Canine Ringworm

What causes ringworm?

Ringworm is a superficial skin disease caused by infection with one of several types of fungus. It typically causes circular areas of hair loss, which is how the condition got its name. It was originally thought that the rounded patches were caused by a worm that curled up in the skin. We now know that this is not correct. The medical term for ringworm is dermatophytosis because the fungi that cause it are called dermatophytes.

These fungi live in the hair follicles, deep within the skin. They damage the hair shaft, which either falls out or breaks off at skin level. As the infection spreads, the hairless area radiates out like the ripple caused by a stone dropped into a pond. Not all ringworm lesions are round; they can be irregular in shape, and quite variable in size. Ringworm can be spread from one dog to another, so the most common sites for ringworm are where dogs contact one another: on the face and body. It usually takes 10-12 days after exposure before lesions develop.

Can I get ringworm from my pets?

The fungi that cause ringworm can be spread from animal to animal, from animal to human, and even from human to animal. In both animals and humans, healthy adults are somewhat resistant to ringworm, but young animals and children are quite susceptible. Healthy adults can still get ringworm, but usually the infection is mild and resolves quickly.

These fungi can also survive in the environment, on contaminated bedding, collars, blankets, and other items that may have been in contact with an infected animal. The fungi can be killed by washing the item with a solution of chlorine bleach (1 pint of chlorine bleach in a gallon of water).

How is ringworm diagnosed?

Your veterinarian may diagnose ringworm a couple of ways:

1. Appearance of the skin lesions

Typical roundworm lesions are rounded patches of hairless skin. The hair may be easily and painless pulled out from around the edges of the hairless area if the infection is still progressing. The bare skin may be scaly, scurfy, or crusty, but in most cases the ringworm lesions are neither itchy nor painful, and the skin is undamaged.

2. Use of a Wood's light (a special type of ultraviolet lamp)

Two of the four species of fungi that commonly cause ringworm in pets glow, or fluoresce, when examined under a Wood's light.

3. Culture of the hair for fungi. With this method, samples of hair from the edges of the lesions are collected and placed on a special culture media. Fungi grow very slowly, so it may take weeks to get a result.

How is ringworm treated?

Treatment depends on how extensive the infection is, whether the dog has any health problems that are compromising its immune function, how many pets are affected, and whether there are children in the household. Treatment options include the following:

1. Antifungal shampoo

There are several antifungal shampoos available. The most effective strategy is to bathe your dog using one of these shampoos every other day for three treatments. You should also bathe any exposed but unaffected pets once. When using an antifungal shampoo, lather up the skin over the dog's entire body (being careful to avoid the eyes, nostrils, and mouth) and leave it on for 5 minutes before thoroughly rinsing the dog.

2. Lime sulfur dip

Lime sulfur is a foul-smelling but inexpensive and effective antifungal product. (It is the same compound often used by gardeners to treat fungal infections in plants.) When using this treatment, thoroughly soak the dog's coat with lime sulfur twice a week for 2 weeks, then once a week for another 4-6 weeks. Be sure to wear gloves when applying this dip; apart from the rotten-egg odor, lime sulfur can tarnish jewelry.

3. Topical antifungal cream

Topical antifungal creams are applied only to the ringworm lesions. The usual recommendation is to apply the cream once daily for 10 days, and rub it well into the skin. Take care when treating lesions close to the dog's eye. It is also a good idea to use an antifungal shampoo or dip to remove any fungal spores from the dog's coat.

4. Oral antifungal medication

Griseofulvin is an antifungal drug that can be given orally as a tablet. Once absorbed from the intestine, the drug is delivered to the skin where it destroys the fungi living deep within the hair follicles. The drug is given daily for at least 30 days.

This drug is not well absorbed from the intestine unless it is given with a fatty meal. The usual recommendation is to give the tablets with a small amount of high-fat food, such as a rich canned dog food, fat from meat trimmings, or some cream. Some dogs cannot tolerate even a small fatty meal. So be sure to let your veterinarian know if fat consumption has caused a problem for your dog or if your dog has had pancreatitis in the past.

5. Body clipping

Removing the infected hair by clipping the dog is an extreme measure that is usually reserved for extensive infections.

Your veterinarian can recommend which treatment option is best for your pet.

When should I expect to see some improvement?

It may take a couple of weeks before you see signs of new hair growth. In fact, you may even see the lesions increase in size for a week or so after you begin treatment. Although it is not obvious, the hair around the edge of the lesion may be infected and usually falls out or breaks off despite treatment. Within a week or two of beginning treatment, hair loss should stop and the bare skin should begin to look healthier (less scaly). It may, however, take several months for the hair that regrows at the lesion site to reach its normal length in a long-haired dog. Call your veterinarian if new lesions appear during or following treatment.

How long will my dog be contagious?

Even with intensive treatment, your dog may be infectious for about 3 weeks. The fungi may persist and remain active for much longer if minimal treatment is undertaken or if you do not complete the course of treatment as recommended. During the first few weeks of treatment, it is a good idea to limit handling of your dog by family members. With a complete course of appropriate treatment, your dog should be cured of this infection.

by William M. Fraser, D.V.M.

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