This paper almost slipped by me. It was published quietly a few weeks ago, and it’s a little eyebrow-raising. From EuroSurveillance, the open-access peer-reviewed bulletin of the European Centre for Disease Prevention and Control (Europe’s CDC): The ST398 strain of MRSA, better known as “livestock-associated MRSA” or just “pig MRSA,” has been found for the first time in milk in England. (And therefore probably in cows, or at least on farms.)
Apparently there has been an ongoing study looking for any evidence of MRSA in UK cows, possibly because of this news from last year (of which more in a minute). Between last January and July, the program tested 1,500 samples of milk from farms’ bulk tanks — that’s the cooler in which milk from a number of cows is collected until it can be picked up by a truck for processing — and found seven of the samples were contaminated by MRSA. All seven isolates were MRSA ST398, the livestock-associated strain. Three came from one farm, so five farms had MRSA in their tanks. According to the paper, this is the first discovery of ST398 in the UK other than one finding in horses in 2009.
Some background: ST398 represents what I think of as the “third epidemic” of MRSA, dating from 2004 and following on hospital-associated (dating from the 1960s) and community-acquired (dating from the 1990s). I told the long story of its emergence and international spread in the book Superbug, but briefly: This strain was discovered in 2004 thanks to the care with which the Netherlands checks for MRSA in people about to be hospitalized; it was found in a toddler whose family were pig farmers, and subsequently in their friends, also farmers, and then in their pigs and their friends’ pigs also. In the eight years since, it has spread into health care workers and hospitalized patients, into people with no connection to farming, and then into retail meat, in Europe, Canada and in the United States.
What makes ST398 distinctive is that it has a signature antibiotic resistance, to tetracycline, which is not present in hospital or community MRSA, and which is easily traced to antibiotics used in livestock raising and especially in swine agriculture.
(For much more about ST398, you could look at my archives here and in this blog’s former location; and you could also peruse the blog of Tara Smith, the University of Iowa professor who has been the sole US researcher to take this seriously.)
So, now, this news: The interesting thing is that this is not the first identification of MRSA in milk in the UK. That finding (which I referred to up above) was made 18 months ago, by the same team responsible for this new discovery. Having made that identification — of what was, at the time, a never-seen MRSA strain — this team from Cambridge and Denmark went on looking in milk for other MRSA strains, and found ST398.
As far as anyone can say, this is the first identification of the farming-associated strain in livestock in the UK (since the earlier identification of ST398 was in horses). But it is important to note that the UK agriculture authorities have been notably resistant to looking for ST398 over the years, despite sustained pressure from the rest of the EU and within the UK from the organic group the Soil Association, and even a raising of the issue in Parliament. What’s curious — and the EuroSurveillance paper isn’t clear — is whether it is absolutely certain that this is being carried by cows. (The samples tested were from the milk tanks, not from cows themselves, so I wonder whether it is possible that there could have been contact contamination of the cows by other livestock, such as pigs; or of the milk by farm workers who had contact with other livestock.)
The paper says, and this is narrowly correct, that there should be no concern over MRSA transmission via milk, because pasteurization will sterilize it. That may be true, but it does not account for the increasing appetite for milk sold raw, nor for raw-milk cheese.
But a larger issue is that the presence of ST398 on UK farms could pose the potential for spread from cows into other animal species, as well as to farm workers. That makes ST398 an occupational health risk for farm workers, who could become infected with this strain — but it also threatens to make farm workers the vector for carrying the strain off farms and into the wider world. ST398 spread from farms in the Netherlands because so many people in the densely farmed, densely populated southeastern part of the country had some tie to farming — a part-time job, a partner’s job, a family member, a co-worker — that there was no hope of keeping it confined.
Despite its longstanding reluctance, it will be really important for the UK to start looking for MRSA on its farms now. Identifying it, and understanding how widely it has spread, might allow it to avoid the wide spread of ST398 that the Netherlands was not able to catch in time.
Cite: Paterson GK, Larsen J, Harrison EM et al. First detection of livestock-associated meticillin-resistant Staphylococcus aureus CC398 in bulk tank milk in the United Kingdom, January to July 2012. Euro Surveill. 2012;17(50):pii=20337. Available online: http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=20337
