A5-year-old boy, who lives on a farm and routinely plays with his pet dogs, presented with these scaly, inflamed macules with a central clearingon the abdomen (A) and forehead. 
Robert L. Bratton, MD, of the Mayo Clinic, Jacksonville, Fla, reports that a Wood lamp examination of the child's forehead showed the characteristic pale green fluorescence of Microsporum canis infection (B, arrow). 
Results of examination with a potassium hydroxide( (KOH) preparation of skin scrapings from a lesion confirmed this finding.
Trichophyton tonsurans--the most common cause of tinea capitis--and M canis account for the majority of cases of ringworm in the United States and Europe.1 Microsporum is typically associated with animals and soil, and the risk of infection in humans is low. Each species of zoophilic dermatophyte prefers specific animal hosts according to the type of keratin found in the animal.2 Cats and dogs are natural reservoirs of M canis; other animal hosts include cattle, sheep, pigs, rodents, and monkeys. Infected cats contaminate the environment with Microsporum through airborne arthrospores. Dogs are more likely than cats to contaminate surfaces and are less likely to cause infections in humans.3
Transmission of Microsporum to humans is prevalent in rural settings, where animal contact is common. The organism is transmitted through direct contact of an exposed area--most commonly, the scalp, face, or arms--with an infected animal or its dander.4 In humans, Microsporum infections are usually suppurative, whereas the animal host may lack clinical signs.
Typically, infection results in annular, erythematous lesions that may appear as scaly, inflamed nodules or macules with central clearing. A Wood lamp reveals a pale green fluorescence that may be subtle and difficult to detect. KOH preparations that show septate hyphae, macroconidia, and microconidia confirm the diagnosis of M canis infection.
Twice-daily applications (for up to 8 weeks) of clotrimazole(, miconazole(, ketoconazole(, econazole, naftifine, terbinafine(, or ciclopiroxolamine cream are usually effective. Lesions may respond slowly. Children with ringworm that is severe or unresponsive to topical therapy may require systemic antifungal therapy with griseofulvin( or fluconazole(. In some patients, oral or topical corticosteroid therapy is needed to reduce the inflammatory response.
This patient's infection resolved after 4 weeks of twice-daily applications of 1% clotrimazole cream.