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Ringworm in Cats




Contributing Factors
Clinical Signs
Diagnosis
Transmission
Treatment of the Cat
Causes of Treatment Failure

Ringworm is a skin disease caused by a fungus (plural: fungi, it is not a "worm!"). Because the lesions are often circular, ringworm was once thought to be caused by a worm curling up in the tissue. However, ringworm has nothing to do with any type of worm.

Ringworm is also known as dermatophytosis. There are four species of fungi that can cause dermatophytosis in cats; however, it is most often caused by the organism called Microsporum canis. The Microsporum canis organism is so well adapted to cats that up to 20% of cats are thought to be asymptomatic carriers, meaning they have the organism but show no outward signs.

Ringworm is actually an infection of the dead layer of the skin, hair, and nails. The organism is able to utilize this dead tissue (keratin) in the skin as a source of nutrition.

Contributing Factors

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Genetic and environmental influences play an important role in feline ringworm infection. In catteries, ringworm can be hard to control because of the numbers of animals involved.

Clinical Signs

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The fungi live in hair follicles. As the organism invades the hair shafts and they weaken, hairs break off at the skin line. Patches of hair loss tend to be round; however, as the fungus multiplies, the lesions may become irregularly shaped and spread over the cat's body. These patches may be associated with scaling and crusting of the skin. The lesions are sometimes pruritic (itchy) but this is not a consistent finding.

The incubation period is variable, but not sooner than 7-14 days. This means that the exposure to the fungus and establishment of infection occurs 7-14 days before any lesions occur.

Diagnosis

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Feline ringworm can be diagnosed by four different methods. In some cases, more than one technique is used.

  1. Identification of the typical "ringworm" lesions on the skin. This is an inaccurate diagnostic method, especially since "typical" ringworm lesions are not always present in cats, the clinical signs of ringworm in cats are extremely variable.
  2. Examination of the scales and hair under the microscope. Some of the fungal elements, such as spores, can be visualized with this technique.
  3. Fluorescence of infected hairs under a special light. This is a screening test that is useful because Microsporum canis will sometimes fluoresce as a bright apple green under ultraviolet light. However, failure to fluoresce does not eliminate ringworm as a potential diagnosis.
  4. Culture of the hair for the fungus. This method is the most accurate way and considered the gold standard for diagnosing feline ringworm. After some hair is plucked from a lesion on the skin, it is placed on a special gel (culture media) to watch for growth of the fungus. Also, the color of the gel will change from yellow to red as the fungus grows. These cultures are checked daily. Most cats with ringworm will have a positive culture within 10 days, but in rare cases, growth may not occur for 14-21 days.

Transmission

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Transmission occurs by direct contact between infected and non-infected individuals. It may be passed from dogs to cats and visa versa. It may also be passed from dogs or cats to people and visa versa. If a child has ringworm, he or she may have gotten it from the pet or from another child at school. Adult humans are relatively resistant to infection unless there is a break in the skin or there is suppression of the immune system (AIDS, chemotherapy, etc). Children are quite susceptible. Consult with your family physician if any family member develops suspicious skin lesions.

Transmission may also occur from the infected environment. The fungal spores may live in the environment for up to 18 months. They may be killed with a dilution of chlorine bleach and water (1part bleach to 10 parts water) where it is feasible to use it. See the section below on treating the environment.

Treatment of the Cat

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There are several methods for treating ringworm. The specific method(s) chosen for your cat will depend on the severity of the infection, how many pets are involved, presence of children in the household, and how difficult it will be to disinfect your cat's environment.

  • Itraconazole is considered the drug of choice to treat ringworm. It is not approved for use in cats, but is generally considered safe and effective. The high cost of the drug is prohibitive in some cases.
  • Other oral drugs that can and have been used include griseofulvin (higher risk of side effects vs. itraconazole, not as easy to find as it used to be), terbinafine (Lamisil) (effective but quite expensive), and fluconazole.
  • Lime Sulfur Dip. This should be done once or twice weekly as prescribed by your doctor. Lime sulfur dip should also be applied to other pets (dogs or cats) in the household to prevent them from being affected. If they develop ringworm lesions, they should be seen by a veterinarian. You should wear gloves when applying the dip and should remove jewelry before you start. Lime sulfur can change the color of some jewelry. This is an effective form of treatment, but the dip has an objectionable odor (rotten eggs).

Treatment will not produce immediate results and the areas of hair loss may get larger before they begin to get smaller. Within 1-2 weeks, the hair loss should stop, there should be no new areas of hair loss, and the crusty appearance of the skin should diminish. If any of these do not occur within two weeks, we should see your cat again.

Once visible skin lesions are gone, we need to start repeating fungal cultures. These can be done every one to two weeks. Treatment must continue until we have two negative fungal cultures in a row, as cats will look cured before they are cured. The endpoint of treatment is a mycological cure (i.e., two consecutive fungal cultures at weekly or biweekly intervals) and a decontaminated environment to prevent reinfection. The duration of therapy, or length of time that owners can expect that the cats and the environment need treatment is usually 60-100 days.

Causes of Treatment Failure

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Many factors can contribute to treatment failure when treating feline ringworm. Since ringworm is expensive to treat, and requires diligence and work on the part of the owners, the more knowledgeable the owner, the less likely treatment will fail and the more quickly the response to therapy will be.

The most common causes of treatment failure include:

  1. Reinfection, usually when a cat is exposed to a contaminated environment or to other cats with positive culture results (which may include cats with no skin lesions at all).
  2. Contaminated environment.
  3. Resistant organisms (rare).
  4. Owner compliance problems (inability to medicate or bathe the cat, drug intolerance by the cat, etc.)
  5. Concurrent illness or drugs that can complicate resolution of ringworm, such as diabetes, kidney disease, steroid administration, chemotherapy, feline leukemia, or immunodeficiency viruses.
  6. Refusal to clip medium- or long-haired cats or any cat with generalized ringworm (i.e., not just one or two obvious spots).
    Treatment of the Environment

This is also a very important part of treatment because if the ringworm spores remain in the environment, they can reinfect the cat at any time. Infective ringworm material can remain viable in the environment for up to 18 months under optimal conditions of temperature and humidity. Additionally, cats shed infected hair and spores into the environment throughout treatment until they test negative. Environmental treatment should continue until all cats test negative on fungal culture.

  1. Thoroughly vacuum and disinfect all nonporous surfaces (countertops, walls, floors, windows, sills, carriers, etc.) with dilute bleach (1:10). Make sure to test for colorfastness where needed.
  2. Destroy all bedding, brushes, combs, etc where possible.
  3. Rugs that cannot be destroyed should be washed with an antifungal disinfectant (test for colorfastness).
  4. Vacuum and disinfect all heating and cooling ducts. Have furnace professionally cleaned. Changed furnace filters weekly.
  5. Vacuum twice daily if possible.


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