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Ringworm (fungal infection) occurs occasionally in swine and may affect any age group. Although cases often are sporadic, occasional outbreaks affect many animals, especially sows. Most ringworm is caused by the fungi Microsporum nanum or Trichophyton verrucosum.

Lesions can occur anywhere but on older swine are usually seen on the neck or behind the ears. Ringworm lesions begin as brown expanding areas a few centimeters in diameter but eventually may enlarge to five to ten centimeters. Mature lesions have a central, brown crust; hair loss is minimal or absent and lesions are non-pruritic. In adult swine, ringworm must be differentiated from sarcoptic mange especially if lesions are behind the ears. In young growing pigs, ringworm must be differentiated from pityriasis rosea and exudative epidermitis (“greasy pig”). Diagnosis of ringworm usually can be confirmed by microscopic examination of skin scrapings or by histopathologic examination of skin lesions.

Ringworm is usually self-limiting but lesions may require months for healing to occur. Lesions are most common during the winter months. Ringworm can be treated orally with nystatin or griseofulvin but typically is left to resolve on its own. Ringworm should be considered contagious to people.