider surveillance, but they appear keen to stress that there is no risk from drinking milk because pasteurization would kill the organism. (Watch out, British raw-milk drinkers.)

But that Dairy UK spokesman also said: “”In the extremely rare cases where infection has occurred, this has been through physical contact, either cattle to cattle or cattle to people, and through cuts or open wounds,” and that raises a critical issue: This new form of MRSA is a potential occupational hazard for farm workers. In the Netherlands, farm workers and farm veterinarians were held to be one of the first avenues by which livestock-associated MRSA, ST 398, moved out into the wider population. That could be true for this new strain as well, which has been dubbed CC (for “clonal complex”) 130. Notably, there has been CC 130 MSSA, drug-sensitive staph, found in the past, but this is the first identification of drug-resistant CC 130.

The most important issue, of course, is what this means for antibiotic use in dairy farming. MRSA in cows is rare, but staph is not; staph mastitis is a common and costly disease, for which antibiotics are used preventatively. The drug class most commonly used for prevention is the beta-lactams, to which methicillin also belongs. In the UK, the nonprofit the Soil Association has put out a detailed briefing sheet; I’ll update when I get a link (Update: press release, fact sheet) , but will end meanwhile with a comment from its director, Helen Browning:

In the relentless drive for increased per animal productivity, and under acute price pressure, dairy systems are becoming ever more antibiotic dependent. We need to get farmers off this treadmill, even if that means that milk has to cost a few pennies more. That would be a very small price to pay for maintaining the efficacy of these life-saving drugs.


  • Alvarez-Garcia L. et al. Meticillin-resistant Staphylococcus aureus with a novel mecA homologue emerging in human and bovine populations in the UK and Denmark: a descriptive study. The Lancet Infectious Diseases, Online First publication, published Online June 3, 2011 DOI:10.1016/S1473-3099(11)70126-8
  • Shore AC et al. Detection of Staphylococcal Cassette Chromosome mec Type XI Encoding Highly Divergent mecA, mecI, mecR1, blaZ and ccr Genes in Human Clinical Clonal Complex 130 Methicillin-Resistant Staphylococcus aureus. Antimicrobial Agents and Chemotherapy, published ahead of print on 2 June 2011, doi:10.1128/AAC.00187-11.



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