Ringworm in Dogs: Symptoms, Treatment, and Prevention

Ringworm in dogs is a highly contagious fungal infection that can affect the skin, hair, or nails. It can spread between dogs, other animals, and people through direct contact or contaminated objects and environments.

Ringworm is treatable and curable, but treatment, quarantine, and environmental cleaning are important to help clear the infection and prevent reinfection.

What Is Ringworm in Dogs?

Ringworm in dogs is a fungal infection that can affect the skin, hair, or nails. Despite its name, ringworm is not actually a worm.

The name “ringworm” comes from the circular, itchy rash that typically appears on the skin of an infected animal. The medical term for this type of infection is dermatophytosis.

The most common fungal organisms that can cause ringworm are Microsporum and Trichophyton. The fungus feeds on dead hair or skin cells.

Although ringworm is most commonly found in cats, it can also affect dogs and humans. Ringworm is highly contagious and zoonotic, meaning it can spread from pets to people and from people to pets.

What Are the Symptoms of Ringworm in Dogs?

Ringworm in dogs commonly affects the face, ears, tail, and feet and may cause circular areas of hair loss, dry or inflamed skin, itching, and changes to the nails.

Symptoms of ringworm in dogs may include:

• Circular areas of hair loss, often with a red and crusty edge
• Broken hair and a poor hair coat
• Dry, scaly skin or areas of excessive dandruff
• Inflamed areas of skin
• Darkened patches of skin
• Itchiness, scratching, or excessive grooming
• Inflamed nail beds or darkened or dry nails
• Dry, brittle, or misshapen nails

What Causes Ringworm in Dogs?

Most dogs with ringworm contract it through direct contact with an infected animal or human. Ringworm can also spread through contaminated objects and surfaces or through soil containing ringworm fungi.

How Ringworm Spreads

Dogs can have the ringworm fungus on their body without showing external signs of the disease. However, they can still pass the disease to humans or other animals.

Ringworm can also be passed through contaminated objects such as brushes, dog beds, and toys, as well as surfaces that are difficult to clean, including rugs, wool, and wood.

Some types of ringworm fungi live in the soil, and a dog can contract ringworm by digging in the dirt. This is especially true in warm and humid environments. The spores of the fungi that cause ringworm are quite hardy and can live in the environment for at least 18 months.

Dogs at Increased Risk for Ringworm

Dogs more likely to become infected with ringworm include:

• Very young or old dogs
• Dogs with diseases that weaken their immune systems
• Dogs that have recently had long-term stays at kennels or shelters
• Boston Terriers, Yorkshire Terriers, and Russell Terriers, which are genetically more prone to ringworm infections
• Dogs with skin conditions

The skin normally functions as a protective barrier, but dogs with an open wound, fresh scratches, fleas, or a chronic skin condition are more susceptible to infection.

Ringworm infection occurs when spores attach to damaged skin. Skin lesions typically appear about one to three weeks after exposure.

How Is Ringworm in Dogs Diagnosed?

Veterinarians may diagnose ringworm in dogs using a Wood’s lamp examination, fungal culture, or PCR test.

Wood’s Lamp Examination

A Wood’s lamp emits long-wave ultraviolet light (a type of “black light”) to help detect bacteria. 

The fungus Microsporum canis, the most common cause of ringworm infections in pets, causes a chemical reaction when it attaches to hair follicles, causing them to glow when a Wood’s lamp shines. No fluorescence will be seen on a pet that is an asymptomatic carrier.

This “bedside” examination is effective in about 72% of M. canis cases with active skin lesions. Fluorescence usually becomes visible about five to 18 days after an infection takes hold. It is a great place to start when a veterinarian suspects a dog may have ringworm.

Fungal Culture

A fungal culture, sometimes called a DTM (dermatophyte test medium), involves collecting a few hairs, scales, or scabs and placing them on a culture medium—a substance for growing microorganisms—to allow the contaminant to grow.

Your veterinarian will then look at the growth under a microscope to determine if one of the fungal species that causes ringworm is present. Positive signs can appear after three to five days, but final results can take 10–21 days.

One benefit of this test is that it does not require an active skin lesion; any hair sample can be tested.

PCR Testing

PCR (polymerase chain reaction) testing is the newest way to diagnose ringworm. This test is also performed on hairs and specifically looks for fungal DNA, which contains the fungus’s genetic information.

PCR is much faster than a culture, with results usually available in three to five days.

PCR is a great test for an initial diagnosis of ringworm; however, it is not suitable for follow-up testing to ensure that ringworm has been fully cleared. PCR testing looks for DNA but cannot distinguish whether that DNA comes from living fungus.

Fungal culture testing is usually the recommended follow-up test to make sure the infection is completely gone.

How Is Ringworm in Dogs Treated?

Ringworm in dogs is treatable and curable if treatment, quarantine, and cleaning procedures are followed. Treatment includes quarantining your dog, topical treatments, oral medications, and environmental decontamination.

Quarantine

Ringworm is highly contagious, so when a ringworm infection is suspected, quarantine your dog. Keep your dog away from other pets and limit their contact with humans.

Ideally, keep your dog in a room that can be easily cleaned, such as a large bathroom. Avoid rooms with rugs or wood floors because these surfaces are difficult to clean.

Topical Treatments

Bathing is a critical part of treatment because it removes and kills fungal spores on your dog's fur. This helps prevent further environmental contamination and cross-contamination with other animals in the home because treated hairs will not be infectious when shed.

There are two main types of topical treatments: lime sulfur dips and antifungal shampoos. Of these, lime sulfur dips are more effective.

Lime sulfur dips can be done at home or at your veterinary hospital. They are quite messy and odorous, so many pet parents elect to have them done at their veterinarian's office.

Antifungal shampoos are often paired with a disinfectant such as chlorhexidine because they work well together to combat ringworm. These typically need to be used twice weekly.

Ointments or other topically applied medications have not been shown to be effective against ringworm infections.

Shaving dogs with very long coats is not recommended because shaving can cause microtrauma (small injuries) to the skin, which can make the dog more susceptible to the infection.

Oral Treatments

Oral medications clear the ringworm infection by making the fungus unable to reproduce and spread. In most cases, oral and topical measures are recommended together.

There are numerous types of oral antifungals with different costs and potential side effects, the most common of which are gastrointestinal.

Your veterinarian will work with you to find the right product and medication for your pet based on their health and diagnosis.

How To Decontaminate Your Home

Infected pets constantly shed fungal spores into the environment, so environmental decontamination is an important part of treating ringworm in dogs.

Isolate your dog and disinfect other areas of the house until a culture test is negative or your veterinarian recommends stopping treatment. Otherwise, your dog could become reinfected with ringworm from the environment.

Thoroughly clean all areas your dog has contacted using an effective disinfectant such as a dilute (1:10) bleach solution, accelerated hydrogen peroxide, or a similar product. Bleach will not properly disinfect a dirty surface, so clean first with an agent such as liquid dish soap and disinfect second.

Vacuum and/or steam clean your floors, and disinfect your vacuum afterward. Items such as dog beds that can be laundered should be run through a washing machine twice in hot water, preferably with bleach.

The area of your home where your pet is confined should be cleaned twice weekly during treatment. When treating a pet at home, wear disposable gloves and wash your hands and clothing after handling an infected pet.

Recovery and Management of Ringworm in Dogs

Treatment for ringworm in dogs generally continues for weeks to months and should not be stopped until follow-up testing shows that the fungal organisms are clear and your veterinarian directs you to stop treatment.

Pets can be silent carriers of ringworm, meaning they are infected but do not show any signs. Because of this, once a pet has been diagnosed with ringworm, all other pets in the home should be tested.

Ideally, a fungal culture should be done every two to three weeks after starting treatment to monitor treatment progression.

How To Prevent Ringworm in Dogs

Ringworm can spread between people, pets, and the environment, so prevention focuses on limiting exposure to potential sources of infection.

• Address human infections. A person infected with ringworm can give it to their pets. If you notice a red, ringed skin lesion on your skin, have your doctor or another medical professional examine the area.
• Isolate new stray pets. When taking in stray dogs or cats, keep them isolated from your pets until they have had a complete medical evaluation by a veterinarian.
• Limit exposure to contaminated soil. Because ringworm can be present in soil, don't let your dog dig outside, especially if there is abundant wildlife nearby, as they can be a source of infection.
• Maintain regular veterinary care. Taking your dog to the veterinarian for twice-yearly exams can also be helpful. Pets with healthy skin are much less susceptible to ringworm.