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Dermatophytosis in Dogs

Dermatophytosis is the medical term for a parasitic fungal infection that affects the skin, hair, and/or nails (claws). The most commonly isolated fungal organisms are Microsporum canis (more commonly referred to as ringworm), Trichophyton mentagrophytes, and Microsporum gypseum. This disease occurs in dogs, cats, and other mammals. It is diagnosed more commonly in young animals than in old.

The condition or disease described in this medical article can affect both dogs and cats. If you would like to learn more about how this disease affects cats please visit this page in the PetMD health library.

Symptoms and Types

Symptoms of dermatophytosis include accumulations of surface skin cells, such as seen in dandruff (scales); poor hair coat; reddened skin (erythema); darkened skin (hyperpigmentation); itchiness (pruritus); and hair loss (alopecia), which may be patchy or circular. Other indications of dermatophytosis that are readily apparent on the skin are raised, rounded, knotty (nodular) lesions known as granulomatous lesions, or boils, and raised nodular lesions that frequently ooze (kerions), the result of ringworm infection. There may also be inflammation of the claw folds -- the folds of skin bordering the nail, and medically referred to as paronychia.

Occasionally, dogs are classified as inapparent carriers -- harboring the disease-causing fungus, but presenting no visible signs of the condition. However, even these dogs are contagious to humans and other animals.

Causes

Dogs most commonly develop dermatophytosis because of infections with the fungi Microsporum canis, Microsporum gypseum, and Trichophyton mentagrophytes. The incidence of each fungus varies according to your geographical location.

Diseases or medications that decrease the body's ability to develop a normal immune response (known as immunocompromising diseases, or immunosuppressive medications, respectively) can increase the likelihood that your dog will be susceptible to a fungal infection of the skin, hair, and/or nails, as well as increase the potential for a more severe infection. Environments that are densely populated with animals (for example, in an animal shelter or kennel), or where there is poor nutrition, poor management practices, and lack of adequate quarantine period, will also increase risk of infection.

Diagnosis

Your veterinarian will perform a fungal culture of skin clippings, a microscopic examination of a sample of hair, and possibly a skin biopsy.