Yeast Infections in Dogs (Malassezia): Primary and Secondary Causes

Malassezia pachydermatis overgrowth causes canine yeast dermatitis. Understanding primary vs. secondary etiology guides treatment.

Primary Causes (Direct Overgrowth Triggers)

• Environment: Warm, humid climates; seasonal flares (spring/fall).
• Breed predisposition: West Highland White Terrier, Cocker Spaniel, Shar Pei, Basset Hound, Poodle, English Bulldog — skin folds, lipid profile, immune genetics.
• Concurrent skin disease: Atopic dermatitis, food allergy, contact allergy → barrier defect, inflammation → Malassezia proliferation.

Secondary Causes (Underlying Systemic Drivers)

• Endocrine: Hypothyroidism (↓ immunity, ↓ skin turnover), Cushing's (hypercortisolism → immunosuppression), diabetes (hyperglycemia → immune dysfunction).
• Immunosuppression: Chemotherapy, corticosteroids, cyclosporine, oclacitinib (paradoxically).
• Skin trauma: Wounds, surgical sites, excessive licking/chewing.
• Otitis externa: Ear canal yeast overgrowth (common in floppy-eared breeds, swimmers).

Diagnosis and Treatment

• Cytology (tape prep, swab) → budding yeasts ("peanut/footprint" shapes).
• Topical: 2% miconazole/chlorhexidine shampoo q2–3d × 2–4 wks; leave-on 10 min.
• Systemic: ketoconazole, fluconazole, terbinafine for severe/refractory.
• Must treat underlying cause or recurrence is certain.

Malassezia is a commensal; disease = host + environment permitting overgrowth.