Ringworm infection in Cats

Don’t be misled by the name. Ringworm has nothing to do with worms. In fact, ringworm is the name for a nasty fungal infection of the skin. The medical name for a ringworm infection is dermatophytosis (literally ‘skin-dweller’ in Greek). Although there are about 40 different species of dermatophyte fungi which can cause ringworm infection, 90 % of the infections in cats are caused by Microsporum canis (M. canis). The fungus makes itself at home in the outer layers of a cat’s skin, hair and nails. The infection was originally called ‘ringworm’ because the skin lesions are round and have a visible ring at the edge. In the past this ring was thought to be caused by a worm embedded in the skin.

Ringworm is contagious. An infected cat can spread the fungus to other cats and also to other creatures, including dogs and humans. Ringworm in humans is unsightly and uncomfortable and still infectious. Though not medically serious, it is best treated immediately. (There are a number of good treatments available.) In cats, the lesions can become irritating and scratching the itch leads to breaks in the skin which are literally open to infection.

How can a cat get infected?

The fungal spores live in feline hair follicles, so a cat does not need direct contact with an infected animal to get the fungus. Spores (which are how the fungus spreads) are shed with hair and can persist in the environment for a long time (as long as 2 years). Infected clippers, grooming tools or bedding and carpets are the most common causes of cross-contamination. The spores (which are not visible to the naked eye) become embedded in a cat’s hair. Thereafter the spores germinate and set about invading skin and hair follicles. Any abrasion in the skin may become a point of entry.

Longhaired cats seem to be more likely to get the infection. It is not clear why, but one possibility is that the spores which are already in cat’s hair are not removed during normal grooming and brushing because cleaning long hair is that much more difficult. Another suggestion is that some longhaired species (e.g. Persians) have a genetic vulnerability.

Ringworm is most common in kittens less than one year old, probably because their immune system has not matured enough to be able to deal with the infection. Older cats with better immune systems can often clear the infection before it becomes established. However senior cats, or cats being treated with immune suppressants (e.g. steriods) are also vulnerable. Just because a cat’s immune system might be able to throw off the infection without help does not mean that an infection should not be treated as soon as it is discovered. Having a cat shed spores around the house or neighbourhood is anti-social and not pleasant for the cat whilst it is infected.

How is ringworm diagnosed?

A classic ringworm infection shows hair loss and circular skin lesions which have a clear red ring at the border. However, not all infected cats visibly lose hair or have clear signs of skin infection. Skin lesions may not be circular, and may not even be ringworm, because different types of infection can look very similar.

There are three methods the vet will use to diagnose the infection:

  1. Ultraviolet light - ringworm fluoresces under UV light, so the skin lesion under low UV light will reveal apple-green coloured hair around the lesion, as well as fluorescent patches of infected skin.
  2. Microscopic examination of the suspected hair makes spores clearly visible. This can give a rapid diagnosis but this is not enough to tell which species of dermatophyte fungus is the villian.
  3. Fungal culture is the most reliable method of diagnosis. Infected hairs are used to culture the spores. This method can determine not only if the infection is by is a dermatophyte but also the particular species. The only problem with this method is that as the fungi grow very slowly the cultures take weeks before yielding results.

Because of the ease with which ringworm spreads, if one cat in the household is diagnosed with ringworm, all your cats will have to be tested.


All affected animals should be treated by with both oral medication and by medication applied directly to the haircoat and skin (topical therapy).

Although there are number of anti-fungal ointments which successfully treat ringworm in humans, these should not be used on a cat. Many anti-fungal remedies are toxic and are definitely not recommended. Nor is putting ointment only on the lesion particularly useful, because the area of infected skin is often considerably larger than what you can see. Therefore the medication should be applied to the whole body.

The only product licensed for topical treatment of ringworm in cats is a chlorhexidine and miconazole shampoo. This is usually applied twice a week. Pet Virkon (Germicidal skin cleanser) can also be useful. Spray the cat’s hair and skin and rinse after 10 minutes. Always check with the vet and follow his advice. Also enquire whether the low-tech route will work with your cat. Vinegar has anti-fungal properties and is non-toxic when applied to the skin.

You can also get anti-fungal shampoos for cats which will help the treatment but you should be aware that by themselves the shampoos are generally not sufficient to clear an active infection. Giving the cat a haircut will make it easier to apply topical therapy. Make sure that at least the hair around the lesion is clipped carefully. Clip generously around the area leaving at least 6 cm of clipped hair around each visible lesion.

Itrafungol pills are often used to treat cats. The drug is normally administered for three weeks with a week’s break after each week of treatment. This is because Itrafungol slowly builds up in the skin during the week of treatment and the accumulated dose is sufficient to keep up the level of medication during the non-treatment period. The product can be used with all cats older then 10 days.

Griseofulvin pills are anti-fungal tablets. As with intraconazole, griseofulvin accumulates in the hair follicles where it can reach an active infection. Normally it is recommended that cats with an active infection are treated for 30 days. However, griseofulvin can have severe side-effects, the most common being vomiting and diarrhea. The drug should not be used in pregnant cats since it may cause birth defects.

Remember that skin lesions can disappear well before the infection has been fully eradicated so it is important that after treatment the vet checks the cat’s hair for spores to make sure that fungus has really gone.

House disinfection

One of the important parts of fighting ringworm infection is to get the spores off carpets, floors etc. Frequent vacuuming is very important in a house where a cat has an active infection. Any stray hair is likely to contain fungal spores. Diluted bleach or Vircon can be used to wipe all hard surfaces.

Make sure that everything the cat touches (bedding, brushes, litter trays) is cleaned regularly.

Remember that ringworm can easily spread to humans, so wear gloves and protective clothing when dealing with an infected cat.

Note: This information is for guidance only. It is not intended to replace consultation with a licensed practitioner.

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