Ringworm is an infection of skin, hair, or claws caused by a type of fungus known as a dermatophyte. In cats, about 98% of ringworm cases are caused by the fungus Microsporum canis. The fungus is spread easily in the environment and often infects people.
The fungi spread to people primarily by contact with infected cats and contaminated objects, such as furniture or grooming tools. Broken hairs with associated spores are important sources for spread of the disease. Contact does not always result in infection. Whether infection is established depends on the fungal species and on host factors, including age, health, immune system, condition of exposed skin surfaces, grooming habits, and nutrition. Infection leads to temporary resistance to reinfection. Under most circumstances, dermatophytes grow only in the dead cells of skin and hair, and infection stops on reaching living cells or inflamed tissue. As inflammation and host immunity develop, further spread of infection stops, but this process may take several weeks. Kittens and longhaired cats may have more persistent and widespread infections.
The appearance of ringworm varies in cats. Kittens are most likely to be infected. Infected cats can develop bald, scaly, crusted patches with broken hairs. The most common areas for ringworm to occur are the face, ear tips, tail, and feet. Some cats develop small, solid bumps on the skin (called miliary dermatitis) that itch. Cats with widespread ringworm often have larger bumps with open sores.
Veterinarians diagnose ringworm by fungal culture, examination with an ultraviolet lamp, and direct microscopic examination of hair or skin scale. Fungal culture of hairs and scrapings from the affected areas is the most accurate method, but it can take up to 3 weeks to get the results. A positive result obtained with an ultraviolet lamp may allow early diagnosis, but the results are not always reliable and need to be verified with a fungal culture.
Ringworm infections can clear up without treatment, but treatment may speed healing and reduce the spread of the disease. Your veterinarian can prescribe anti-fungal medications that can be applied to the body in the form of dips and medicated shampoos. Such treatments are not always effective, however. Cats with widespread ringworm infections usually need to be treated with oral antifungal drugs. Your veterinarian may recommend that your cat be shaved, especially if it has long hair or a widespread infection. The cat's environment also needs to be cleaned thoroughly with a dilute bleach solution. Your veterinarian can provide you with information about any treatment that may be appropriate for your pet and advise you regarding precautions you should take to avoid ringworm infection in yourself and members of your family.
Also see professional content regarding dermatophytosis.