Ringworm is a skin disease caused by a group of fungi called dermatophytes. In humans it causes a red ring-like lesion that is itchy and was therefore misnamed a worm. The more proper terminology is not ringworm, but dermatophytosis. Ringworm fungi are soil-borne organisms that affect many species of animals including humans. They are highly contagious, and spores from infected individuals can fall off the host and contaminate the environment. Some individuals can be carriers without showing any clinical signs.
Cats tend to be affected more readily than dogs, and kittens are the most easily infected because of their immature immune systems.
Clinical signs in pets tend to be non-specific and can look like most any other form of skin disease. Scabs appear on the skin that may or may not have the red ring around them. Most cases are diagnosed when a pet has not responded to an appropriate course of antibiotics to treat the much more common staph infection.
Diagnosis is made definitively with fungal cultures. Screening tests include fluorescence under a Wood’s lamp and looking at hair samples microscopically. Most ringworm cases will fluoresce under a Wood’s lamp, but not everything that fluoresces is ringworm. Microscopic identification can be difficult, so most veterinarians do ringworm culture tests when ringworm is suspected.
Treatment is aimed at minimizing shedding of the spores into the environment while treating the patient. Some cases require shaving to facilitate topical treatment with antifungal shampoos. Oral antifungal medication: itraconazole and griseofulvin are the mainstays of successful treatment but it can be expensive and typically must be given for at least 6 weeks. Infected animals should be segregated from other pets and human family members to reduce the risk of infection and to prevent shedding of infective spores throughout the environment. Lime sulfur dips can be effective, but are very foul smelling (rotten eggs), and can stain clothes and carpets. Some pets fight off the infection without treatment, but those cases are rare. The environment can be disinfected by using dilute bleach to disinfect animal bedding and carpets.