Public health impact and antimicrobial selection of meticillin-resistant staphylococci in animals

Whilst meticillin-resistant Staphylococcus aureus (MRSA) infections reported sporadically in companion animals and cattle in the 1970s were probably of human origin, the recently emerged livestock-associated MRSA (LA-MRSA) and meticillin-resistant Staphylococcus pseudintermedius (MRSP) strains clearly have animal origins and their isolation from humans is usually associated with exposure to livestock and companion animals, respectively. LA-MRSA is primarily an occupational health risk to farm workers and veterinarians. The risk that this zoonotic agent may spread in the community is particularly acute in countries with high livestock production and low MRSA prevalence in the human population. MRSP is primarily a threat to animal health, and cases of human infection are rare but may be overlooked in diagnostic laboratories. There is no conclusive evidence of antimicrobial selection associated with the sudden emergence of LA-MRSA and MRSP. However, the rapid global spread of these bacteria has presumably been favoured by antimicrobial selective pressure. Tetracyclines, zinc and extended-spectrum cephalosporins (including extra-label use) are the most likely selective drivers implicated in the spread of LA-MRSA, whilst increased use of broad-spectrum β-lactams and fluoroquinolones, partly enhanced by extra-label use and the introduction of cheap generics, may have played an important role in the rapid dissemination of MRSP. Control of LA-MRSA and MRSP requires a dual approach aimed at reducing antimicrobial consumption and preventing transmission between animals and from animals to humans or vice versa. Restricted use of fluoroquinolones and cephalosporins in livestock, and national practice guidelines for rational antimicrobial use both in food and companion animals are warranted.