Ringworm (dermatophytosis) — despite its name, a fungal infection rather than a worm — is a skin disease of dogs, cats, and other animals with significant welfare implications and zoonotic potential (transmissible to humans).
Microsporum canis is the most common dermatophyte in dogs (and even more so in cats). Trichophyton mentagrophytes (rodent-associated) and Microsporum gypseum (soil-associated) also cause infection. Dermatophytes invade keratinised tissues — the dead outer layers of skin, hair, and nails — and do not penetrate to living tissue in immunocompetent animals. Their metabolites cause an inflammatory host response that produces the characteristic lesions.
Classic ringworm presents as circular patches of hair loss with scaling and sometimes mild erythema. The name 'ringworm' comes from the circular lesion pattern. However, presentation is highly variable: diffuse hair loss, folliculitis (inflamed hair follicles), kerion (raised, moist, secondarily infected lesions), and occasionally widespread disease. Many dogs carry dermatophytes asymptomatically without developing clinical signs. Breed predispositions: Yorkshire Terriers are particularly susceptible.
Pruritus (itching) is variable — some lesions cause significant itching, others very little. Kerion lesions are painful. Secondary bacterial infection of lesions causes additional discomfort. The major welfare concern beyond direct skin effects is the duration of infection before diagnosis and the protracted treatment period required. Dermatophytosis can persist for months without treatment.
Ringworm is readily transmissible to humans, causing circular, itchy lesions on skin. Children and immunocompromised individuals are most susceptible. Owners of affected dogs should be informed of the zoonotic risk, advised on hygiene measures (handwashing after handling the pet, not sharing bedding), and seek medical advice if they develop skin lesions. This dual human-animal welfare dimension makes prompt treatment important.
Wood's lamp examination (UV light) causes some M. canis strains to fluoresce apple-green, but sensitivity is only 50–60% — a negative result does not rule out infection. Microscopic examination of plucked hairs detects fungal elements but requires skill. Fungal culture (DTM or Sabouraud's agar) is the gold standard — sensitivity approaches 100% but requires 7–14 days for growth. PCR testing provides more rapid, sensitive diagnosis where available.
Systemic antifungals are required for generalised infection or in multi-pet/kennel environments: itraconazole (pulse dosing) or terbinafine are first-line options. Topical therapy alone (miconazole shampoo, lime sulphur dip) is inadequate for generalised disease but useful as an adjunct reducing environmental contamination. Environmental decontamination is essential — dermatophyte spores persist in the environment for months. Vacuuming, washing bedding at high temperature, and disinfecting with dilute bleach or enilconazole reduces reinfection risk.
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