Ringworm in Dogs

By Ryan Llera, BSc, DVM; Ernest Ward, DVM; Updated by Amy Panning, DVM

Infectious Diseases, Medical Conditions, Zoonosis & Human Health, Pet Services

What is ringworm?

close-up view of ringworm lesions above a dog's nose

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' marking the boundary of inflammatory lesions in people infected with the disease. The common name of ringworm is somewhat misleading, in that it is not an infection caused by a worm, and the infected areas are not always ring-shaped.

The fungi responsible for ringworm infections belong to a specialized group known as dermatophytes, so the medical name for this disease is dermatophytosis. There are several distinct species of dermatophytes. Some species of dermatophytes are species-specific with infection, meaning that they will 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 on combs, brushes, food bowls, furniture, bedding, carpet, or other environmental surfaces for up to 18 months. 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.

"Ringworm is contagious and transmission occurs by direct contact with the fungus."

What does ringworm in dogs look like?

In the dog, 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 nails may occur. 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?

Veterinarian scanning a puppy's fur with a Wood's lampSome cases of canine ringworm will glow with a yellow-green fluorescence when the skin and coat are examined in a dark room under a special ultraviolet lamp called a Wood's lamp. However, not all cases show clear fluorescence and some other species of dermatophytes do not fluoresce under a Wood’s lamp. Therefore, additional diagnostics may be needed to confirm that there are ringworm fungi present.

The most accurate method for diagnosing ringworm in dogs is by a culture of 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.

"The most accurate method for diagnosing ringworm in dogs is by a culture of the fungus in a laboratory."

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 be eliminated. Ringworm cultures will be taken periodically after the start of treatment to determine if your pet is still infected.

DO NOT stop treatment unless your veterinarian has made this recommendation. Stopping treatment too soon can result in a recurrence of the fungus.

If there is more than one pet in the household, try to separate infected from non-infected animals and just treat the infected ones. In some situations, it may be preferable to treat all the pets. Your veterinarian will advise you on the best treatment given your individual circumstances.

Topical Treatment
Occasionally, topical therapy is used alone for the treatment of ringworm, but more commonly it is used in combination with oral medications. 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 are aided by using a chlorhexidine + miconazole-based shampoo or a lime-sulfur dip that can be used twice weekly.

Shaving the hair in small areas (if only one or two areas are affected) or all of your dog's hair may be recommended by your veterinarian. It is extremely important to only use preparations that have been specifically provided or recommended by your veterinarian for topical treatment of dogs. Topical treatment will usually be necessary for a period of several weeks to several months.

Oral Treatment
In most cases of ringworm, effective treatment will require the administration of an oral anti-fungal drug. The most widely used drug for this purpose is griseofulvin (Fulvicin®, Gris-Peg®, Grisovin®), although newer drugs such as itraconazole (Itrafungol®, Sporanox®, Onmel®) or terbinafine are being used more frequently and are often preferred since they have fewer side effects. The response of individual dogs to treatment varies and if therapy is stopped too soon, the disease may recur. Usually, treatment lasts for a minimum of six weeks, and in some cases much longer therapy is required.

Environmental Cleaning
Infected hairs contain numerous microscopic fungal spores that can be shed into the environment. Infection of other animals and humans can occur either by direct contact with an infected dog or through contact with fungal spores in a contaminated environment. In addition to minimizing direct contact with an infected dog, it is important to keep the environment as free of spores as possible. Clipping the hair (with its careful disposal) combined with topical antifungal treatment of affected areas of skin may help to reduce environmental contamination. It is important to remove pet hair from floors or furniture, as it may be contaminated with fungal spores. It is worthwhile to restrict the dog to rooms of the house that are easy to clean.

"Infection of other animals and humans can occur either by direct contact with an infected dog or through contact with fungal spores in a contaminated environment."

Environmental contamination can be minimized by thorough damp mopping or vacuum cleaning of 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 using the dilution of one pint of chlorine bleach (500 ml) in a gallon of water (4 liters) where it is practical to use it.

How long will my dog be contagious?

Infected pets remain contagious for about three weeks if aggressive treatment is used. The ringworm infection will last longer and remain contagious for an extended time if only minimal measures are taken or if you are not compliant with the prescribed approach. Minimizing exposure to other dogs or cats and to your family members is recommended during this period. Two consecutive negative fungal cultures will indicate successful treatment of your dog.

Will my dog recover from a ringworm infection? 

The majority of dogs, if treated appropriately, will recover from a ringworm infection. Symptoms may recur if the treatment is discontinued too early or is not aggressive enough (i.e., only topical treatment was used), or if the pet has an underlying disease compromising the immune system. Occasionally, despite appropriate treatment, the infection persists. In this situation, your veterinarian may have to try alternative anti-fungal drugs.

What is the risk to humans?

Ringworm can be transmitted quite easily to humans, especially young children, and 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 and cause clinical signs in humans who have a depressed immune system. If any people in the house develop skin lesions such as small patches of skin thickening and reddening with raised scaly edges, seek medical attention immediately. Ringworm in humans generally responds very well to treatment. 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 hands thoroughly afterward.

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