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Dr. Lila Miller, D.V.M., ASPCA



Ringworm, otherwise known as Dermatophytosis, is a very exasperating problem for shelters. This disease, which is one of the most common infectious skin diseases of young, old and long-haired cats, is actually caused by a fungus, not a worm. It is much less common in dogs and this discussion will focus on cats. Although it does not cause fatal disease in individual cats, there are several reasons why this disease presents major headaches to shelters and should be seen as a serious health threat to everyone.

  • It can take up to 2 weeks or more to make an accurate diagnosis.
  • It has an inapparent carrier state during which time the cat may appear clinically normally and still spread the disease.
  • Treatment protocols vary, but a cure generally requires at least 6 weeks, and can take up to 6 months.
  • It is very resistant to inactivation, surviving for months to even years in the environment.
  • There is no effective vaccine or fast, reliable screening test.
  • It is highly contagious and can spread to other species, including humans.

Although the disease itself is often mild, may consist of localized lesions only and is considered to be self-limiting, (meaning it will resolve in most healthy, short haired animals on its own in about 3 months), many shelters are forced to euthanize cats with ringworm because it is zoonotic, highly contagious and extremely difficult to eradicate. It has been estimated that as many as 50% of the people exposed to cats with ringworm may develop the disease, although this is much more likely to occur in children, the elderly, the immune compromised or people with breaks in their skin. (Shelter workers should observe common sense rules of precaution to avoid contagion when handling any animal with skin lesions, but particularly kittens.)

Although there are three genera of fungi that can cause ringworm in dogs and cats, this article will focus on the one found in 98% of cat cases and 70% of dog cases, Microsporum canis, or M. canis. (The other 2 genera are Trichophyton mentagrophytes and Microsporum gypseum). M. canis is highly contagious and lives in the superficial layers of skin, hair and claws. The fungus is not a normal skin inhabitant and should always be considered to be a cause of disease (pathogen) when found. As stated before, it infects humans and may also occasionally infect other species, including rabbits and guinea pigs.

M. canis is highly contagious. The fungal spores can survive in the environment for 18 months or longer, in dust, ventilation ducts, heating and air filters, carpets, drapes, floors, etc. Although it had been previously reported that ringworm was susceptible to many disinfectants, current research shows that the only agents that can be relied upon to kill ringworm spores are 1% formalin, 10% bleach or enilconazole. The 1:32 dilution of bleach that is commonly used in shelters as a disinfectant will not kill M. canis.

Ringworm is usually spread through direct contact with infected hairs. Asymptomatic carrier cats represent a real problem for shelters because the lesions are not readily visible. These cats pick up the disease from other contaminated cats or a contaminated environment and then spread it to others. The disease is also readily spread through contact with fomites, which are inanimate objects that may harbor and spread infectious spores. Some of the most common fomites are routinely overlooked- hands and clothing, toys, cat carriers, brushes, flea combs, collars, bedding, carpet, scratching posts, furniture, litter boxes, clippers, etc. There is some evidence that fleas may be capable of spreading the disease as well. In addition, the spores may be carried by air currents and on dust particles.

Cats that develop cases of ringworm may have other underlying problems that compromise the immune system and thus render them more susceptible to the disease. The list includes many of the problems often found more frequently in shelter animals than in animals in individual homes, such as stress, malnutrition, endo or ectoparasitism, feline leukemia (FeLV), feline immunodeficiency virus (FIV), pregnancy, lactation, etc. Excessive bathing or moisture on the skin as often occurs during routine cage cleaning can predispose cats to infection. It should also be suspected after prolonged antibiotic or steroid usage. It occurs more readily in overcrowded, humid or poorly ventilated environments, conditions that are also not uncommon in many shelters. These factors are important to know because of the important role they play in control measures for shelters.

The incubation period (time from infection to appearance of lesions) for ringworm is variable, and can range from as short a period as 4 days, and up to 6 weeks. It is encountered most frequently in kittens. It is uncommon in adult dogs unless they are immunocompromised. The duration of immunity after recovery from infection is unknown, but cats can be reinfected if exposed to enough spores.

Clinical signs
The clinical signs of ringworm are variable. Although the classic sign is a circular area of hair loss (alopecia) with crusts and scaling, it may show up with large areas of hair loss, with or without scaling or as broken, frayed hairs. The primary disease does not normally cause itching (pruritis), although this may occur secondarily and be intense. Lesions may present symmetrically and the skin may be reddened. It is often found on the face, head, ears, paws (especially the claws), forelimbs and tail of cats, and may be implicated in other skin diseases such as feline acne or stud tail. It must be remembered that ringworm can be confused with other skin diseases, (especially Staphylococcal skin infections) and should be suspected in any young cat or kitten with hair loss and non- pruritic skin lesions. Associated signs may include vomiting, constipation, hairball problems and excessive grooming and shedding.

There are 3 methods commonly used to diagnose ringworm. They include the Wood’s lamp for screening, direct examination of infected hairs under the microscope and culture. Culturing is the most reliable method and should be employed by any shelter that is serious about dealing with the disease. The drawback is that it may take 2 weeks to confirm the diagnosis.

The Wood’s Lamp is an ultraviolet source of light that causes about 50% – 80% of some strains of M. canis to fluoresce as a bright apple or yellow green. Although widely used, this is not a very reliable screening method if the result is negative since not all fungal strains fluoresce. To further complicate matters, many other things can cause florescence, such as dandruff, kitten milk replacers, oily skin conditions (seborrhea), etc. This renders positive results suspect as well. If a Wood’s lamp is used, it should be for screening purposes only and these limitations must be kept in mind. Proper procedure requires using the lamp in a dark room only, after warming up for 5 minutes. The lamp should be held within 1-2 inches of the suspected lesion. Positive screenings should be followed up by direct microscopic examination of the hair or other diagnostic procedures. It should not be considered a stand-alone diagnostic tool.

Direct examination of hairs under the microscope requires accurate sampling of suspect hairs and the ability to recognize infected hairs, a skill that requires practice to develop. This is why many shelters and veterinary practices tend to skip this step. Animals that test positive should also be given a thorough physical exam to detect other underlying health problems, and screened for FeLV, FIV and internal parasites. A detailed description of the procedure can be found in many veterinary dermatology texts.

Culturing is the most accurate method to diagnose this disease. Representative samples of infected hair should be collected by plucking and placed on the culture medium. In the absence of distinct lesions, a toothbrush culture may be performed to obtain a broad sampling of hairs. A new toothbrush or gauze sponge should be used for each animal, brushing vigorously for 30 strokes or until there is ample hair and scales on the brush. The collected material is then placed (stabbed or embedded) in the culture medium. Unfortunately, toothbrush cultures cannot distinguish between true clinical disease and environmental contamination of an asymptomatic, mechanical carrier. (Once ringworm has gained a foothold in a shelter, toothbrush cultures may yield many false positives.) The hairs are placed on Dermatophyte test medium or DTM (Derm Duets are recommended), kept in a dark area and observed daily at room temperature for a minimum of 10 days for a color change to red and growth of the white or pale colored fungus. Other organisms can cause a red color change, and not all strains of M. canis cause the color change, so both false positives and false negatives can occur. This necessitates further testing of cats with suspicious lesions. Biopsy and histopathology, or direct examination of the hairs may be necessary to confirm the diagnosis. Pictures of the fungus can be found in many dermatology textbooks for identification.

Next: Treatment and prevention strategies.

© 2004 ASPCA

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