Ringworm is caused by a fungus. The disease with the misleading name (it has nothing to do with a worm) is a common infection that often causes itchy red patches on the skin. Dogs, cats, and humans can be affected by the disease, which causes hair to fall out in affected areas. It’s easily transmitted between people and pets, and if someone (or some pet) in a household has it, all should be tested and treated, if needed. Treatment ranges from oral medications to topical products and can take weeks or months to resolve.


Ringworm in pets is most often caused by the fungus Microsporum canis. Although two other species of fungus can also cause ringworm infections, they tend to do so less frequently.

These fungi (also known as dermatophytes) invade the superficial layers of the skin, hair, and/or nails. Because fungi thrive in moist environments, dermatophytes are especially persistent in humid climates and damp surroundings.

The ringworm infection caused by dermatophytes is also known as dermatophytosis. It’s not only contagious to other animals, it’s considered a zoonotic disease, meaning it can be transmitted from animals to humans (and vice versa). Children and those with compromised immune systems are most at risk.

Signs and Identification

In pets, the fungal infection causes the hair to become brittle and break off, resulting in bald spots. These occur most commonly on the face, ears, and legs. Within these hairless patches, the skin may be crusty or mildly inflamed, especially around the edges (hence, the ring-like appearance). If claws are affected, they may become deformed as they grow, just as in a human with a fungal infection of the nails.

Typically, the infection itself is not itchy, though secondary bacterial infections (pyoderma) may elicit significant pruritus (itchiness). Some animals may show no signs but may be sources of infection nonetheless, shedding fungal spores into the environment and serving as a reservoir for infection.

Ringworm is typically spread by contact with an infected animal. Because animals can shed fungal spores and infected hairs into the environment, touching objects the infected animal has been in contact with, including bedding and brushes, can also lead to infection. Organisms that are shed into the environment can remain infectious for months.

The best way to diagnose ringworm infection in an animal is by fungal culture. The veterinarian will pluck a few hairs from several lesions and place them on a culture medium where the organism can grow. Because it takes time for fungal growth, results may not be available for two weeks or more. Preliminary results, however, may sometimes be obtained within five days.

Veterinarians might also examine skin lesions under a Wood’s (ultraviolet light) lamp. In some cases — but not all — the organism may glow yellow-green. Because this test is not always accurate, a fungal culture is still the preferred method of diagnosis. Nonetheless, Wood’s lamp testing can be helpful.

In households in which people are diagnosed with ringworm, all the family pets should be tested. The same goes for multi-pet households in which one pet has been diagnosed with ringworm. Other pets should be tested and treated if positive in order to eliminate sources of ongoing infection.

Affected Breeds

All breeds of dogs and cats are equally susceptible. Dogs and cats with compromised immune systems may be predisposed to ringworm infections.


In healthy animals, the infection may be self-limiting, meaning that it will eventually resolve without treatment. However, treatment can hasten resolution of the problem and limit the spread of infection to other animals and people in the household.

Pets may be treated with topical products, oral medications, or both. Before applying a topical treatment, veterinarians may recommend shaving or clipping the infected area. Topical treatments include lime sulfur dip, anti-fungal sprays/creams or antifungal shampoos.

There are a number of oral medications for ringworm, such as griseofulvin and itraconazole. Griseofulvin should never be given to a pregnant animal because it may cause birth defects in developing puppies or kittens. It may also cause bone marrow suppression in cats, especially those with feline immunodeficiency virus (FIV) or feline leukemia virus (FeLV). Administration of griseofulvin may require periodic blood monitoring tests. Itraconazole is rapidly becoming the preferred oral treatment for cats because it has fewer side effects.

Thorough cleaning and treatment of the home environment is important to prevent recurrence and spread of the infection to pets and people. To eliminate fungal organisms in the environment:

  • Clip affected areas on the pet and dispose of all hairs.
  • Confine infected pets to one area of the house.
  • Thoroughly vacuum areas that were highly trafficked by the pet and dispose of the vacuum bag outside. Wash all bedding and toys in hot water.
  • Dispose of any carpets or rugs, if possible.
  • Clean exposed areas and kennels with chlorine bleach that has been diluted 1:10 or with an antifungal spray recommended by your veterinarian.
  • Repeat vacuuming and surface treatment at least monthly until infection is resolved.

Treatment may be required for six weeks or longer. Once skin lesions have resolved, fungal cultures should be performed again. Treatment should not be stopped until fungal cultures are negative. Discontinuing treatment based only on resolution of lesions may result in recurrence of the infection.


It’s difficult to prevent ringworm infection 100% in pets that enjoy outdoor life. These organisms are in the soil and might be on other dogs in the dog park, for example. Keeping cats indoors is an obvious solution, but even they can be infected by humans or dogs in the household.

Once characteristic lesions are seen, however, owners should take care not to touch the lesions and take their pets quickly to the veterinarian to prevent the spread of this skin infection.

This article has been reviewed by a Veterinarian.

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