Ringworm (dermatophytosis) is a fungal infection that can affect the hair, skin or nails of cats, dogs and humans. It is the most common contagious skin infection in cats. In humans, the infection often causes classic ring-like lesions, but these are seen less commonly in cats and dogs. In most patients, ringworm is self-limiting. It can be transmitted from cats and dogs to other animals and also to people.

Most common organisms - Microsporum canis, Microsporum gypseum, or Trichophyton species. The fungus is most commonly found either on an infected animal or in the living quarters of infected animals. Spores from infected animals can be shed into the environment and live for up to 24 months. Humid, warm environments encourage growth of the fungus. Spores can be on brushes, bedding, furniture, or anything that has been in contact with an infected animal or the animal's hair. Cats can be asymptomatic carriers and harbor and shed the organism without showing signs of infection.

Transmission occurs by direct contact with fungal spores. These spores can be found on infected animals, infected grooming equipment or brushes, and in a contaminated environment where an infected animal has visited. Ringworm spores can survive in the environment for long periods of time. Most healthy adult cats have some natural resistance and never develop clinical signs. Cats under a year old are most often infected. Cats with a suppressed immune system, senior cats, free-roaming, those under stress or malnourished appear to be at increased risk. Genetic factors may play a role; Persians appear to be more susceptible. Exposure to or contact with a dermatophyte does not necessarily result in an infection and infection may not result in clinical signs. An affected animal may remain as an asymptomatic carrier for a prolonged period of time and some animals never become symptomatic.

Clinical signs
The classic sign is a small round lesion that is devoid of hair and often scaly skin in the center. The lesion may start as a small spot and continue to grow in size. It may or may not be irritated and itchy. Lesions are most commonly found on the head, ears, and tail. In some infections, the fungus will not be in a circle and can spread across the face, lips, chin, or nose and look like other generalized skin diseases. Occasionally, the infection will occur over the entire body and create a generalized scaly or greasy skin condition. Hair loss may be mild or severe. In some cases the first sign may be excessive shedding. Scratching at the ears is also common.

One method is through the use of a specialized black light called a Wood's lamp. Several species of the ringworm fungus will glow a fluorescent color when exposed. However, it is estimated that up to half of the most common species of M. canis do not fluoresce and T. mentagrophytes does not fluoresce. Other substances may also fluoresce resulting in a false positive reading. Another method is to pluck hairs from the periphery of the lesion and examine them under the microscope. False negative results are common. The most reliable way to identify ringworm is by collecting scales and crust from the skin and coat and performing a fungal culture.

Treatment and Environmental Control
The recommended topical treatment is lime sulfur dips. Small isolated lesions can often be treated with a topical cream containing an antifungal such as miconazole or thiabendazole.

In more severe cases, a combination of oral and topical treatments is generally used. Oral antifungal agents may also be recommended when there is no response to topical therapy after 2-4 weeks of treatment. Treatment is generally continued until there have been two negative cultures a week apart.

Ringworm can survive for such long periods in the environment, it is critical that an effective cleaning plan be used in all infections. Spores are very light and are carried in the air, so wherever there is dust and hair, there may be spores.

Transmission to People
Ringworm can be transmitted between cats and people. Persons with suppressed immune systems and those undergoing chemotherapy may be especially vulnerable. Gloves should be worn when handling affected animals.