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MeThicillin-ResisTanT sTaPhylococcal infecTions | METHICILLIN-RESISTANT STAPHYLOCOCCI Staphylococcus pseudintermedius S pseudintermedius colonizes the skin and mucosa of healthy dogs and is the most common cause of canine pyoderma.12 MRSP has been recognized as a cause of pyoderma, surgical site infections (particularly following orthopedic procedures), and urinary tract infections, among others.9,13 Classification Investigators have recently used molecular techniques to classify 3 closely related staphylococcal species—S intermedius, S pseudintermedius, and S delphini—as the S intermedius group. It appears correct to assume that all previously classified S intermedius isolates from dogs, cats, and humans were actually S pseudintermedius isolates.12-15 Prevalence The prevalence of MRSP infections in veterinary patients has increased substantially over the past decade. Although reported sporadically in the 1990s.16,17 MRSP infections are now commonly reported in the veterinary literature and increasingly isolated by veterinary microbiology laboratories. Colonization MRSP has also been isolated from carriage sites (skin and mucous membranes, including nares, oral mucosa, and rectal mucosa) of healthy dogs and cats.17,18 Several studies support a carriage rate of: • 1.5% to 3% in healthy dogs19-21 • 0% to 4% in healthy cats.18,22 Overall, the literature suggests that MRSP isolation rates from clinical specimens may significantly exceed the prevalence of MRSP colonization in healthy animals, although prevalence may vary by sampled population and geographic area. Staphylococcus schleiferi S schleiferi is an emerging cause of infections in veterinary patients. Classification Two variants have been described based on coagulase production: • S schleiferi subspecies schleiferi (coagulase negative) • S schleiferi subspecies coagulans (coagulase positive). Recent studies suggest the 2 subspecies are not distinct by genotype or clinical behavior; therefore, both should be considered important pathogens.23,24 Coagulase-positive and coagulase-negative S schleiferi are associated with pyoderma (especially recurrent pyoderma) and otitis in dogs with allergic dermatitis, although infections of other body sites in nonallergic dogs have also been reported.24 Prevalence Methicillin resistance is particularly prevalent in clinical isolates of S schleiferi, with reported rates often exceeding 50%.1,22-26 Fluoroquinolone resistance is also common in methicillin-resistant S schleiferi (MRSS) isolates.9,23,24,27-29 MRSS has been isolated from: • Dogs and cats with inflammatory skin disease18-20,30-32 • Carriage sites (skin or mucosal) of healthy dogs and cats. Staphylococcus aureus Human Prevalence S aureus colonizes approximately 30% of the human population worldwide and is a major cause of skin and soft tissue infections in humans.33 The number of human infections caused by methicillin-resistant S aureus (MRSA) has increased dramatically since the 1960s.34 While human MRSA infections were once primarily nosocomial, community-associated MRSA infections of healthy individuals are being diagnosed more frequently.34,35 Veterinary Prevalence MRSA infections have also been reported in a variety of companion and exotic animal species.9,36-41 • In dogs, prevalence of S aureus colonization appears to be substantially lower than that of S pseudintermedius.42 In addition, S aureus infections are far less common than S pseudintermedius infections.9 • In cats, reports conflict as to whether S pseudintermedius or S aureus is the primary colonizing coagulasepositive staphylococcal species.43-46 • Healthy dogs and cats may be colonized by MRSA, although this colonization is likely transient.47,48 As is the case with MRSP, MRSA isolates are often resistant to non–beta-lactam antibiotics, especially:9 • Fluoroquinolones • Lincosamides • Macrolides. Zoonotic Potential MRSA infections in companion and exotic animals are often associated with human hospital- or community-acquired clonal strains, suggesting, but not proving, human-toanimal transmission.38,39,49 Although the possibility of MRSA transmission from colonized or infected humans to animals, or vice versa, has frequently been suggested, the true direction of transmission is difficult to prove.30,49,50 While human-to-animal transmission is usually assumed, the infected or colonized human in the household may not be identified as the MRSA source for the pet.48 Coagulase-Negative Staphylococci The clinical importance of coagulase-negative staphylococci (CoNS), other than the coagulase-negative variant of S schleiferi, has not been well elucidated in veterinary patients. Historically, CoNS were considered commensal organisms or contaminants with limited pathogenic potential; however, they are becoming more frequently associated with nosocomial infections in humans. CoNS are frequently methicillin resistant and may produce a number of virulence factors.51,52 When CoNS are isolated from clinical specimens, practitioners are encouraged to interpret results in light of culture technique: CoNS isolated from culture samples of closed sites, such as intact pustules or joints, are more likely to be true pathogens than those obtained by swabbing the skin's surface. May/June 2013 Today's Veterinary Practice 27