The opportunistic Staphylococcus aureus and S pseudintermedius bacteria have significant veterinary implications. Methicillin-resistant S aureus (MRSA) infection is an important public health concern in companion animals, horses, and food animals. MRSA-associated infections include wound infections, surgical site infections, pyodermas, otitis, and urinary tract infections. Administration of antimicrobials, particularly fluoroquinolones, may be risk factors for infection in dogs and cats. MRSA can also be detected in a small percentage of healthy dogs and cats, and colonization may be transient. In addition, most studies identify humans as MRSA sources.

MRSA has also been detected in horses, and recent reports have identified both larger numbers of MRSA infections involving horses in veterinary hospitals and primary community-onset infections; most horses are transiently colonized. Many studies evaluating MRSA in food animals have examined pigs; pig farming is a risk factor for nasal S aureus colonization, and the colonization rate among pig farmers was 760 times the rate of the general population in the Netherlands. There is minimal information describing MRSA in cattle; however, MRSA may be involved in enterotoxin-associated staphylococcal food poisoning and may be transmitted via handling of contaminated meat.

Animals may act as MRSA reservoirs with subsequent transmission to people. S pseudintermedius is currently the primary pathogen of dogs and may be isolated from the nares, mouth, anus, groin, and forehead of dogs and cats. Studies have demonstrated that the prevalence of methicillin-resistant S pseudintermedius (MSR [PI]) colonization in dog populations varies from 1.5% to 2%, with higher percentages identified in dogs with inflammatory skin disease. However, the prevalence in 200 healthy cats was reported as 4% but was 0% in cats with inflammatory skin disease.

S pseudintermedius colonization is rare in humans. Studies have suggested that S pseudintermedius has mistakenly been identified as S aureus and its real incidence in humans is unknown. S pseudintermedius is a common and potentially invasive pathogen in dog bite wounds and may be involved in food intoxication. At this time, extensive clinical and epidemiological studies involving humans and animals are required to better understand and quantify the role of both MRSA and MRS(PI) in public health and establish evidence-based prevention and control measures.

Commentary: The pet-owning public is increasingly aware of the importance of methicillin-resistant staphylococci, whether it is from a news show, an Internet source, or a popular television medical drama. There is a great deal of information (both accurate and inaccurate) available to clients, and veterinarians must provide core information to clients when a MRS is isolated from a pet. From my own experience, most pet owners are unaware of the difference between MRSA and MRS(PI). Although the article discusses MRS with a global perspective, we must educate clients about the exact pathogens isolated from their pets, the appropriate treatment options, and the true risk for zoonotic transmission.—Karen A. Moriello, DVM, Diplomate ACVD

Methicillin-resistant S aureus and S pseudointermedius in veterinary medicine. Weese JS, van Duijkeren E. VET MICROBIOL 140:418-429, 2010.