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.
Some dogs scent mark by urinating small amounts on vertical surfaces, usually while raising a leg. Both female and male dogs can urine mark. Dogs who urine mark might do so in a number of situations, including while on walks, when in their own homes and yards, and during visits to other locations. A dog must be at least three months of age to urine mark.
Ringworm takes its name from its classic appearance: a spreading circle of
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
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.