Ringworm (dermatophytosis) is a skin disease that is caused by a fungus, not a worm. It can affect cats, dogs, other animals, and people. Long ago, it was thought that a parasite (worm) was responsible, but it is now known that ringworm is caused by a type of fungus called dermatophytes, which infect tissues containing keratin. Keratin is a protein produced by skin cells. The outer most layer of skin cells contains keratin. Hair and claws/nails are also made of keratin. Therefore, dermatophytosis is a contagious fungal infection that can affect the skin, hair, and claws.
Ringworm skin infections tend to enlarge in a circular pattern as the organisms continuously infect more skin and hair on the edges of the area—hence the “ring” in the name ringworm. Microsporum canis, is usually transmitted from one pet to another. The other two, Microsporum gypseum and Trichophyton mentagrophytes, are normally found in soil and on rodents, respectively, but can infect pets as well. People can become infected with any of the three types of ringworm from contact with infected pets.
Three types of ringworm/dermatophyte infections are common in dogs and cats. One,
Skin and hair ringworm infections cause hair loss in a circular pattern. The ringworm infection weakens the hairs,causing them to break off easily, leaving the skin bare in affected areas. Small red bumps, scales (dandruff), and hyper pigmentation (darkening) of the skin may occur.