Carlo Vitale,     San Francisco, California

Incidence

Historically, Staphylococcus intermedius was the most common bacteria isolated from cases of pyoderma. It has been shown, however, that most strains previously identified as S. intermedius actually are S. pseudintermedius, as shown through multilocus sequence typing (Bannoehr et al, 2007) (see Chapter 100).

The clinical presentation of superficial pyoderma caused by methicillin-resistant organisms generally does not differ from that of routine cases. However, it is the author’s experience that pyoderma caused by methicillin-resistant Staphylococcus aureus (MRSA) in dogs and cats differs markedly and characteristically is severe and deep. Articles have been published documenting methicillin resistance in cases of S. pseudintermedius infection (Cole et al, 2004; Gortel et al, 1999; Morris et al, 2006; Ruscher et al, 2010; Bryan et al, 2012). Interestingly, in another study only 2 of 57 S. intermedius isolates were resistant to methicillin; however, about half of the total expressed PBP2a, which indicates that the genetic capability was present to be resistant under more optimal conditions (Kania et al, 2004).

Since most of the public’s attention has been given to MRSA in animals, it is clear that there has been a definite increase in the number of reports of MRSA infections in pets (Baptiste et al, 2005; Gortel et al, 1999; Kania et al, 2004; Loeffler et al, 2005; Morris et al, 2006; Weese et al, 2010; Vitale et al, 2005). In addition to MRSA, Staphylococcus schleiferi recently has been associated with cases of canine pyoderma and has demonstrated methicillin resistance (May et al, 2005; Morris et al, 2006). MRSA and methicillin-resistant S. schleiferi infections appear to be uncommon in dogs. Many strains of methicillin-resistant Staphylococcus are not multidrug resistant.

Zoonosis

MRSA is considered to be a reverse zoonosis (human to animal transmission). There have been several documented cases of likely transmission of MRSA from humans to animals (Manian, 2003; O’Mahony et al, 2005; Tomlin et al, 1999; van Duijkeren et al, 2004; Weese et al, 2006). Transmission of MRSA to pets via contact with infected humans likely results in colonization (primarily nasal and rectal), infection, or both in dogs and cats. In addition, some of these reports have shown that pets can serve as a colonized or infected reservoir, which allows transmission back to the humans. The author has seen several cases of nasal colonization with MRSA in dogs that acted as a source of reinfection of humans who had intimate contact with the affected pet.