“Ringworm” is the common name for the skin infection caused by a special group of fungi; it is not caused by a worm at all. Three of the most common fungi responsible for the development of ringworm in your pet are Microsporum canis, Microsporum gypseum and Trichophyton mentagrophytes. Ringworm treatment is generally continued until there have been two negative cultures a week apart. The point being made here is that some degree of self trauma is probably required to enable fungal infection to develop and that ectoparasite infestation may be an additional predisposing factor. Ringworm invades the dead, outer layers of the skin, claws & hair.