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.Infected nails become deformed and may have a fluid or gummy discharge at the base of the nail. Some infected cats show no symptoms at all but can be carriers, transmitting the ringworm infection to dogs or other cats. Other cats can develop miliary dermatitis, a condition where small crusts (scabs) can be felt beneath the hair coat.
The signs and symptoms of ringworm infection are similar to several other skin diseases so the diagnosis of ringworm cannot be made by appearance alone. Your veterinarian may use an ultraviolet lamp called Wood’s light to screen suspicious areas for ringworm. Some dermatophytes produce substances that glow with a green color under Wood’s light. However, this is a screening tool only, since not all dermatophytes show this response and conversely, some substances on the skin that are not ringworm can cause a false positive reaction. The best test for diagnosing ringworm infection is fungal culture. For this simple test,the haircoat is brushed with a disposable toothbrush or a few hairs are plucked from the affected area, and the specimen is incubated in a special culture medium called dermatophyte test medium, or DTM. A ringworm fungus tends to grow slowly when it is off the body, so results will not be known for a few days or up to two weeks. Microscopic examination of the fungus growth on the DTM can identify which of the three dermatophyte organisms is causing your pet’s infection, which is helpful in determining the source (i.e., the risk of reinfection).
Microsporum canis is the most contagious type of ringworm. It is important to realize that the environment and all infected pets in the household must be treated to eliminate ringworm and prevent its spread to other pets and to people. Dermatophyte organisms on shed hairs can remain infectious (contagious) for weeks. Other pets in the household should be inspected for infection. Since asymptomatic cats may be carriers, they should be tested by sampling the hair (brushing the haircoat with a brand new disposable toothbrush and then submitting the entire toothbrush and its collected hairs in a clean, zip-seal plastic bag for DTM culturing by your veterinarian. Infected pets should be isolated from other pets and from people, especially immunocompromised people including persons receiving cancer chemotherapy, people with human immunodeficiency virus (HIV) infection, and others. If your pet has contracted ringworm due to Microsporum gypseum or Trichophyton mentagrophytes, avoiding exposure to contaminated soil and rodents or rodent burrows will help prevent re-occurrence.
Most pets will have an immune response to the fungus that will eventually eliminate the infection. However, treatment is needed to speed the process and to prevent the spread of the ringworm organism to other pets and people. If for any reason, your pet is receiving any medications that suppress the immune system, such as corticosteroids, they should ideally be stopped; you should discuss this with your veterinarian to find suitable alternative medications as necessary. Long-haired pets should have their entire haircoats shaved off and the hair carefully collected and disposed of. Fungicidal dips (soaking the pet in diluted, purpose-made fungicidal shampoo or solution) help reduce the infectiveness of remaining hairs. Frequent, thorough vacuuming and cleaning of the area where the pet is kept are needed to remove infected hairs from the environment. All pet bedding and grooming equipment should be disinfected or destroyed. The entire house should be carefully cleaned to remove pet hairs. Consult your veterinarian for recommendations of cleaning agents that will destroy the organisms.
Pets affected with mild cases of ringworm (small, isolated skin lesions) can be treated with antifungal creams and lotions (prescription topical medications), which are applied directly to the affected areas of skin. It is important to wear gloves, such as disposable latex or rubber medical gloves, to avoid contracting the infection from the pet when applying these topical treatments.Oral antifungal medication is required for pets if topical treatment is not successful or if the hair loss and skin lesions are very extensive. Several oral drugs are available as well, and oral treatment, topical treatment, or a combination of both may be most effective depending on the case. Never give topical medications orally, as dips and ointments and creams may be harmful or fatal if swallowed.
Ringworm is often tenacious, and treatment typically takes about three months. Nail/claw infections require much longer treatment—often from 6 to 12 months.
A vaccine is licensed for control of Microsporum canis infection in cats. It has not proven to be very effective, and is not recommended for use except in some cattery situations.