When to perform culture and susceptibility testing in canine pyoderma
5-year-old atopic West Highland White Terrier with superficial pyoderma treated with cefalexin at the correct dose and frequency for 3 weeks. The owner confirms no doses were missed, yet new pustules are appearing on the abdomen. The dog has had two other antibiotic courses this year.
What is the most appropriate next step?
chevron_right Show answer and explanation
A. Switch empirically to amoxicillin-clavulanate
If the failure is due to a methicillin-resistant staphylococcus, it will also be resistant to amoxicillin-clavulanate and all cephalosporins. Switching blindly between beta-lactams makes no sense.
B. Confirm the infection with cytology and take a culture and susceptibility sample from an intact pustule or the edge of a collarette
Correct. Culture and susceptibility testing are indicated: new lesions are appearing despite correct treatment and there has been recent antibiotic use, both risk factors for MRSP. Open the pustule with a sterile needle and collect its contents with the swab, without preparing the skin with antiseptic.
C. Double the cefalexin dose and extend treatment for 3 more weeks
If the bacterium is resistant, increasing the dose will not solve it and will increase selection pressure. A failure with good compliance requires a microbiological diagnosis.
D. Add prednisolone, because atopic pruritus is causing the lesions
New pustules indicate active, uncontrolled infection. Immunosuppressing without controlling it can make it worse; managing the atopy matters, but later.
lightbulb Key takeaway
Culture if there are rods on cytology, deep pyoderma, recent antibiotic use, a history of MRSP, new lesions despite correct treatment or an incomplete response. While awaiting the result, step up topical treatment.
References: ISCAID: Guidelines for the diagnosis and antimicrobial therapy of canine superficial bacterial folliculitis (Hillier et al., 2014)
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Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.