Ringworm in Cats and Dogs

Ringworm is fungal disease of the skin, hair and nails of animals and humans. On the skin the infection often spreads outward in a circular pattern, hence the name. The majority of cases in small animals are caused by 3 species: Microsporum canis, Microsporum gypseum or Trichophyton mentagrophytes. M. canis is the most common fungus of dogs and cats; it is rare in dogs and very common in cats.


Ringworm is spread by exposure to spores, either by direct contact with an infected animal, or indirectly by contaminated grooming equipment, cages or bedding. Environmental contamination is low or non-existent for dogs but high for cats. Spores can survive for up to 2 years in the environment, allowing re-infection. Young or old animals, or animals with a suppressed immune system from disease or medication are more susceptible to contracting ringworm. Most infections are self-limiting because of the host’s inflammatory response.

Signs of Ringworm

Signs of ringworm are very variable and may first appear as one or more small areas of hair loss that may be reddened or inflamed. Crusts may form on the areas of hair loss and patches may increase in number and size. Some animal infected with ringworm may appear normal.


The best and most accurate way to diagnose ringworm is by laboratory culture of skin and hair debris. Examination with a Wood’s Lamp may cause ringworm to fluoresce, however about 50% of the most common species, M. canis, will fluoresce. Microscopic examination of infected hairs is a poorly diagnostic.


The hair is clipped from small, isolated lesions, bathed and treated with a topical fungacide. Clipping the whole body is essential in long haired animals. A topical antifungal wash is commonly used in dogs but oral medication is required in cats.


Treatment of the environment is essential as hair and skin debris may remain infectious for several years. Vaccuum clean thoroughly and disinfect with bleach or a specific environmental fungicide. It may be prudent to destroy bedding, grooming equipment, leashes, etc.

Transmission to People

Ringworm can be transmitted between animals and people. People with a suppressed immune system and those undergoing chemotherapy may be especially vulnerable. Wear gloves when handling affected animals and wash hands well afterwards.

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