Ring Worm

Ring Worm
By Dr. Ed Mapes
Stonebridge Animal Hospital

The term “ringworm” refers to a fungal infection that can involve the skin, hair, and toenails in animals and humans. It is concerning when we diagnose pets with ringworm due to the possibility of spread to family members.

There are three fungal organisms responsible for the vast majority of infections in animals, and all are transmissible to humans. These are Trichophyton Mentographytes, Microsporum canis, and Microsporum gypseum. These fungal organisms that affect the skin, hair, and nails are known as Dermatophytes; the correct term for infection is actually dermatophytosis.

The fungi are present in soil and can exist in the home for months once they are introduced. Fungal spores are very hardy and can persist in carpeting, furniture, hair brushes, etc. for months. Infection occurs by way of direct contact of the spores with hair or skin.

Infections of the skin can take on a somewhat characteristic appearance. Hair shafts, hair follicles, and skin are affected by the organisms. Hair falls out or is very easily broken, so hairless areas on the skin appear. As the infection spreads from a central point on the skin, a red, circular ring of inflamed skin appears (hence the name “ringworm”).

The ring of inflammation continues to extend outward as the infection spreads, and sometimes hair begins to re-grow in the central portion of the hairless area. The lesions often develop a crusty appearance as skin cells are damaged and slough off. Infected toenails become brittle and discolored, and the toe above the nail becomes red and swollen.

The lesions do not usually itch, which can help differentiate this condition from other skin disorders that do cause scratching. Secondary bacterial infection will cause the skin to itch, however, so lesions must be inspected closely to differentiate and not misdiagnose these as an allergy or pyoderma.

Diagnosis is done by examination of the lesions and then setting up a test in Dermatophyte Test Medium (DTM); samples of sloughed skin and hair are placed in bottles containing a specific growth medium. Testing usually takes about 5 days to conclude; a positive test is identified when a white, puffy growth appears on the surface and the medium changes color from orange to a deep red.


A DTM bottle on left before testing begins. At right, a positive sample shows the white, puffy fungal growth and change of the medium from orange to red.
Ringworm can be treated with several oral medications, dips, shampoo and spray conditioning agents depending on the extent of the infection over the body, severity of lesions, and overall medical condition of the pet. Topical creams applied directly to lesions are not usually recommended because patients tend to lick them off.

Treatment is extended to approximately six weeks in order to ensure complete resolution of infections; complete cure can take months in some cases. Concurrent bacterial infection is usually treated for two weeks with an appropriate antibiotic.

Fungal spores in the home can be a source of continual infection to pets and to people. Thorough cleaning of floors, bedding, and furniture can minimize spores in the home. Fungal spores can be killed using bleach mixed with water at a 1:30 ration of bleach to water.

Clipping of haired areas near lesions will minimize contamination, and gloves should be worn if dips are to be applied in the treatment regimen.

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