MRSA clone in food animals worrisome, expert says

Jan 4, 2010 (CIDRAP News) – The emergence and wide spread of a new clone of methicillin-resistant Staphylococcus aureus (MRSA) in food animals is a worrisome development that should be watched closely, one of the strain's lead researchers has warned in a medical journal.

Writing in Clinical Microbiology and Infection, Dr. Jan Kluytmans of Amphia Hospital in Breda, the Netherlands, recounts the identification of MRSA multilocus sequence type 398 (or ST398).

The new strain, which was first noted in French livestock in 2005, is widely distributed. It was successively found in the Netherlands in pigs, in agricultural workers and their families, and in unrelated healthcare workers and hospital patients—and then in a variety of livestock species, and in retail meat, in Europe, Canada, and the United States.

On Dutch farms, from 23% to 81% of pigs have been found colonized with the strain, carrying it without being made sick by it. When farmers on those farms were surveyed, they were colonized with an identical strain.

ST398 appears to be less virulent and less transmissible than the community-associated strains of MRSA common in the United States, which have very low prevalence in the Netherlands. "The impact of ST398 on [human] public health may be limited," Kluytmans writes, "but close monitoring of its evolution over time will be required."

The strain's presence in meat after slaughter—it has been found in beef, lamb, and chicken and other birds in addition to pork—raises uncertainties over the degree of its movement into the food chain. While staph species are known to cause staphylococcal food poisoning, MRSA ST398 to date lacks the toxin-producing genes that would produce similar illness.

The chief concern, Kluytmans writes, is the possibility that people preparing MRSA-contaminated meat for cooking will become colonized with the organism on their skin or mucous membranes, moving it into a broad new ecological niche and positioning it as a possible cause of further human infections.

Because the novel strain has spread so widely and has already been identified as a cause of hospital outbreaks, it should not be allowed to spread further without surveillance, Kluytmans argues.

"It is unlikely that this reservoir will be eradicated easily," he writes. "Considering the potential implications of the reservoir in food production animals and the widespread presence in meat, the epidemiology of [MRSA] ST398 in humans needs to be monitored carefully."

Kluytmans JAJW. Methicillin-resistant Staphylococcus aureus in food products: cause for concern or case for complacency? Clin Microbiol Infect 2010 Jan;16(1):11-5 [Abstract]

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