Clinical Exposures: Canine dermatophyte infection - Veterinary Medicine
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Clinical Exposures: Canine dermatophyte infection



The puppy had an uneventful recovery from surgery. No other treatment was given for the dermatophytosis. The lesions on the head resolved over several weeks without intervention. Eight months later, the puppy was in a new home and doing well. No new skin lesions had developed.


Dermatophytosis, also called ringworm, may be caused by several keratinophilic fungal organisms, but the most common dermatophytes in dogs and cats are M. canis, Microsporum gypseum, and T. mentagrophytes. These dermatophytes invade the hair follicle, the hair, and, less frequently, the epidermis.1,2

The infective form, the arthrospore, is transmitted by direct contact with an infected animal or with fomites, which contain a hair, piece of skin, or scale of an infected animal. In addition, asymptomatic dogs can be carriers and may spread dermatophytosis to other dogs, cats, or people. Arthrospores cannot penetrate the dermis because of healthy skin's innate fungistatic properties; however, if there is a breach in the epidermal layer, arthrospores may bond tightly to keratin and germinate within six hours of adherence.1 Wet or humid conditions may enhance an arthrospore's ability to penetrate compromised skin and germinate.1,2

Most dermatophytes are not components of the normal canine fungal flora. Microsporum canis is well-adapted to dogs; however, unless the infected animal is very young, very old, or immunocompromised, it rarely causes inflammation or infection. Infections with M. gypseum and T. mentagrophytes tend to cause more serious dermatologic disease.1-3


Dermatophytosis is commonly diagnosed in puppies and young dogs. Localized or generalized disease may be present. Dogs younger than 1 year old have a greater risk of dermatophytosis than do older dogs, but mild or localized disease may be self-limiting in some young dogs.1 Older dogs may be at increased risk for disseminated disease if they have concurrent immune system dysfunction (secondary to endocrinopathy, autoimmune disease, or neoplasia) or if they are receiving chemotherapy or immunosuppressive therapy. No sex or breed predilection has been reported,1-3 but the coats of longhaired breeds may trap arthrospores, promoting infection.1,3


The classic dermatophyte lesion is a ring of alopecia expanding from an erythematous border that surrounds a central area of healing. The lesions are often pruritic. Other possible dermatologic changes associated with infection include scales, crusts, and hyperpigmentation.1-3

Although dermatophyte lesions may occur anywhere on the body, the face and forelimbs are most commonly affected. Dogs infected with M. gypseum or T. mentagrophytes may have symmetrical facial lesions with marked exfoliation and crusting. Trichophyton mentagrophytes may cause onychodystrophy or paronychia .2,3

Other possible presentations include nodular and granulomatous forms. The nodular form, which was seen in this case, is characterized by well-circumscribed nodules called kerions. Kerions occur on the face and extremities and may develop draining tracts. Kerions are often associated with M. gypseum or T. mentagrophytes infection.1-3

Diagnostic testing

A Wood's lamp examination is a commonly used in-house diagnostic test for dermatophytosis. The lamp uses an ultraviolet light to detect hair shafts presumably contaminated with certain strains of M. canis. Test results are positive if the characteristic greenish fluorescence is noted. Unfortunately, this test has poor sensitivity and only detects 50% of M. canis infections.4 False positive results may be caused by cellular or acellular debris, scales, or some topical medications.1,4


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