Most canine skin infections are caused by coagulase-positive Staphylococcus intermedius, which commonly produce β-lactamase. Other staphylococcal species have been described, including S aureus, S schleiferi, S hyicus, and S pseudintermedius. There does not appear to be any difference in the disease patterns or clinical signs produced by the different species, although species-specific differences in antimicrobial resistance profiles have been seen in North America, with S intermedius and S aureus showing more resistance than S schleiferi coagulans. Species identification requires molecular techniques such as PCR detection of species-specific thermonuclease genes (nuc) or 16S rDNA sequencing, as phenotypic differentiation is unreliable.
Occasionally, Proteus spp, Pseudomonas spp, and Escherichia coli are secondary invaders of the dermis. Pasteurella multocida and β-hemolytic streptococci are the most common bacteria isolated from the epidermis of cats. Actinomycetes and mycobacteria are rare opportunistic invaders in dogs and cats. Bactericidal drugs expected to be effective against these bacteria should be used when treating the first occurrence of pyoderma in an animal.
Bacterial skin disease in large animals may be caused by Dermatophilus congolensis, staphylococci, Corynebacterium spp, Actinomyces, and rarely Bacillus spp or Pseudomonas spp. Draining tracts or abscesses in the skin of sheep or goats may be caused by Corynebacterium pseudotuberculosis. Fusobacterium spp and Bacteroides spp are the primary invaders in interdigital necrobacillosis (footrot). The spirochete Borrelia suilla is a secondary invader of skin lesions caused by sarcoptic mange or ear biting in swine. Clostridial diseases in cattle and erysipelas in swine are disorders that involve the integumentary system and cause serious economic losses.
If the exudative cytology shows the presence of an active infection with coccoid organisms, empirical antibiotic treatment should begin. In most clinics worldwide, canine isolates of S intermedius have excellent (>95%) sensitivity to oral cephalosporins, fluoroquinolones, antistaphylococcal penicillin (cloxacillin, oxacillin), and amoxicillin-clavulanate. Erythromycin, lincomycin, clindamycin, and chloramphenicol have good (>75%) efficacy, while potentiated sulfonamides have shown variable efficacy (see Systemic Pharmacotherapeutics of the Integumentary System: Dosages of Antistaphylococcal Antibiotics).
Duration of therapy varies with the type of infection present. In general, superficial infections should be treated for 7 days beyond surface healing; deep infections should be treated 7–21 days beyond resolution, which may require treatment durations of 8–12 wk if continued improvement is seen. Culture and sensitivity testing should be done in all cases of refractory or recurrent pyodermas or if exudative cytology shows a mixed infection.
Recent reports from the USA have documented increased antimicrobial resistance in staphylococcal species in dogs (and less commonly cats) to multiple antimicrobial agents, including methicillin (a marker for resistance to all β-lactam antibiotics including penicillins, cephalosporins, and carbapenems), fluoroquinolones, macrolides, and a range of other antibiotics. One European study described the emergence of mecA-positive (methicillin resistant) S intermedius resistant to all systemic antimicrobial agents available and licensed for use in dogs and cats. Antimicrobial resistance may lead to treatment failure in the individual and pose a potential zoonotic risk to owners, particularly with methicillin-resistant S aureus and S intermedius. Thus, cases of suspected staphylococcal infection should be cultured if the first empirical therapy fails, rather than sequentially trying multiple antibiotics.
Last full review/revision March 2012 by Michael Shipstone, BVSc, FACVSc, DACVD