Middlesex Veterinary Center

Ringworm in Cats

Ringworm is a common fungal infection that can affect cats of all ages and breeds. Despite its name, ringworm is not caused by a worm, but rather a fungus that lives on the surface of the skin. It is highly contagious and can easily spread from one cat to another, as well as to humans and other animals.

What Is Ringworm?

Ringworm is a skin disease caused by a fungus (plural: fungi). 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. Microsporum canis 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 in the dead layer of the skin, hair, and nails. The fungus is able to utilize this dead tissue (keratin) in the skin as a source of nutrition.

What cats are likely to get ringworm?

Genetic and environmental influences play an important role in feline ringworm infection. A significant amount of research remains to be done on this disorder. It appears that Persian cats are affected most frequently. In catteries, ringworm can be hard to control because of the numbers of animals involved.

What are the clinical signs?

The fungi live in hair follicles. As the organism invades and weakens the hair shafts, 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 10-14 days. This means that the exposure to the fungus and establishment of infection occurs 10-14 days before any lesions occur.

How is ringworm diagnosed?

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 the least accurate method since other skin diseases may have the same appearance.
  2. Examination of the scales and hair under the microscope. Some of the fungal elements, such as spores, can visualized with this technique.
  3. Fluorescence of infected hairs under a special light. This is a screening test which 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 to diagnose 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.

How is it transmitted?

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 acquired it from a 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 bedding or carpet for several months. They may be killed with a dilution of chlorine bleach and water (1 cup of chlorine bleach in a gallon of water) where it is feasible to use it.

How is it treated?

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.

  1. Griseofulvin. This is a drug, in tablet form, that is concentrated deep in the hair follicles where it can reach the site of active fungal growth. Griseofulvin should be given daily. Cats with active lesions should receive the tablets for a minimum of 30 days. At that time, the cat should be rechecked to be sure the infection is adequately treated. These tablets are not absorbed from the stomach unless there is fat in the stomach at the time they are given. This can be accomplished by feeding a high fat diet, such as a rich canned cat food or a small amount of fat trimmings from meats (often available at the meat departments of local grocery stores upon request of the butcher) or by allowing the cat to drink some rich cream. Griseofulvin is the most important part of the treatment. If you are not successful in giving the tablets, please call us for help. Griseofulvin is the only drug approved for systemic (oral) treatment of feline ringworm. It is very effective and has successfully treated the vast majority of affected cats. However, it does have some potentially significant side-effects.
    1. Gastrointestinal signs of vomiting and diarrhea are the most common side-effects. These signs may stop if the dosage is divided over two or three treatments in a day.
    2. It is a potent teratogen, meaning it can cause birth defects. As such, it should not be used to treat pregnant cats or cats which might become pregnant during the treatment period. It can potentially cause abnormalities in the breeding tomcat, as well, so may be best avoided for all reproductively-active cats. There are no reported effects for queens who become pregnant after griseofulvin therapy is completed.
    3. Suppression of the bone marrow is a rare, but potentially fatal, complication of griseofulvin therapy. Toxicity cannot be predicted and may appear without warning.

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