Ringworm in Dogs

What is ringworm?

Ringworm is the common name of an infection of the skin, fur or nails. These infections can occur in humans and all species of domesticated animals. In dogs, about 70 percent of ringworm cases are caused by the fungus Microsporum canis, 20 percent by Microsporum gypseum, and 10 percent by Trichophyton mentagrophytes.

The name comes from the classical appearance of the round, red, raised “ring,” which marks the boundary of the inflammatory lesions in people affected with the disease. The name “ringworm” is somewhat misleading, in that the infection is not caused by a worm and the infected areas are not always ring-shaped.

The fungi responsible for ringworm infections belong to a specialized group known as dermatophytes, so the medical term for this disease is dermatophytosis. Some species of dermatophytes are species-specific with infection. This means they will only infect one species, such as dogs or cats, whereas others can be spread between different species of animals or from animals to humans.

How is ringworm transmitted?

Ringworm is contagious and transmission occurs by direct contact with the fungus. Most cases are spread by handling infected animals or contaminated objects. The fungus produces infective seeds called spores, which are quite hardy and difficult to get rid of in the environment. Spores may remain dormant on combs, brushes, food bowls, furniture, bedding, carpet or other environmental surfaces for up to 18 months.

Contact with ringworm fungus does not always result in an infection. Whether an infection develops depends on the fungal species and on other factors involving the host, such as age, health, condition of exposed skin, grooming behavior and nutrition. After an infection, there will be a short-lived resistance to reinfection.

Under most circumstances, dermatophytes grow only in the dead cells of skin and hair, and infection stops when it reaches living cells or inflamed tissue. As inflammation and host immunity develop, further spread of infection stops, but this process may take several weeks.

What does ringworm in dogs look like?

Infected dogs will have bald, scaly patches on their skin with brittle, broken hairs and nails. Dogs may also develop acne-like bumps on the skin. Other skin conditions, such as staph infections, may have a similar appearance.

The most common sites affected by ringworm are the face, ear tips, tail and feet, but they can appear on any part of the body. Some dogs may have ringworm fungi present on their fur or skin without showing any signs of the disease. These dogs can spread ringworm to other animals or people despite having no obvious skin lesions.

How is a ringworm infection diagnosed?

Some cases of canine ringworm will glow with a yellow-green fluorescence when the skin and coat are examined in a dark room under a special ultraviolet lamp called a Wood’s lamp. However, not all cases show a clear fluorescence, and some other species of dermatophytes do not fluoresce under a Wood’s lamp. Additional diagnostics may be needed to confirm a diagnosis of ringworm.

The most accurate method for diagnosing ringworm in dogs is by culturing the fungus. Samples of fur from the affected area are placed on a special gold-colored agar media. If there is ringworm fungus present, the agar will turn bright red, usually in 24 to 48 hours. In some cases, the fungal spores may be slow to grow, and culture results can take up to three weeks. Samples for cultures may be sent to the laboratory or done in your veterinarian’s office, if they have the DTM (Dermatophyte Testing Media) containers.

What is the treatment for ringworm in dogs?

The most common way to treat ringworm in dogs is to use a combination of topical therapy, such as creams, ointments or shampoos, along with systemic therapy, which is oral anti-fungal medication. The extent of the infection will determine how much of each is required. For treatment to be successful, all environmental contamination must be eliminated.

If there is more than one pet in the household, try to separate the infected pets from the non-infected ones. This way you can just treated the infected ones. If separation is not possible, or if all the pets have been exposed, it may be best to treat all of them. Your veterinarian can advise you on the best plan for your situation. Ringworm cultures should be repeated during treatment to determine if your dog is still infected.

In mild cases, topical therapy may be used alone for the treatment of ringworm. Various creams and ointments containing miconazole (Micaved), terbinafine (Lamasil) or clotrimazole (Otomax) are available to apply to localized areas of the skin affected by ringworm. These can be combined with a chlorhexidine and miconazole-based shampoo or a lime-sulfur dip (horrible smell) that can be used once or twice weekly.

It may be necessary to shave the fur in small areas around the spots; if the infection is all over the body, you may need to shave the entire dog. Topical treatment may be necessary for a period of several weeks to several months.

More severe cases of ringworm will require the administration of an oral drug to cure the fungal infection. The most widely used drug for this purpose is griseofulvin (Fulvicin, Gris-Peg, Grisovin). Newer drugs such as itraconazole (Itrafungal, Sporanox, Onmel) or terbinafine are being used more frequently and are often preferred since they have fewer side effects.

The response of individual dogs to treatment varies, and the disease may recur if therapy is stopped too soon. Usually, treatment lasts for a minimum of six weeks, and in some cases, much longer therapy is required.

Griseofulvin is a known teratogen, meaning it will cause birth defects in fetuses if taken during pregnancy. When given during the first trimester, such defects such as cleft palates, and skeletal and brain malformations have been reported. Since ringworm is not a life-threatening infection and other therapies are available, griseofulvin should be considered contraindicated during pregnancy.

Infected hairs contain numerous fungal spores that can be shed into the environment. Infection of other animals and humans can occur either by direct contact with an infected dog or through contact with fungal spores in a contaminated environment. The fur should be clipped and carefully discarded. Topical antifungal cream can be applied to affected areas of skin to help reduce environmental contamination.

It is important to remove pet hair from floors or furniture, as it may be contaminated with fungal spores. It will help to restrict the infected dog to rooms of the house that are easy to clean.

Contamination of the environment can be kept to a minimum by daily damp mopping or vacuuming the areas accessible to the dog. Fungal spores may be killed with a solution of chlorine bleach and water using the dilution of one pint of chlorine bleach (500 milliliters) in a gallon of water (4 liters).

How long will my dog be contagious?

Infected pets remain contagious for about three weeks if aggressive treatment is used. The ringworm infection will last longer and remain infectious for an extended time if only minimal measures are taken. Avoiding exposure to other pets and family members is recommended during this period. Two consecutive negative fungal cultures will indicate the infection has been resolved.

What is the risk to humans?

Ringworm can be transmitted easily to humans, especially young children. It is important to take appropriate steps to minimize exposure to the fungus while the dog is being treated. Humans who have a depressed immune system are more susceptible to ringworm infection.

Medical attention should be sought immediately if any people in the house develop skin lesions such as small patches of thickened, red skin with raised scaly edges. Ringworm in humans generally responds well to treatment.

The ringworm fungus can remain infectious for up to 18 months in the environment and re-infection may occur. It is important to wear gloves when handling infected animals and wash hands thoroughly afterward.