RINGWORM (M Canis)

Ringworm is a fungal infection caused by a fungus that grows in the dead, surface layers of the skin, hair or claws.It has nothing to do with worms. The scientific name for ringworm is dermatophytosis and the fungi which cause the disease are called dermatophytes.

Some cats can have ringworm and show no signs of it. Signs can include patches of hair loss and/or a red,scaley circle on the skin. If you have more than one cat, it will not matter which one has is – ringworm is contagious, you will have to treat all of the animals anyway. Treatment usually lasts about 6-8 weeks, but you must stick with it. Here are some suggestions on how to treat the condition.

There are approximately 40 different species of dermatophyte, with 119 known strands, each tending to cause infection in particular species of hosts. In the cat, the cause of more than 90% of cases of ringworm is the dermatophyte Microsporum canis (M canis). This organism can also cause infection in many other species, including dogs and humans. Other dermatophytes that may cause ringworm in cats are Trichophyton mentagrophytes and Microsporum persicolor, which can be acquired by contact with infected wild rodents.

Ringworm infections are often self-limiting, clearing up in 6 to 8 weeks whether they are treated or not. However, this is not always the case and there are cats who have very persistent problems with ringworm. In addition, many of the cats who no longer have ringworm lesions are still carriers of the disease. For this reason, we favor treatment of cats when we grow ringworm organisms from cultures.

Dept of Small Animal Medicine, College of Veterinary Medicine, University of Georgia, evaluated seven commonly used topical antifungal products (i.e., lime sulfur [Lymdyp], chlorhexidine, captan, povidone-iodine, sodium hypochlorite, and enilconazole solutions, and ketoconazole shampoo). These products were evaluated for their antifungal activity on Microsporum canis-infected hairs from dogs and cats in an in-vitro study. Hairs were soaked or shampooed in each product for five minutes twice a week for four weeks. Of the seven products used in this study, lime sulfur and enilconazole solutions were superior in inhibiting fungal growth; no growth occurred on fungal cultures after two treatments with either product.

Chlorhexidine and povidone iodine solutions were effective after four treatments, and sodium hypochlorite solution and ketoconazole shampoo inhibited fungal growth after eight treatments. Captan did not inhibit fungal growth during the test period.

White-Weithers N, Medleau L.

Department of Small Animal Medicine, College of Veterinary Medicine, University of Georgia, Athens 30602, USA.

PMID: 7634061 [PubMed – indexed for MEDLINE]

How do cats become infected with M canis ?

Ringworm is contagious. Spores are the infectious stage of dermatophytes and are produced by M canis during an infection. They are typically found in clusters around infected hairs and can only be seen using a microscope. Infected hairs are shed into the cat’s environment. Cats may become infected either by direct contact with an infected animal or by exposure to a contaminated environment or object, such as grooming tools, clippers or bedding. Spores in the environment are very robust and without treatment can remain infectious for approximately two years.

Spores attach to the skin and germinate to produce hyphae that invade skin and hair. It is not known how many spores are needed to start an infection. Self-grooming, particularly licking, may be an effective way of harmlessly removing spores from the skin and haircoat. Intact skin is very resistant to infection. Cats with pre-existing skin disease or other conditions resulting in skin trauma such as flea, lice or mite infestations are much more likely to become infected following exposure to spores.

Ringworm seems to be more common in young cats less than one year old, and in long-haired cats. The reasons for this are unknown. It is speculated that young cats may have immature immune defense mechanisms which limits their ability to resist infection, and too young kittens just don’t spend very much time in washing themselves. In long-haired cats grooming is less efficient and the skin surface is more protected from exposure to the sun (which dermatophytes don’t like) than in short-haired cats.

What does a cat with ringworm look like?

The appearance of cats with ringworm is very variable. Some cats have severe skin disease while other cats have only very minor lesions or no lesions at all and look completely normal. Typical skin lesions are discrete, roughly circular, areas of hair loss, particularly on the head, ears or extremities of the paws. The hairs surrounding affected areas appear broken. The affected skin is often scaly and may look inflamed. However, ringworm can look very similar to many other feline skin diseases, such a flea allergic dermatitis, symmetrical alopecia and feline acne. Some loss of hair is usually involved, but the amount of inflammation, scaling and itchiness can be very variable. In very unusual cases cats may appear just to have an ear infection or infection of the claws.

How is ringworm diagnosed?

It is impossible to diagnose a cat as having ringworm based on its appearance alone because this is so variable and can easily be confused with other skin diseases, or look like a normal cat. Diagnostic tests are used to confirm the presence of M canis or other dermatophytes. Most veterinary dermatologists will use at least one of these tests on any cat with skin disease to investigate the possibility that ringworm might be involved.

1.The ultraviolet Wood’s lamp can be used to examine cats suspected of having ringworm. It is shone onto the haircoat in a dark room and infected hairs may fluoresce (or glow) with a characteristic apple-green colour. The fluorescence is thought to be caused by a substance produced by M canis. Unfortunately, not all dermatophyte species, or varieties of M canis,fluoresce, so failure to demonstrate fluorescent hairs does not rule out the possibility of ringworm. In addition, extraneous substances may cause a similar fluorescence. For these reasons the result of Wood’s lamp examination is not definitive, but it can provide a very useful method of selecting hairs for further examination, either by fungal culture or microscopic examination.

2. Microscopic examination of suspect hairs can provide a very rapid positive diagnosis. The observer looks for fungal elements and spores associated with hairs. Interpretation can be difficult and it is best performed by an experienced mycologist. It is not possible to determine which species of dermatophyte is involved. A negative result is unreliable and may only mean that the sample of hairs examined was not representative and did not include infected hairs.

3. Fungal culture is the most reliable way of diagnosing ringworm. Cat hairs are collected and used to inoculate plates of a special culture medium, which are then incubated in a laboratory. Hairs for culture can be selected because they are damaged or closely associated with skin lesions or because they fluoresce when examined with the Wood’s lamp. Hairs are collected from cats that look completely normal by whole body brushing using a sterile toothbrush or massage brush. Culture enables precise identification of the species of dermatophyte involved, but because dermatophytes are slow growing it may take several weeks for laboratories to report a result. A positive result indicates that the cat is infected with ringworm or is carrying dermatophytes on its coat (due to exposure to an infected environment). If one cat in a household is diagnosed as having ringworm then all of the other animals will need to be examined, even if they seem to be completely unaffected. In most cases all cats in a household will be culture-positive and require treatment.

How is ringworm treated?

Although in most healthy cats ringworm infection will resolve spontaneously after many weeks, treatment is necessary in all cases to speed this up because of the risk of infection of humans and contact animals. Some cats will not eliminate infection unless they are treated. In some cases, prolonged courses of treatment will be needed to achieve a cure. Treatment can be broken down into several elements, all of which are essential.

  1. Treatment of any other skin conditions
    Any pre-existing skin condition or parasitic infestation (fleas, lice, mites) which causes skin damage should be treated specifically as it could be causing skin damage which predisposes the cat to ringworm.
  2. Treatment of ringworm

All affected animals should be treated by administration of (a) tablets (systemic therapy) and (b) treatment applied directly to the haircoat (topical therapy).

(a) Systemic therapy : Griseofulvin is the drug most commonly used for the treatment of dermatophytosis and is the only anti-fungal medicine that is licensed (FDA approved) for use in the cat. Absorption of the drug can be improved by giving fat, oil or a fatty meal at the same time as the tablets. Griseofulvin should not be used in pregnant animals because it can cause neo-natal abnormalities. Pregnant women should avoid handling griseofulvin. Griseofulvin can cause other unwanted side effects, such as causing liver damage, so if a cat receiving treatment becomes i,ll this medicine should be stopped and veterinary advice sought. Cats with feline immunodeficiency virus infection (FIV) are much more susceptible to dangerous side effects, so your vet may recommend a screening test for FIV before starting therapy. Alternative drugs are now available and may be used in cats that will not tolerate or are unresponsive to griseofulvin. These are all human medicines and are expensive.

Internal medications are most effective at treating infection. Systemic therapy with griseofulvin, terbinafine, ketoconazole, itraconazole, fluconazole, or lufenuron is recommended for inflammatory varieties. The following regimens are recommended:

Drug Regimen
Griseofulvin 500 mg once daily for 4-6 weeks
Fluconazole 150 mg once daily for 4-6 weeks
Itraconazole 100 mg daily for 1-2 weeks
Terbinafine 250 mg daily for 2-4 weeks
  • Lufenuron (Program Rx), Cats and dogs: 80-100 mg/kg, Cats in catteries: at least 100 mg/kg
    The treatment should be repeated once every two weeks until at least two consecutive fungal cultures are negative over a period of two weeks.

(b) Topical therapy : Spot therapy with one of the human anti-fungal creams is not recommended for animals because the area of infected skin is often considerably wider than the skin lesions might suggest. Topical therapy is best applied to the whole body by either shampooing or dipping (using a sulpher based product called Lymdyp which is available without a prescription on the internet). Clipping of cats will make this much easier, particularly for long-haired cats, and also reduce environmental contamination. Clipping should be done carefully in order to avoid damaging the skin, as this can spread infection and make the skin lesions look worse for a short time. It is normally necessary to sedate cats to clip them safely. Infected hair should be disposed of by burning and clippers should be decontaminated properly using one of the products suggested below. There is a “vaccination” for ringworm. It is effective at controlling clinical symptoms of ringworm but not at preventing the actual infection. In a humane society situation this may not be a satisfactory solution to ringworm infection since suppressing the symptoms would make it even more difficult to identify carriers. Cleanliness and routine disinfection are still the best options for controlling ringworm in a shelter situation.

The only product licensed for the topical treatment of ringworm in cats, in conjunction with griseofulvin, is a shampoo (Malaseb: Leo Labs). It is usually used twice a week. The coat is first completely wetted and the shampoo is then lathered on and massaged well down into the skin. The shampoo is left in contact with the skin for 10 minutes (timed with a clock) before rinsing. Most cats tolerate this remarkably well. Leo labs have produced a leaflet giving tips about shampooing cats.

3.  Decontamination of the environment and contaminated objects

If possible, infected cats should be restricted to one easily cleanable room. This makes decontamination much easier and will reduce exposure of humans to the cats and sources of infection. All areas of the house to which infected animals have had access will require decontamination, but the majority of effort can then be concentrated on the room in which the cats are confined.

Any contaminated objects such as collars, baskets, igloos, bedding, soft toys and grooming tools which cannot be disinfected should be disposed of, preferably by burning. Many chemical disinfectants claim to have good activity against fungi but few are very effective against ringworm spores. Products that do work include Clorox Bleach (original, not scented) ( 1 part bleach to 11 parts water has some activity), Enilconazole and Virkon (a disinfectant powder) both available from vets.

These disinfectants should be used daily on hard surfaces and objects such as feeding bowls, but cannot be used on carpets and soft furnishings. On these surfaces daily thorough vacuuming is the only practical method of reducing contamination. Vacuum bags should be disposed of by burning. Steam cleaning is of limited use because the temperature of the water delivered is insufficient to kill spores.

How long will it take for my cat to get better?

Treatment should be continued until all of the affected animals have recovered and are negative on fungal cultures. Skin lesions will often resolve before the cats have eliminated the fungal infection (usually 4 weeks after lesions are gone), so it is necessary to monitor progress by taking hair samples (whole body brushing) for fungal culture. If treatment is stopped prematurely the ringworm may seem to reoccur after a time, although in fact it was never eliminated. In most cases, cats will need treatment for a minimum of six weeks, and in some cases much longer. Typically, the more cats in a household the harder it is to resolve the problem.

M canis infection in humans

Ringworm can easily be spread from people to people and people to cats. Children are particularly at risk. Direct contact with infected animals should be minimized. Gloves and protective clothing should be worn when administering treatment. Efficient environmental decontamination will reduce exposure to dermatophyte spores.

Dermatophytosis in humans presents as circular patches of thickened, inflamed skin or hair loss with scaling. These may be itchy. Lesions may occur anywhere on the skin or scalp. If any skin lesions develop the family doctor should be consulted. Ringworm in humans usually responds well to treatment. Topical therapy is useful in cases of noninflammatory lesions. This can play a very important role in reducing environmental contamination.

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