Chapter 100 Staphylococcus species are both normal resident cutaneous microflora and opportunistic pathogens frequently associated with pyoderma in dogs and cats. Staphylococcus species usually are divided into coagulase-positive and coagulase-negative species, with the former most often associated with skin infections. Recently, Staphylococcus intermedius has been reclassified, with Staphylococcus pseudintermedius being recognized as the common coagulase-positive canine staphylococcus (Sasaki et al, 2007). Staphylococcus aureus, Staphylococcus schleiferi, and coagulase-negative staphylococci occasionally are isolated from skin infections of animals. Often these isolates are methicillin- and multidrug resistant, which most likely is attributable to empiric treatment with multiple classes of antibiotics before culture (see Chapter 103). Methicillin resistance implies resistance to all β-lactam antibiotics including cephalosporins and amoxicillin/clavulanic acid. S. aureus, S. schleiferi, and coagulase-negative staphylococci, unlike S. pseudintermedius, also are human pathogens or opportunistic invaders. Identification of the causative organism is important to determine the likelihood of resistance as well as the source and possible human risk. Formerly known as S. intermedius, S. pseudintermedius is now accepted as the organism that colonizes canine and feline skin and is the organism within the S. intermedius group that is associated mainly with canine and feline skin and ear infections; S. intermedius, in the strict sense, is associated primarily with pigeons. The S. intermedius group consists of S. intermedius, S. pseudintermedius, and S. delphini. In recent years there has been an increase in methicillin and multidrug resistance in S. pseudintermedius, which makes treating skin infections associated with this organism a therapeutic challenge (see Chapter 103). S. pseudintermedius is an infrequent pathogen in people and most often is associated with dog bite wounds. S. aureus is a resident organism and the predominant pathogenic staphylococcus of humans. S. aureus also colonizes healthy cats and has been associated uncommonly with skin infections in dogs and cats. In addition, wound infections, surgical site infections, otitis, and urinary tract infections caused by S. aureus all have been reported. Animals are thought to acquire the infection from people and then may serve as a source for human infections. When methicillin-resistant S. aureus (MRSA) is isolated from dog or cat skin infections, it usually is genetically related to the human hospital-acquired MRSA isolates for that region based on genetic typing. Although deep concern often follows a diagnosis of MRSA in a pet, the organism was likely acquired from the owner or another resident of the household. Because the numbers of dogs or cats infected with S. aureus have remained low, it is unlikely that permanent colonization occurs, which makes pets unlikely true reservoirs for the bacteria. MRSA carriage in a dog is not maintained for long periods when the dog is moved to a clean environment, which suggests that attempts at decolonization are not necessary. S. aureus has been shown to preferentially colonize human keratinocytes, whereas S. (pseud)intermedius preferentially colonizes canine keratinocytes (Simou et al, 2005).
Staphylococci Causing Pyoderma
Staphylococcus pseudintermedius
Staphylococcus aureus
