Ringworm is a fungal infection that invades the hair and hair follicles. Most cases are caused by Microsporum canis. Ringworm in dogs is primarily a disease of puppies and young adults. Typical areas of involvement are the face, ears, paws, and tail.
Ringworm is transmitted by spores in the soil and by contact with the infected hair of dogs and cats, typically found on carpets, brushes, combs, toys, and furniture. Humans can acquire ringworm from pets, and vice versa. Children are especially susceptible.
Ringworm takes its name from its classic appearance: a spreading circle of hair loss with scaly skin at the center and a red ring at the periphery. Note, however, that many cases of circular hair loss thought to be ringworm are actually localized demodectic mange or hair follicle infection. Black fly bites in the groin area in the spring will also have this ringed appearance. Atypical ringworm is common, with irregular areas of hair loss associated with scaling and crusting.
Ringworm by itself is not an itchy skin condition, but secondary bacterial infection with scabs and crusts can provoke licking and scratching. Ringworm can invade the nails. This results in nails that are dry, cracked, brittle, and deformed.
A kerion is a round, raised, nodular lesion that results from a ringworm fungus in combination with a bacterial infection that invades the roots of the hair. In most cases the fungus is Microsporun gypseum and the bacteria is a type of Staphylococcus. Kerions occur on the face and limbs. Therapy involves treating the bacterial component, as described in Folliculitis.
Ringworm mimics many other skin diseases, so an accurate diagnosis is essential. Hair infected by Microsporum canis may fluoresce green under an ultraviolet light (called a Wood's light), but false positive and false negative results are common. Ultraviolet light is used as a screening tool only. Microscopic examination of hairs plucked from areas that fluoresce can sometimes provide an immediate diagnosis, but the most reliable method of diagnosing ringworm is by fungal culture. Some hairs from the abnormal area are plucked and placed on a special medium to grow out any fungus that is present. Results may take up to two weeks.
Treatment: Although mild cases resolve spontaneously in three to four months, all cases of ringworm should be treated to prevent progression and spread to other pets and people in the household.
With only one or two areas of involvement, apply a topical antifungal agent containing miconazole 2 percent cream or 1 percent lotion twice a day in the direction of the lay of the hair. Continue treatment until the skin is healed. Be ready to treat the dog for at least four to six weeks.
When several sites are involved, repeat the treatment just described, and add an antifungal shampoo containing miconazole or another shampoo labeled for the treatment of ringworm. Continue to treat for two weeks beyond apparent cure. Longhaired dogs may need to be shaved for effective treatment.
In difficult cases your veterinarian may prescribe an antifungal drug called Griseofulvin. Ketoconazole, itraconazole, and other drugs of the imidazole group are also effective, but may not be approved by the FDA for treating ringworm in dogs. Griseofulvin, itraconazole, and ketoconazole have serious possible side effects and should not be given to pregnant bitches because they may cause birth defects. Antifungal drugs require close veterinary counseling and supervision.
Prevention: Ringworm spores can survive for up to one year and should be eliminated from the environment. Discard the dog's bedding. Sterilize grooming equipment in a 1:10 dilution of bleach. Vacuum the carpets at least weekly to remove infected hair. Mop and wash hard surfaces (floors, countertops, dog runs) using diluted bleach. Technical Captan can be used as a spray in a dilution of 1:200 in water to treat a kennel.
There is a vaccine for cats for ringworm, primarily used in catteries with longstanding problems, but nothing for dogs at this time.
Strict hand-washing precautions are necessary to prevent human contamination. Children should not be allowed to handle pets with ringworm. Wash contaminated clothing and fabrics with bleach.
This article is excerpted from “Dog Owner’s Home Veterinary Handbook” with permission from Wiley Publishing, Inc.