New MRSA variant detected in cow's milk that can evade some existing detection methods

June 02, 2011

An Article published Online First by The Lancet Infectious Diseases reports detection of a new variant of meticilin-resistant Staphylococcus aureus in cow's milk--genetically different to existing MRSA strains--from the UK and Denmark. This new variant is associated with clinical disease in people, yet some existing testing methods would wrongly identify this new variant as meticillin-susceptible, leading to prescriptions of the wrong antibiotics. Furthermore, the study reveals indirect evidence that cows could be a reservoir of MRSA that could infect humans. The study is by Dr Mark A Holmes, Department of Veterinary Medicine, University of Cambridge, UK, and colleagues.

The resistance found in variants of MRSA is usually conferred by the mecA gene, which can be transferred between S. aureus strains. In today's era of genetic testing, a positive test for the mecA gene has become the gold standard for identifying an MRSA. But in this study, Holmes and colleagues found an MRSA with a new mecA gene (a mecA homologue) that was not detected by the usual confirmatory test for mecA. The authors say that, when existing detection methods such as the polymerase chain reaction (PCR) or monoclonal antibody methods are used as the only method to detect MRSA, or when these methods are used to confirm provisional detection of MRSA, then the new variant will be wrongly diagnosed as meticillin-susceptible.

The study also shows indirect, although not conclusive, evidence that cows could be an important reservoir of this new-variant of MRSA infection in humans. Firstly, the isolates found in humans were either of a strain thought to be unique in animals or other strain types detected in cattle but not previously in humans. Secondly, none of the strain types come from lineages previously associated with human MRSA carriage or infection. Thirdly, in England, where data from both cattle and human beings are available, there is evidence of geographical association between human and bovine isolates (human and bovine ST425 in the southwest, human and bovine ST130 in the east, and human and bovine ST1526 in the northeast). Finally, the UK Veterinary Laboratories Agency survey looking for MRSA in dairy cattle noted no other (ie, human) MRSA.

The authors say "Such evidence suggests that a bovine reservoir exists, from which mecALGA251 MRSA is transmitted to people. Pasteurisation of milk will prevent any risk of infection via the food chain but individuals in close contact with cattle could be at higher risk of carriage. Further research is needed to test this hypothesis."

They add: "The discovery of this previously undetected mecA homologue is potentially of public health importance. Diagnostic protocols, whether for clinical or epidemiological purposes, should consider the ramifications of not detecting S aureus strains that carry this new mecA homologue."
-end-
Dr Mark A Holmes, Department of Veterinary Medicine, University of Cambridge, UK. Please contact Genevieve Maul, Media Relations. T) +44 (0) 1223 765542 / +44 (0) 7774 017464. E) Genevieve.Maul@admin.cam.ac.uk

Lancet

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