Cat Ringworm: Diagnosis and Treatment

In cats, ringworm is a very common contagious skin infection — and it’s not actually a worm at all. The medical term for ringworm infection is dermatophytosis (derm means “skin” and phytosis means “an infection or a disease caused by a parasitic plant”). Ringworm is a fungus that infects the skin, fur or nails of cats, dogs and even humans. It’s called “ringworm” because the typical skin lesions in humans (and in some animals) can appear as a circular patch of hair loss with crusting in the shape of a worm.

How is ringworm in cats spread?

There are more than 40 different fungi that can cause ringworm, but only three are common in felines. During infection, thousands — or even millions — of microscopic spores (think of these as dust particles) are produced around infected hairs, serving as the main source of infection for other pets. Infected hairs and spores are shed into the pet’s environment, and infection is caused either by direct contact with the infected animal or by exposure to objects (such as furniture or a pet comb) in the contaminated environment. Spores can remain infectious for up to two years, especially in a moist, humid environment, and they are difficult to kill.

Fortunately, most healthy adult cats have some natural resistance to ringworm. Kittens and cats under a year old are the ones most affected, since their immune systems are not yet fully developed. Other felines who are more at risk of contracting ringworm include those with a suppressed immune system from diseases or overuse of steroids, senior or free-roaming cats, and cats who are under stress, malnourished or ill. Genetic factors may also play a role, as Persian cats appear to be more susceptible.

In most animals, ringworm is self-limiting, in that it will cure itself over time. However, because this infection can be transmitted from cats to other animals and to people, most veterinarians recommend treatment.

What does ringworm look like on a cat?

Ringworm lesions can vary. Some pets have severe skin disease, while others have only very minor lesions or almost no obvious lesions at all. The affected area may start as a small spot and continue to increase in size, and may or may not be irritated. Classic skin lesions are discrete circular areas of hair loss, usually on the head, ears or legs. The hairs surrounding these areas may be broken and the skin is often scaly or red and inflamed. Ringworm can also occur in the nails, causing them to grow malformed. 

How is ringworm diagnosed?

Because ringworm can look like flea allergy dermatitis or feline acne, ringworm cannot be diagnosed by simply looking at a lesion. Three diagnostic tests are commonly used:

• Wood’s lamp examination: This test uses a special lamp that emits ultraviolet light of a particular wavelength. Hairs infected with one type of ringworm fungus will fluoresce an apple-green color.
• Fungal culture: Hairs are carefully plucked from the suspicious lesions and inoculated onto the test medium, a special fungal culture material. This is the most reliable way of diagnosing ringworm infection, but it can take up to three weeks to get a result.
• Polymerase chain reaction: This test checks for the fungal DNA in the sample submitted and must be done at a commercial veterinary laboratory. The test can provide results in five days, although it can show false positives or false negatives.

For more about the pros and cons of each test, we recommend that you talk to your veterinarian.

How is ringworm treated?

In cats who have small, isolated lesions, ringworm is often treated with a topical anti-fungal cream. In more severe cases, a combination of oral medications and whole body “dips” in a medicated solution is employed. Typically, a dip called “lime sulfur” is used and is very effective and safe. However, it has the odor of rotten eggs and can temporarily turn the coat yellow if the cat has light-colored fur.

If a cat has very long fur (such as a Persian or Himalayan), the areas with lesions are clipped so that the dip can contact the skin. Some veterinary dermatologists recommend that all long-haired cats be shaved when treating ringworm.

Oral medications are commonly used for any cat with severe generalized lesions, for long-haired cats and in cases where the nails are infected. It is important to discuss oral medications with your veterinarian, since they can affect the liver, and liver enzymes should be checked both before and during therapy. Treatment is generally continued until there have been two negative cultures one week apart.

Can my other pets get ringworm from my cat?

To prevent your other animals from getting ringworm, a thorough cleaning is in order. Keep in mind, however, that not all animals housed together will get ringworm, even if they are exposed. If your other pets have strong immune systems and low stress levels, it’s highly possible that they will not experience infection.

First, restrict the cat to one room or area of your home during treatment. Then clean all other areas thoroughly. The spores are very light and are carried in the air, so wherever there is dust and hair, there may be spores. Avoid sweeping and other types of cleaning that could spread spores through the air. Vacuuming, damp mopping and using a Swiffer-type mop are recommended. Carpets should be steam-cleaned where possible. After vacuuming or mopping all hard surfaces, clean with water and a detergent solution. Next, wipe the surfaces with diluted bleach (one part bleach to nine parts water) and leave on for at least 10 minutes. All bedding, kennels and carriers should be thoroughly cleaned and disinfected with bleach, too.

Even if they have no lesions, your other pets should be tested. We recommend bathing them just before or after cleaning your home, to remove any spores from their coats that could cause a false positive result. Once test results are back, you’ll know whether treatment is needed for them as well. Although ringworm is labor-intensive to treat and requires patience, we can be thankful it is treatable and usually has no long-term consequences.