Ringworm in Dogs

What is ringworm?

Ringworm is the common name given to a fungal infection of the skin, hair, and nails. Ringworm infections can occur in humans and in all domesticated species of animals. The name comes from the classical appearance of the round, red, raised “ring” that marks the boundary of inflammatory lesions in people infected with the disease. The common name of ringworm is somewhat misleading, because it is not an infection caused by a worm, and the infected areas are not always ring-shaped.

The organisms that cause ringworm infections belong to a specialized group known as dermatophytes, so the medical name for this disease is dermatophytosis. Some types of dermatophytes are species-specific, which means they only infect one species, whereas others can be spread between different species of animals or from animals to humans.

How is ringworm transmitted?

Ringworm is contagious and transmission occurs by direct contact with the fungus. It may be passed by direct contact with an infected animal or person, or by touching contaminated objects or surfaces. The fungal spores may remain dormant for up to 18 months on combs, brushes, food bowls, furniture, bedding, carpet, or other environmental surfaces. Contact with ringworm fungus does not always result in an infection. The amount of environmental contamination is an important factor in the development of a ringworm infection, as is the age of the exposed animal.

What does ringworm look like?

In dogs, ringworm lesions usually appear as areas of hair loss (alopecia) that are roughly circular. As these circular lesions enlarge, the central area heals, and hair may begin to regrow in the middle of the lesion. The affected hair shafts are fragile and easily broken. These lesions are not usually itchy, but sometimes they become inflamed and develop a scabby covering. In most cases, there are several patches scattered throughout the body. Occasionally, fungal infections of the claws may occur, and the claws become rough, brittle, and broken.

Some dogs may have ringworm fungi present in their hair or skin without showing any clinical signs of disease. These dogs can spread ringworm to other animals or people despite having no obvious skin lesions.

How is a ringworm infection diagnosed?

The most accurate way to diagnose ringworm in dogs is to culture the fungus in a laboratory. To do this, samples of hair and skin scrapings are taken from the dog. A positive culture can sometimes be confirmed within a couple of days, but in some cases, the fungal spores may be slow to grow, and culture results can take up to three weeks. For this reason, PCR testing (looking for fungal DNA) may be recommended, as it is faster. However, this method can have false positives and may fail to identify the fungus species, which can be important for determining the source of the infection or its risk of human infection.

If your veterinarian suspects ringworm, they may examine the skin and coat in a dark room under a special ultraviolet lamp called a Wood’s lamp. Some cases of ringworm will glow with a yellow-green fluorescence under this lamp. However, not all cases show clear fluorescence and some dermatophyte species do not fluoresce under a Wood’s lamp, so additional diagnostics may be needed to confirm that ringworm fungi are present. 

There are numerous causes of hair loss in dogs. Before making a diagnosis of ringworm, your veterinarian may recommend additional testing to rule out other possible causes.

How is ringworm in dogs treated?

The most common way to treat ringworm in dogs is to use a combination of topical therapy (application of creams, ointments, or shampoos) and systemic therapy (administration of anti-fungal drugs by mouth). For treatment to be successful, all environmental contamination must also be eliminated. Ringworm cultures will be done periodically after the start of treatment to determine if your pet is still infected. DO NOT stop treatment unless your veterinarian tells you to. Stopping treatment too soon can result in a recurrence of the fungus.

If there are other pets in the household, try to separate infected animals from non-infected animals and treat only the infected ones. In some situations, it may be preferable to treat all the pets. Your veterinarian will advise you on the best treatment for your circumstances.

Topical Treatment

Occasionally, topical therapy is used alone to treat ringworm, but more commonly it is used in combination with oral medication. Various creams and ointments containing miconazole (Micaved®), terbinafine (Lamisil®), or clotrimazole (Otomax® or Otibiotic®) are available to apply to localized areas of skin affected by ringworm. Often, these creams and ointments are used with a chlorhexidine + miconazole-based shampoo along with a lime-sulfur dip that can be used twice weekly. Topical treatment will usually be necessary for a period of several weeks to several months.

Shaving the hair in small areas may be sufficient if only one or two areas are affected. If there is more generalized disease, your veterinarian may recommend clipping all your dog's hair. It is extremely important to only use preparations that have been specifically provided or recommended by your veterinarian. 

After bathing or treating your dog, be sure to wash your hands and sanitize any surfaces your dog may have been in contact with using a dilute bleach solution.

Oral Treatment

In most cases of ringworm, effective treatment requires the administration of an oral anti-fungal drug. The most commonly used drugs for this purpose are itraconazole (Itrafungol®, Sporanox®) and terbinafine (Lamisil®), since they have fewer side effects. Griseofulvin (Fulvicin®, Gris-Peg®) may also be used, although it has a greater risk of side effects. The response to treatment varies and if therapy is stopped too soon, the disease may recur. Treatment usually lasts for a minimum of six weeks, but in some cases, therapy is required for much longer.

Environmental Cleaning

Infected hairs contain numerous microscopic fungal spores that can shed into the environment. Other animals and humans can be infected by direct contact with an infected dog or through contact with fungal spores in a contaminated environment. It is important to minimize direct contact with an infected dog and to keep the environment as free of spores as possible. It is worthwhile to restrict the dog to rooms of the house that are easy to clean.

You may help reduce environmental contamination by clipping the dog’s hair and carefully disposing of it, while also using topical antifungal treatment on affected skin. Use clippers, not scissors, to remove your dog’s hair, as you may accidentally cut your dog’s skin. Remove any pet hair from floors or furniture, as it may be contaminated with fungal spores.

You can further minimize environmental contamination by thoroughly vacuuming and mopping all rooms or areas that are accessible to your dog; this should be done daily. Fungal spores may be killed with a solution of chlorine bleach and water: one pint of chlorine bleach (500 ml) in a gallon of water (4 liters). Although ringworm is self-limiting (resolves on its own) in many dogs, treatment of the disease is always necessary to minimize the risk of spreading infection to humans, especially children and other pets.

How long will my dog be contagious?

Infected pets remain contagious for about three weeks if aggressive treatment is used. Ringworm will last longer and remain contagious for longer if only minimal measures are taken or if you do not follow the prescribed approach. It is recommended that you minimize your dog’s exposure to other pets and family members during this time. Two consecutive negative fungal cultures will indicate successful treatment of your dog.

Will my dog recover from ringworm?

Most dogs, if treated appropriately, recover from ringworm infection. Symptoms may recur if you discontinue treatment too early, or if treatment is not aggressive enough (i.e., only topical treatment was used), or if your dog has an underlying disease that compromises its immune system. Occasionally, despite appropriate treatment, the infection persists. In this situation, your veterinarian may try alternative anti-fungal drugs.

What is the risk to humans?

Ringworm can be easily transmitted to humans, especially young children, so it is important to take appropriate steps to minimize exposure to the fungus while the dog is being treated. Ringworm is more likely to be transmitted to humans with a depressed immune system. Seek medical attention immediately if anyone in the house develops skin lesions, especially small patches of skin thickening and reddening, with raised scaly edges.

Ringworm in humans generally responds well to treatment. However, the ringworm fungus can remain infectious for up to 18 months in the environment and re-infection may occur. It is important to wear gloves when handling infected animals and wash your hands thoroughly afterward.