Ringworm (Microsporum canis) is a treatable, common disease of cats and people. This fungal skin infection can be spread by direct contact with an infected animal or person, or by contact with spores that are shed into the environment. Environmental spores can live up to one year if not thoroughly destroyed.

Several factors are important in the spread of the disease. A weak immune system predisposes cats to contracting the disease as well as living in a crowded or multiple cat environment. The time from contact to development of disease can be as little as four days or as much as four weeks.

Most commonly, ringworm on cats appears as a crusty lesion on the head or front legs. Occasionally infected nail beds or tail seborrhea are seen. Since fleas, allergies and injuries can all look like ringworm; tests will be done to determine if your cat has ringworm or one of these other diseases.

Approximately 70% of M. canis species will cause infected hair to glow apple green when exposed to a black light. If the hair glows, other tests may not be needed; however, the most definitive way to diagnose ringworm is through culture or cytology. Humans who may have the disease should consult their family doctor or a dermatologist.

Cats with ringworm should be treated both topically and systemically. Topical medication (antifungal rinses) will help cut down on spores shed into the environment while systemic therapy will attack the fungal infection deep in the hair shafts. Rinses and orally medication can be prescribed by a veterinarian after a definitive diagnosis has been made. Topical ointments are of limited value when treating a cat for this condition.

Eradicating environmental spores will prevent re-infection. This is accomplished various ways depending on the environment. Any area that can be bleached should be cleaned with 1:10 dilution of bleach. Areas that cannot be bleached should be vacuumed and the vacuum bag thrown away after each cleaning. Other items such as toys and combs that cannot be easily cleaned should be disposed of to prevent continued spread of spores.

Ringworm is a common, but treatable disease of cats and people. Treatment requires diligent cleaning of the environment and isolation of the infected cat(s) until fungal cultures are negative for growth of M. canis.