I. What is Ringworm?

Ringworm is not actually caused by a worm at all. Ringworm is an infection of the outer layers of the skin, hair and nails caused by one of several fungi called dermatophytes. The most common fungi that cause ringworm in a dog are Microsporum canis, Micorsporum gypseum, and Trichophyton mentagrophytes. Because these dermatophytes prefer a warm, moist environment, ringworm is seen more commonly in dogs living in hot, humid climates but dogs anywhere can be infected.

Dogs get ringworm infections by coming into contact with contaminated soil or coming into contact with another infected animal. Not all dogs that are exposed to one of these fungi will become infected with ringworm. Age, stress level, status of the immune system, nutritional status, and the presence of other diseases will influence whether or not a dog gets ringworm when it is exposed. Dogs younger than one year of age, older dogs and those dogs that have other diseases are more likely to develop a ringworm infection.

II. What to Watch for at Home

• Circular areas of hair loss are the most common symptom. There may be one or many.


• Crustiness or scaling in and around the areas of hair loss can be seen.


• The areas of hair loss may be itchy.


• The skin in healing areas may turn dark or black.


• If the nails are affected, they can be cracked, brittle, or have black colored discharge on them.

A dog with many ringworm lesions

III. The Veterinary Visit – How to Tell if a Dog has Ringworm

There are two commonly used tests to diagnose ringworm. The first is a black light or Wood’s lamp. Approximately 50% of M. canis organisms will turn a bright green color (fluoresce) when viewed under a Wood’s lamp. If the Woods lamp test is negative, a fungal culture is the next step. Hairs from around the edges of the hairless areas are plucked and placed in a special culture container. This container is then monitored for 10-20 days for growth of a fungus. The fungal culture is the best test for diagnosing ringworm.

A positive ringworm culture

IV. Treatment

Because people can get ringworm from infected dogs, it is important to treat all ringworm cases aggressively using a three-pronged approach.

• Topical therapy – Topical therapy consists of antifungal shampoos used every five days and antifungal creams or ointments. A full body clip should be given to long-haired animals and animals with extensive disease. In some cases with small or very few lesions, topical therapy alone can be used.


• Oral Therapy – Oral antifungal medication should be used in all dogs over 12 weeks old. Puppies younger than 12 weeks should not be give oral antifungal medication


• Environmental Treatment – Any surface that can be should be cleaned with a 1:10 dilution of bleach. Do not use this on any fabric or surface that would be discolored or ruined by applying bleach. Wash all of the dog’s bedding and thoroughly vacuum carpet and furniture that the dog has contacted. Change or clean all air filters.

If other pets are in the household, they should be treated also. If there are long-haired cats in the household, their hair should be cultured. Long-haired cats can be carriers of the fungal organisms without developing symptoms. Wash your hands thoroughly after petting or touching your dog and after medicating your dog. Some antifungal medications require that you wear gloves while handling them. Follow all directions carefully. See your doctor immediately if you develop any skin lesions.

The information in this article is not intended to provide a diagnosis or treatment for your pet and is not a substitute for examination and appropriate treatment by your veterinarian.