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| MeThicillin-ResisTanT sTaPhylococcal infecTions RISK FACTORS Recent antimicrobial exposure appears to be the most critical risk factor for acquisition of methicillin-resistant staphylococci.13 • In a recent prospective study, Beck and colleagues demonstrated that, following antimicrobial therapy, isolation of MRSP from the skin and mucosal sites of dogs with pyoderma caused by methicillin-susceptible S pseudintermedius (MSSP) is common.31 • Similarly, in a large retrospective study, recent administration of beta-lactam antimicrobials was found to be a risk factor for dogs with MRSS clinical isolates.24 • Recent administration of beta-lactams or fluoroquinolones is a risk factor for MRSA infection in dogs and cats.37,53 Additional risk factors for MRSA infection include: 37,53 • Contact with ill or hospitalized human • Intravenous catheterization • Multiple antimicrobial courses • Prolonged hospitalization • Surgical implants. It is likely that alteration of commensal staphylococcal flora by systemic antimicrobial treatment, particularly beta-lactam antibiotics, allows for subsequent colonization by methicillin-resistant strains. With the increasing prevalence of antimicrobial resistant strains, practitioners are encouraged to consider the potential for infection associated with methicillin-resistant staphylococci even in the absence of risk factors, such as recent antimicrobial administration or hospitalization. CULTURE & SUSCEPTIBILITY Given the increasing prevalence of methicillin- and multidrug-resistant staphylococci, culture and susceptibility testing (Figure 1) is likely an underutilized tool, particularly in the management of pyoderma, urinary tract infections, wounds, and surgical site infections. Clinical indications for performing culture and susceptibility testing include: • Clinical lesions consistent with deep pyoderma, Figure 1. Collection of a sample for culture and susceptibility testing using a culturette swab; taken from a dog with superficial pyoderma 28 Today's Veterinary Practice May/June 2013 such as furuncles, nodules, and draining tracts • Any surgical site infection, particularly those associated with orthopedic procedures13,54 • Presence of a nonhealing wound • Cytologic evidence of mixed or nonstaphylococcal infection (eg, intracellular bacterial rods) • History of prior antimicrobial-resistant staphylococcal infection • History of recent antimicrobial therapy • Lack of response to appropriate empiric antimicrobial therapy • Recurrent or relapsing infection. EMPIRIC THERAPY Despite the importance of culture and susceptibility testing in management of staphylococcal infections, empiric antimicrobial therapy remains common practice for first-time infections or treatment-naïve patients, especially patients with pyoderma. • Beta-lactam derivatives, especially cephalosporins, are frequently considered first-line choices in treatment of pyoderma due to their bactericidal activity, tissue penetration, and low incidence of adverse effects.55 • Preferential selection of other antimicrobials, such as macrolides, lincosamides, or potentiated sulfonamides, as first-line therapies for staphylococcal infections has been suggested due to concerns regarding potential colonization with methicillinresistant strains due to beta-lactam antimicrobial INDUCIBLE RESISTANCE & CLINDAMYCIN • Inducible resistance, in which the presence of an inducing agent, such as erythromycin, promotes expression of a resistant Figure 2. Double phenotype, has been disk diffusion test (D reported in MRsa isolates test) for detection of from humans and animals inducible clindamycin and some MRsP isolates.56,57 • Clindamycin use in infections resistance; note the Dshaped zone around caused by isolates exhibitthe clindamycin disk ing inducible resistance may (CC) due to its close 56,57 result in treatment failure. proximity to the eryth• Microbiology laboratories can romycin disc (E) test for inducible clindamycourtesy dr. david a. Bemis, cin resistance using a douUniversity of Tennessee veterinary Bacteriology ble disk diffusion test (D -test) laboratory with adjacent erythromycin and clindamycin disks (Figure 2). • in the absence of this test, clinicians may predict inducible resistance based on susceptibility results, indicating erythromycin resistance and clindamycin susceptibility.56