Livestock-Associated Staphylococcus aureus: The United States Experience

Staphylococcus aureus is a gram-positive bacterium that colonizes a variety of animal species [1]. S. aureus infections in animals are most commonly reported as a cause of mastitis in dairyproducing animals (including cattle and goats) and “bumblefoot” in chickens [2], as well as being identified as a pathogen of farmed rabbits [3]. Most reports characterizing animalassociated S. aureus have demonstrated that strains affecting animals are distinct from those infecting humans, suggesting that there are host-specific lineages which only rarely cross species boundaries [4]. Livestock-associated strains may evolve on farms because of the use of antibiotics in animal husbandry. These may be used as feed additives for growth promotion in industrial livestock and poultry [5], for prevention of disease within a herd, or for treatment of an existing disease outbreak. Agricultural-use antibiotics include many classes that are relevant for human health, including tetracyclines, macrolides, penicillins, and sulfonamides, among others. Antimicrobial resistance generated during animal husbandry may then be spread to the general human population in a number of different manners: contact with contaminated meat products (via handling or ingestion); occupational contact (farmers, meat packers, butchers, etc.) and potential secondary spread into the larger community from those who are occupationally exposed; entry into and transmission via hospitals or other health care facilities; or spread via environmental routes including air, water, or manure in areas in proximity to live animal farms or crop farms where manure has been used as a fertilizer (Fig. 1). While methicillin resistance has been the most commonly investigated phenomenon and will be the main topic of this review, resistance to any of these antibiotics can occur and can potentially be a threat to successful treatment of S. aureus infections and therefore to human health outcomes. As such, my research group and others have begun to look more broadly at any S. aureus present on farms, including those that may be susceptible to methicillin but resistant to other antibiotics.