Are all meticillin-resistant Staphylococcus aureus (MRSA) equal in all hosts? Epidemiological and genetic comparison between animal and human MRSA

Meticillin-resistant Staphylococcus aureus (MRSA) continues to pose a major threat to human health. In animals, MRSA has become established as a veterinary pathogen in pets and horses; in livestock, it presents a concern for public health as a reservoir that can infect humans and as a source of transferrable resistance genes. Genetic analyses have revealed that the epidemiology of MRSA is different in different animal hosts. While human hospital-associated MRSA lineages are most commonly involved in pet infection and carriage, horse-specific MRSA most often represent 'traditional' equine S. aureus lineages. A recent development in the epidemiology of animal MRSA is the emergence of pig-adapted strains, such as CC398 and CC9, which appear to have arisen independently in the pig population. Recent insight into the genome structure and the evolution of S. aureus has helped to explain key aspects of these three distinct epidemiological scenarios. This nonsystematic literature review summarizes the structure and variations of the S. aureus genome and gives an overview of the current distribution of MRSA lineages in various animal species. It also discusses present knowledge about the emergence and evolution of MRSA in animals, adaptation to different host species and response to selective pressure from animal-specific environments. An improved understanding of the genetics and selective pressure that underpin the adaptive behaviour of S. aureus may be used in the future to predict new developments in staphylococcal diseases and to investigate novel control strategies required at a time of increasing resistance to antimicrobial agents.