Journal Scan: Methicillin resistance and pyoderma: What's the big deal?

WHY THEY DID IT

Methicillin-resistant organisms are being recovered more and more frequently from canine skin cultures, and concerns about transmission are increasing. This report attempts to describe the current understanding of these organisms, particularly infections with methicillin-resistant Staphylococcus pseudintermedius (MRSP), and zoonotic risks.

OVERVIEW

Staphylococci are indigenous flora of the skin and mucous membranes in all healthy dogs but may cause opportunistic infections, particularly of the skin. Staphylococcus pseudintermedius is the predominant cause of pyoderma, otitis, and opportunistic surgical site infections in dogs, while Staphylococcus aureus is isolated less often in dogs. Staphylococci that carry the methicillin-resistant gene are resistant to all beta-lactam drugs.

Diagnosis. Pyoderma caused by MRSP is clinically indistinguishable from that caused by methicillin-susceptible strains of staphylococci and is not inherently a more virulent infection. Successful therapy, however, will require appropriate antimicrobial selection based on culture and susceptibility testing. An aspirate from an intact pustule is the best sample to submit for culture, although in some cases of deep pyoderma, a tissue biopsy may be required.

Treatment and zoonotic potential. Avoid beta-lactams, cephalosporins, and amoxicillin-clavulanic acid as they are ineffective against MRSP. The study authors also note and reference that fluoroquinolones can be a poor choice for long-term treatment since susceptible staphylococci strains can quickly develop resistance to this drug class.

The authors note the importance of differentiating between methicillin-resistant S. aureus (MRSA) and MRSP. These are not the same infection. Colonization of dogs with S. aureus is possible but not common since it is not an indigenous component of normal canine flora. If a dog becomes colonized, the pet may serve as a reservoir for infection and yet not demonstrate clinical signs. Veterinary personnel who come in contact with colonized pets may then become MRSA carriers, which is why hygiene in the practice setting in so important. Staphylococcuspseudintermedius on the other hand is not a commensal organism in people and poses little zoonotic risk.

Preventing spread of infection. The authors recommend infection control measures for all patients with methicillin-resistant infections. These include using gloves and washing hands thoroughly between patients. Routine disinfection of hospital surfaces and equipment is also critical. In addition, all wounds should be kept covered to decrease the risk of environmental contamination.

Hand hygiene and environmental disinfection at home are just as important as in the clinic. Pets with known MRSP infections should not be allowed to share a bed with their owners, and owners with MRSA infections should not allow their pets to lick their wounds or share their beds.

TAKE-HOME MESSAGE

When treating dogs with pyoderma, the authors recommend the following:

• Treat any underlying or predisposing conditions.

• Reevaluate all empirically treated patients during therapy to ensure appropriate response to treatment.

• If empirical therapy fails or reinfection occurs, perform culture and susceptibility testing.

• Avoid use of drugs such as vancomycin and linezolid because of their importance in treating human MRSA infections.

• Hygiene and disinfection protocols in the clinic and at home should be stringently followed.

Gold RM, Patterson AP, Lawhon SD. Understanding methicillin resistance in staphylococci isolated from dogs with pyoderma. J Am Vet Med Assoc 2013;243:817-824.