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Today's Veterinary Practice May/June 2014 44 DermaTology DeTails Peer reviewed tvpjournal.com P yoderma is a common skin disor- der in small animal practice. Now that the major canine pathogen, Staphylococcus pseudintermedius, has acquired methicillin resistance, treat- ment has become more challenging and more expensive. Keys to success in treat- ment and prevention require: • Prompt identification and treatment of the underlying cause • Use of culture and sensitivity to guide antibiotic use • Increased reliance on topical therapy. WHY ARE DOGS SUSCEPTIBLE TO SKIN INFECTIONS? Of all the species with which we work, dogs seem uniquely predisposed to bacterial skin infections. 1-5 Dogs are more susceptible to skin infections due to basic structural features, such as: • Lack of a follicular lipid plug, which acts like a drain stopper • Fragile skin barrier • Alkaline pH. Table 1 lists underlying skin disor- ders that predispose dogs to staphylococcal skin infec- tions. 6 Dogs with atopic dermatitis are especially suscep- tible due to: • A defective skin barrier, which is represented by the stra- tum corneum and one of the first physical and chemical defenses against microbial infection 7-9 • Potentially decreased levels of defensins—cationic anti- microbial proteins that defend against bacterial infec- tions as part of the innate immune system. 10 WHICH BACTERIA CAUSE PYODERMA IN DOGS? The major canine skin pathogen is S pseudintermedius; 11 however, Staphylococcus schleiferi, Staphylococcus aure- us, and Pseudomonas aeruginosa have also been identi- fied in canine pyoderma. S aureus —a human pathogen— has been identified in a low percent- age of dogs. However, this bacterium has received a great deal of attention due to its methicillin resistance in hu- mans and potential role as a zoonotic agent—dogs infected with methicillin- resistant S aureus (MRSA) most likely acquired the infection from a human. S pseudintermedius , while not as virulent, shares many characteristics with S aureus, in- cluding: • Enzyme and toxin production • Ability to adhere to matrix adhesive proteins • Ability to form biofilms. Methicillin-resistant S pseudintermedius (MRSP) is un- likely to cause human infection, unless a person is very young, very old, or immunocompromised. S schleiferi was first identified from human clinical spec- imens in 1988, and has now been identified as a cause of pyoderma and otitis externa in dogs. 12-14 P aeruginosa —while not common—has been identi- fied on the skin of dogs, particularly in lip fold pyodermas and postgrooming folliculitis. 15,16 Identifying the particular Staphylococcus species in- Valerie A. Fadok, DVM, PhD, Diplomate ACVD North Houston Veterinary Specialists, Spring, Texas TreaTing resisTanT skin infecTions in dogs Table 1. Underlying Disorders in staphylococcal skin infections • atopic dermatitis • other allergic skin conditions • Disorders of keratinization • endocrinopathies • Parasitic diseases A Figure 1. 10-year-old male neutered poodle with atopic dermatitis: Severe gener- alized crusting associated with superficial pyoderma failed to respond to amoxicil- lin–clavulanate and cefpodoxime; culture revealed methicillin-resistant s pseudin- termedius sensitive to amikacin and rifampin (A). Three weeks after topical therapy, consisting of clipping and washing with chlorhexidine and twice daily topical appli- cation of amikacin (5 mg/mL) in Tris EDTA; no systemic antibiotics were used (B). B TVP_2014-0506_DermDetails_Infections.indd 44 5/23/2014 3:24:56 PM