Investigators compared bacterial isolates from healthy dogs and dogs with pyoderma collected from 1999–2002 with similar isolates collected in 2009 to determine the prevalence of Staphylococcus pseudintermedius, antimicrobial susceptibility changes, presence of the mecA gene in methicillin-resistant isolates, and fluoroquinolone resistance. S pseudintermedius was isolated from 76% of samples from dogs with pyoderma during both time periods. In 1999–2002, S pseudintermedius was isolated from 34.8% of healthy dogs, while in 2009 it was isolated from 69.6% of healthy dogs. In the 1999–2002 period, all isolates from normal dogs and dogs with pyoderma were both oxacillin and fluoroquinolone susceptible. In 2009, by contrast, 50% of isolates from dogs with pyoderma and 30% of isolates from normal dogs were fluoroquinolone resistant. Twenty-one isolates were oxacillin-resistant and all carried the mecA gene. Organisms resistant to 1 fluoroquinolone mutated in the quinolone resistance determination region of grlA; those resistant to 2 or more fluoroquinolones had mutations in quinolone resistance determination regions of grlA and gyrA. There was significant increase in antimicrobial resistance between the 2 time periods, presumably as a result of antibiotic use but increased resistance was also noted in isolates from healthy dogs.

Changes in bacterial susceptibility over a 10-year period documented what most veterinarians have been experiencing in clinical practice: susceptible S pseudintermedius isolates becoming increasingly resistant to cephalosporin, oxacillin, and quinolone antibiotics. The explanation for this is antibiotic exposure. During this period, second- and third-generation cephalosporins became available and quinolone antibiotics entered the veterinary market. Antibiotic resistance is changing how I practice; increasingly, aggressive topical shampoo therapy with a chlorhexidine shampoo 2 to 3 times a week with 2% chlorhexidine spray solution on nonshampoo days is my first recommendation. If antibiotic therapy is necessary, I recommend culture and susceptibility testing.—Karen A. Moriello, DVM, DACVD

Antimicrobial resistance of Staphylococcus pseudintermedius isolates from healthy dogs and dogs affected with pyoderma in Japan. Onuma K, Tanabe T, Sato H. VET DERMATOL 23:17-e5, 2012.