Nasal swab for aerobic culture is considered the best sample.

People: Use topical ointment (Bactroban¨, mupiricin), nasally, q 12 h for 5 d.

Pets: Infections with MRSA in dogs have been reported but their potential role as a reservoir for human infection is controversial. All MRSA reported to date in pets bare the genomic signature of human isolates, indicating a human-to-pet migration. Staph. aureus is not normally a long-term colonizing organism in pets. If found in a nasal or skin swab of an asymptomatic pet, the organism is more likely a transient pathogen from a human host rather than a true colonizing organism. Identification and or treatment of all humans in the household is of primary concern. The role of pets as chronic carriers of human pathogens is not well understood, although prudence is wise. Pets should not be allowed direct contact with any individual who is immunocompromized until the pets have been cultured negative for MRSA. No procedure has been demonstrated to be effective in decontaminating pets. Eradication of the MRSA carrier state from people in the household usually will be effective in removing the organism from the pet.

Of more concern in veterinary medicine is the potential for host adapted Staphylococcal species to acquire the resistance patterns seen in MRSA. Staph. intermedius is the coagulase-positive commensal Staph. spp. of dogs and cats.

The vast majority of Staph. intermedius isolates in dogs and cats do not show methicillin resistance.

Literature reports on Staph. intermedius isolates from dogs and cats and methicillin resistance have given conflicting results: Data from University of Tennessee with 57 Staph. intermedius isolates, only 2 were methicillin resistant, although 50% had the mecA gene detected by PCR testing, suggesting that it may not have been expressed. From the University of Illinois, of 25 methicillin-resistant Staph. intermedius isolates, 23 had mecA gene, and the non-methicillin-resistant isolates did not have the mecA gene. Further work is needed to determine the value of mecA gene detection on veterinary isolates.

Coagulase negative Staphylococcus spp. isolates (such as Staph. schleiferi) in dogs and cats appear to be more commonly detected with methicillin-resistance than do the coagulase positive isolates. These are usually reported from canine pyodermas.

Lack of clinical response to use of any b-lactam antibiotic (e.g. penicillins or cephalosporins)—even if in vitro testing indicates that they are sensitive—indicates that these antibiotics should not be used to treat an MRS infection.

Other choices:

• TMS
  Better than average success against MRS, but not a drug commonly recommended in veterinary medicine, because of potential for adverse reactions.
  
• Quinolones
  
• Vancomycin
  Several disadvantages: very nephrotoxic in dogs; needs to be administered q 6 h by IV route.
  
• Zyvox (Linezolid)
  Member of new family of antibiotics (oxazalinozodes); can be administered enterally or parenterally. Pharmacokinetic evaluation in dogs indicates the effective dose is 20-30 mg/kg q 12 h. Low toxicity, but very expensive. Use only if no other choice; for example, in multi-drug resistant MRS. Zyvox is also effective against vancomycin-resistant enterococcus.

Contributed by Dr. David Aucoin, Zoasis Corp., Diplomate ACVCP