Livestock-Associated MRSA and Its Current Evolution

Livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) is a relatively recent phenomenon in veterinary medicine. Although in the beginning it was restricted to a single clonal complex (CC), CC398, it has expanded into several clonal complexes, and the diversity of subtypes in the clonal complexes is increasing also. The prevalence of each type is determined somewhat geographically; for instance, themost prevalent clonal complex in Europe is CC398, whereas in Asia, it is CC9. Although few data exist regarding North America, the situation appears to be mixed there. The SCCmec cassettes detected in LA-MRSA are limited mainly to SCCmec IVa and SCCmec V, although non-typeable cassettes and SCCmec type XI, containing mecC, also have been found. The source of the SCCmec in LA-MRSAwas discovered to be animals. In searching from which bacteria the SCCmec cassettes in LA-MRSA have been transferred, themost obvious species to consider are the methicillin-resistant non-S. aureus staphylococci (MRNaS). However, very few data are available from those species in animals, and the data that do exist are not detailed enough to determine the origin. Nevertheless, similar cassettes were found in MRNaS, indicating a possible origin that needs to be investigated further.