Dermatophytosis is the medical term for the fungal infection affecting the skin, hair, and/or nails (claws) that is more commonly referred to as ringworm. The most commonly isolated fungal organisms are Microsporum canis, Trichophyton mentagrophytes, and Microsporum gypseum. This disease occurs in dogs, cats, and other species of animals, including people. It is diagnosed more commonly in young individuals than in aduld.
The condition or disease described in this medical article can affect both dogs and cats. If you would like to learn more about how this disease affects cats please visit this page in the petMD health library.
Symptoms of Ringworm in Dogs
Symptoms of ringworm in dogs often include some combination of the following:
- Hair loss (alopecia), which may be patchy or circular
- Broken hairs and poor hair coat
- Reddened or ulcerated skin
- Dandruff (scales)
- Darkened skin
- Crusting of the skin
- Itchiness (pruritus) may or may not be present
Less frequently, dogs develop a raised nodular lesion that may ooze called a kerion. The nails and claw folds (the skin bordering the nail) may also be infected by ringworm fungus, which results in brittle or misshapen nails.
Occasionally, dogs are classified as asymptomatic or silent carriers. In other words, they harbor the disease-causing fungus but present no visible signs of the condition. These dogs can still pass the disease on to humans and other animals.
How Do Dogs Get Ringworm?
There are a few ways that dogs can get ringworm. Dogs most commonly are infected with the fungi Microsporum canis, Microsporum gypseum, and Trichophyton mentagrophytes. The incidence of these and the less common species that cause ringworm varies according to your geographic location. Dogs often catch ringworm through direct contact with animals or people who have ringworm themselves, some of whom may have little or no clinical evidence of the disease. Ringworm fungus can also be spread through contaminated objects like bedding, brushes, clippers, and cages. Some species of ringworm live in the soil, and dogs can become sick after contacting dirt that is home these organisms.
Anything that decreases the body's ability to mount an effective immune response (such as young age, immunocompromising diseases, or immunosuppressive medications) increases the likelihood that your dog will develop ringworm, as well as increase the potential for a more severe infection. Environments that are densely populated with animals (for example, in an animal shelter or kennel), or where there is poor nutrition, poor management practices, and lack of an adequate quarantine period, also increase risk of infection. Finally, disruptions to the normal protective barrier of the skin, like wounds or a flea infestation, increase a pet’s susceptibility to ringworm.
Your veterinarian will perform a fungal culture of plucked hairs or skin scales, a microscopic examination of a sample of hair, or possibly a skin biopsy if he or she suspects ringworm. Sometimes veterinarians will use a Wood’s lamp to identify where to take samples from. Some types of ringworm fungus fluoresce when exposed to light from a Wood’s lamp, but others do not. Additional testing may also be necessary.
Ringworm Treatment for Dogs
Most dogs can be treated for ringworm on an outpatient basis, but quarantine procedures should be considered due to the contagious and zoonotic (transmissible to humans) nature of many types of ringworm. In mild cases, topical treatment may be all that is needed to speed recovery and reduce the chances that the disease will spread to other animals or people. Shaving a pet with a long coat can help topical medications reach the skin. Options include lime sulfur dips, enilconazole rinses, and miconazole shampoos.
For more severe cases, a veterinarian will also prescribe oral anti-fungal drugs like itraconazole, griseofulvin, fluconazole, terbinafine, or ketoconazole. Treatment often must continue for several months and should not be stopped until followup diagnostic testing shows that the dog is free of ringworm. If an underlying condition (e.g., malnutrition, administration of immunosuppressive drugs, etc.) is thought to be playing a role in the dog’s development of ringworm, it should be addressed as well.
Living and Management
Repeated fungal cultures are the best way to monitor your dog's response to treatment. Some animals will look better with treatment, but ringworm is still present in their fur, skin, or nails. If treatment is stopped too soon, the dog may relapse and continue to pose a risk to other individuals. Most veterinarians will wait until a dog has no clinical signs of ringworm and at least one negative fungal culture before recommending that treatment be stopped. Also, monthly checks of blood work may be indicated for dogs receiving ketoconazole or itraconazole as these drugs can be toxic to the liver.
It may be necessary to screen or treat other animals (and people) in the home that have had contact with a ringworm positive pet to prevent reinfections from occurring.
Pets undergoing treatment for ringworm need to be isolated to prevent the spread of the disease to other animals or people. Wear disposable gloves and wash your skin and clothes after handling an infected pet. To decontaminate your home, thoroughly vacuum floors and upholstery and clean hard surfaces with an effective disinfectant like a dilute bleach solution. Your veterinarian can put together an appropriate plan for treatment, monitoring, and environmental decontamination based on the specifics of your dog’s case.