Viewpoint: On report published in Lancet Infectious Diseases on MRSA strains in cattle and humans

Dr Nicola Williams, from the University’s Institute of Infection and Global Health, said: “The recent report of the same strains of meticillin-resistant Staphylococcus aureus (MRSA) isolated from both cattle and humans in the UK and Denmark has added to the debate on the role of animals in the epidemiology of MRSA.

“The discovery of a new version of the gene (mecA) which encodes resistance to the important beta-lactam antibiotics is very interesting. The current molecular tests to detect mecA and determine if an isolate is MRSA would not detect this new variant of the gene. New PCR assays, as described in the report, would be required to detect this gene and confirm the presence of such MRSA strains in clinical cases, both in animals and humans.

“The paper doesn’t give direct evidence for the transmission of these strains from cattle to people, and no indication is given if any of the people carrying or infected with these strains had contact with livestock. Further work is needed to address how common the new gene variant is in cattle and if it can be transmitted through contact with livestock or via the foodchain.

“Previous work on the pig and livestock associated MRSA strains, however, have shown that the greatest risk for transmission between animals and humans is via direct contact, with little evidence for transfer via the foodchain. In addition, the isolates only appear to be resistant to the beta-lactam antibiotics, unlike many other strains of MRSA, therefore such isolates could be easily passed over in diagnostic laboratories as not being MRSA.

“The fact that it isn’t resistant to other classes of antibiotics is of interest, especially as overuse of antibiotics in farming is being cited as one of the reasons for its emergence. Beta-lactam antibiotics however are heavily relied upon in cattle medicine, especially for the treatment of mastitis and have been cited as also being responsible for the emergence of another resistance determinant, extended-spectrum beta-lactamases, found in E. coli and other Gram-negative bacteria.

“At the University of Liverpool we have started a study investigating antibacterial use in cattle and are looking at prescribing practice, of which there is currently very little data. This study should help inform on how antibiotics are being used in cattle and lead to improved guidelines for their rational use.”

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