| In dogs, ~70% of cases are caused by
Microsporum
canis
, 20% by
M
gypseum
, and 10% by
Trichophyton
mentagrophytes
; in cats, 98% are caused by
M
canis
. The Wood’s lamp is useful in establishing a tentative diagnosis of dermatophytosis in dogs and cats but cannot be used to rule out this type of infection. Definitive diagnosis is established by DTM culture (see
Dermatophytosis: Introduction). Detection of infection in asymptomatic carrier animals is facilitated by brushing the coat with a new toothbrush and then inoculating a culture plate by pressing the bristles to the surface of the medium. |
| The clinical appearance of ringworm in cats is quite variable. Kittens are affected most commonly. Typical lesions consist of focal alopecia, scaling, and crusting; most are around the ears and face or on the extremities. Cats with clinically inapparent infections can still serve as a source of infection to other cats or people. Occasionally, dermatophytosis in cats causes feline miliary dermatitis and is pruritic. Cats with generalized dermatophytosis occasionally develop
cutaneous ulcerated nodules, known as dermatophyte granulomas or pseudomycetomas. |
| Lesions in dogs are classically alopecic, scaly patches with broken hairs. Dogs may also develop regional or generalized folliculitis and furunculosis with papules and pustules. A focal nodular form of dermatophytosis in dogs is the kerion reaction. Generalized ringworm in adult dogs is uncommon and is usually accompanied by immunodeficiency, especially endogenous or iatrogenic hyperadrenocorticism. Differential diagnoses in
dogs for classic ringworm lesions include demodicosis, bacterial folliculitis, and seborrheic dermatitis. |
| Dermatophytosis in dogs and shorthaired cats is usually self-limiting, but resolution can be hastened by treatment. Another primary objective of therapy is to prevent spread of infection to other animals and people. However, whole-body topical therapy is controversial, and recent studies have not confirmed that any currently available topical rinse or shampoo is truly effective. Enilconazole, a rinse not currently available in North America, is most likely to be effective. Local
lesions can be treated effectively with topical miconazole or clotrimazole. For chronic or severe cases and for ringworm in longhaired breeds of cats, systemic treatment is indicated. The microsized formulation of griseofulvin can be used in dogs (25-100 mg/kg, sid or divided doses) and in cats (25-50 mg/kg, daily in divided doses). These dosages are higher than those approved by the FDA. The ultramicrosized formulations used in human medicine can be used at lower
dosages (10-15 mg/kg). Cats may develop bone marrow suppression, especially neutropenia, at higher doses or as idiosyncratic reactions. In both dogs and cats, GI upset is a fairly common sequela of griseofulvin administration. Alternative and effective treatments include terbinafine (30 mg/kg) or itraconazole (5-10 mg/kg, sid), but neither of these drugs is approved for use in domestic animals. Systemic and topical treatments for dermatophytosis should be continued
for 2-4 wk past clinical cure or until a negative brush culture is obtained. This may require treatment for 1-3 mo with griseofulvin or for ≥1 mo with azole antifungals. A killed fungal cell wall vaccine is approved for treatment and prevention of
M
canis
ringworm in cats. The vaccine hastens clinical resolution but apparently does not affect time to mycologic cure. It also reduces the severity, but not the frequency, of infection in kittens that are subsequently exposed. Use of the vaccine in management of dermatophytosis in pet cats or multicat facilities remains to be defined. Recent reports of the efficacy of lufenuron in treating ringworm in dogs and cats have not been confirmed in controlled studies. |
