Staphylococcus can be a very common cause of infection of the skin and ears of all animals and humans.
Staphylococcus is divided into coagulase positive and coagulase negative species. Coagulase positive species—S. pseudintermedius, S. aureus, and S. schleiferi subspecies coagulans—are more commonly seen; however, coagulase negative species are identified in infections with growing frequency.
S. pseudintermedius is the most common commensal organism in dogs and cats and, subsequently, the most common isolate from canine superficial pyoderma. Staphylococcus sp. infections may also cause pneumonia, urinary tract infections, soft tissue infections, surgical site infections, ocular infections, and endocarditis.
Staphylococcal organisms are typically treated with cephalexin, cefpodoxime, clindamycin, and amoxicillin-clavulanic acid. It is important to note that studies have shown up to 84 percent of Staphylococcus sp. contain the blaZ gene, which codes for beta-lactamase. This means that they are going to be inherently resistant to penicillin.
A large and increasing concern in human and veterinary medicine is methicillin resistance. A mutation occurs on the mecA gene, which encodes for an altered penicillin-binding protein, PBP2a. This altered penicillin-binding protein results in a loss of target affinity for all beta-lactam antibiotics (penicillins, cephalosporins, and carbapenems) used in veterinary medicine.
Methicillin resistance is identified by culture and interpreting whether the organism is susceptible to oxacillin, if it is S. aureus and S. pseudintermedius, or cefoxitin for S. aureus and coagulase negative Staphylococcus sp. Even though some beta-lactam antibiotics may be interpreted as susceptible on culture, they are all presumed to be resistant, regardless of the in vitro results.
While methicillin-resistant S. pseudintermedius (MRSP) infections have been difficult to resolve without the correct antibiotic, they do not require any additional treatment time compared to methicillin-sensitive Staphylococcus sp. Monitoring with cytology is needed to evaluate resolution; monitoring with culture is not accurate. Animals that have been treated successfully for MRSP have been known to be carriers and culture positive for up to 1 year after resolution.
In rare cases, methicillin-resistant Staphylococcus sp. has been transmitted to humans and other animals, although most of these situations involve humans or animals that are immunosuppressed. MRSP has been cultured in households up to 6 months after clinical resolution of the infected animal. Clinically, we have seen numerous cases in which an animal with MRSP shared a residence with other animals that did not become infected.
Veterinarians are at a higher risk to become transient carriers of MRSP, but rarely become infected. If proper infectious disease control policies are not in place, veterinary equipment can transmit the pathogen to animal patients.
Methicillin-resistant S. aureus (MRSA) is rarely a problem in small animal patients, but does occur in farm animals and horses. If small animals are infected, they likely became infected from an infected human.
Guidelines for antimicrobial usage in superficial pyodermas were published in 2014 in Veterinary Dermatology. First-tier choices for superficial pyoderma include clindamycin, cephalexin, and amoxicillin-clavulanic acid. Cefpodoxime and cefovecin are first- or second-tier choices. Additional choices beyond these antibiotics should be based on culture and sensitivity. Most of our cultures in the dermatology service for MRSP have responded only to chloramphenicol, rifampin, and amikacin, making treatment extremely difficult. In all of these cases, topical therapy can be extremely beneficial.
—Jason Pieper, DVM, DACVD