Ringworm in dogs: symptoms, spread and treatment

Overview

Dermatophytes, the fungi causing ringworm, feed on keratin in the skin's surface layer and hair. They can also grow in claws because these contain keratin too. As fungus breaks down hair keratin, the hair becomes brittle and may break near the skin's surface. Follicular inflammation and hair damage can leave a clearly defined bald patch.

Most dermatophytes grow in the dead cell layer of skin and hair. The body's immune response may stop fungal progression in the skin. Spores can also be carried on an animal's coat without fungal growth in its hairs.

Causes

Dermatophytes and spores

Dermatophytes found in dogs include Microsporum canis and Trichophyton mentagrophytes. A cat can shed Microsporum canis spores without obvious skin lesions. Nannizzia gypsea, formerly called Microsporum gypseum, is a soil-dwelling dermatophyte. Acquiring it from soil can cause an inflamed skin lesion.

Fragmentation of fungal filaments produces small infectious spores. Fragments breaking from infected hairs can spread spores. Under favourable conditions, spores can remain infectious for over 12 months.

How does infection spread?

Direct contact with an infected animal is the most common route. Spores can also transfer from shared brushes, bedding and collars. Exposure alone does not always lead to skin infection. Skin moisture may favour infection after exposure. A small skin injury can facilitate infection if equipment carrying spores contacts it. Spores can begin germinating and entering skin or hair within 6–8 hours under favourable conditions. Visible skin signs may not appear until 1–3 weeks after exposure.

Risk factors

Ringworm is more common in young dogs than healthy adults. Very old or immunocompromised animals are also more susceptible. Working and hunting dogs may encounter spores in contaminated environments. Ringworm is more common among animals kept in crowded conditions.

In Portugal, 9.1% of dogs investigated for suspected ringworm had a positive fungal culture. Yorkshire Terriers were more often positive than other breeds in that investigation. Persistent, widespread ringworm has also been described in Yorkshire Terriers.

A Turkish study cultured dermatophytes from 13 of 45 dogs with skin lesions. Five of the 13 findings were Microsporum canis. Trichophyton rubrum was found in 2 dogs. This dermatophyte is common in people but rare in animals.

Typical symptoms

Canine ringworm often affects the face, ear tips, tail or paws. In hunting dogs, lesions may occur particularly on the muzzle and feet.

• Round patches of hair loss may appear.
• Dry scales may be visible on the patches.
• Hairs may break over the affected area.
• Skin may become red and crusted.
• Small pimple-like spots may appear.
• Itching can occur, but its intensity varies.

A round patch may start healing from its centre, creating a ring-like appearance. Ringworm can also cause a raised, red, discharging nodule called a kerion. Lesions may affect one side of the body or several sites. A claw may grow abnormally if affected by the fungus.

Complications and consequences

With a deeper inflammatory reaction, hair fragments containing fungal spores can escape from follicles. A kerion can contain intense inflammation with an accumulation of inflammatory cells. Generalised ringworm can cause numerous patches across the body, particularly if your dog has another disease or is under physical strain. An itchy patch may cause repeated licking or scratching of the same area. Canine ringworm is also a zoonosis, meaning it can cause skin lesions in people. In one northern European survey in 2009, 4.2% of responding vets reported having had ringworm. Reported Microsporum canis infections in that survey were linked to cats. There is no reliable overall estimate of the frequency of animal-to-human ringworm transmission.

Diagnosis at the vet

Skin examination and sampling

Your vet assesses skin lesions and asks about possible exposure. Hairless patches can resemble demodicosis caused by follicular mites. Bacterial folliculitis can also look like ringworm. More than one method is often used to confirm the diagnosis.

UV light from a Wood's lamp can make Microsporum canis-infected hairs fluoresce apple green. Fluorescing hairs can be selected for microscopy or culture. As treatment progresses, only the hair tip may still fluoresce. Soap residue or lint can also fluoresce, but its colour differs from the apple-green glow of infected hair. Not all dermatophytes cause fluorescence, so a negative lamp examination does not exclude ringworm. Hairs or scales are examined microscopically for fungal structures. A biopsy from a deep nodule can be specially stained to reveal fungi.

Culture and PCR testing

Culture establishes which dermatophyte grows from a hair or scale sample. Your vet may collect samples from lesions with a new, clean brush. A negative culture may take at least 4 weeks to allow time for slow-growing fungi. A positive result can be issued once the fungus is identified.

PCR looks for fungal genetic material in hairs or skin crusts. It can also detect remains of dead fungi after successful treatment. In a Canadian shelter study, 18.75% of 48 dogs investigated for suspected ringworm had positive cultures. Results for all 9 culture-positive dogs were available within 7 days. PCR detected Microsporum canis in 88.9% of the 9 dogs in which it grew on culture. In the same comparison, PCR was negative in 38 of 39 dogs without Microsporum canis growth.

For a kerion, adhesive tape can collect surface material for microscopy. In a small study, tape sampling identified dermatophytes in 90.9% of 11 dogs with kerion. Microscopy of plucked hairs detected infection in 36.4% of the same group.

In another study, Microsporum persicolor grew from all 16 dogs despite negative hair microscopy. This species grows in surface skin keratin, so scales may be a better microscopic sample than hairs.

Treatment

Treating the skin and coat

A healthy adult dog may recover without treatment, but treatment can shorten the infectious period. An oral antifungal acts on active infection. Medicated shampoo or rinses reduce infectious spores on the coat. Topical treatment often covers the whole coat because spores can travel beyond the visible patch. An antifungal cream applied to an individual patch can support other treatment.

Itraconazole and terbinafine are oral options. Your vet chooses treatment according to lesion extent and your dog's health. Fluconazole is less effective against dermatophytes than the commonly used alternatives.

Recovery and follow-up samples

Fungal test results may take 4–6 weeks or longer to become negative. In one treatment study, all 20 dogs were culture-positive for Microsporum canis before treatment. In the small group receiving topical and oral antifungals, clinical recovery was confirmed by culture by week 6. Clinical recovery means existing lesions disappear and no new patches appear. A follow-up hair sample is still used to assess whether fungal infection has ended.

In an otherwise healthy dog whose symptoms have resolved, 1 negative culture may support an assessment that infection has cleared. Another monitoring approach confirms recovery with 2 negative follow-up samples. The interval between samples may then be about a month.

Prevention

Clean areas used by your dog during treatment to reduce infectious hairs and spores in the environment. Vacuuming removes loose hairs and skin cells from places your dog spends time. Wash hard surfaces with detergent after removing loose dirt. Disinfection is less effective when hair or other organic debris remains. Dog bedding and towels can be machine-washed without bleach. Vacuuming carpets removes loose hair, but infectious material may remain in the fabric. Deep carpet cleaning can use a carpet washing machine or steam cleaning.

Laundry must be able to move freely in the washing machine so hairs and spores detach from fabric. In one experiment, 65% of towels directly exposed to hairs from infected cats still cultured dermatophytes after the first wash. After a second wash, cultures from the same towels were negative.

Limit close contact between an infected dog and other animals during treatment. A puppy's socialisation needs must also be considered when arranging contacts. During infection, pause close contact with other dogs at day care or training groups. Examine other pets' coats and skin if ringworm is diagnosed in the household.

Important for dog owners

Wear gloves and long sleeves when handling an infected dog or applying prescribed medicine. Wash your hands with soap and water afterwards. Keep the affected dog's brushes separate from other animals' equipment because brushes can carry spores.

If a new ring-shaped, scaly patch appears on your skin, contact a healthcare professional. Mention your dog's ringworm so it can be considered when investigating your rash. Keep children and immunocompromised household members away from the infected dog during treatment.

Frequently asked questions

Is every round bald patch ringworm?

A round, hairless, scaly patch is consistent with ringworm, but mites and bacteria can cause similar lesions. Your vet can collect hairs and scales for microscopy or fungal culture. Culture can establish whether live dermatophytes grow from the lesion sample.

Can a pet without symptoms carry ringworm?

Dermatophyte spores can contaminate an unaffected animal's coat from the environment. An infected long-haired cat may have mild or no symptoms while still shedding spores. If ringworm is diagnosed at home, other pets' skin should also be checked.

Is examining the coat under a UV lamp enough?

A Wood's lamp helps find fluorescing hairs for further sampling. Apple-green fluorescence is particularly associated with Microsporum canis-infected hairs. If hairs do not glow, your dog may still have an infection caused by another dermatophyte species.

When is recovery confirmed?

Clinical recovery means old patches have disappeared and no new lesions develop. Culture or PCR can help assess clearance after the skin heals. The number of follow-up samples needed depends on the monitoring approach and your dog's other conditions.

How can I reduce spread to family members?

Wash your hands with soap and water after handling your dog. Wear gloves and cover your forearms when treating affected skin. Vacuum areas your dog uses to remove loose hairs and machine-wash their bedding.