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Ringworm

Veterinary & Aquatic Services Department
Drs. Foster & Smith, Inc.

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Ringworm is a well known fungus that can infect cats, dogs, and humans. Many people have either had or known someone that has had a ringworm infection. There are several different forms of the fungus, all of which can infect either you or your pet. The diagnosis and treatment is fairly straightforward for all species, however, some species of ringworm in cats can be much more difficult. Every pet owner should be aware of the symptoms, transmission, and treatment of ringworm.

Where is it found?

Several different fungi found throughout the world can cause ringworm, however, the vast majority of cases in cats are caused by Microsporum canis, Microsporum gypseum, or Trichophyton mentagrophytes. Among these three most common species, M. canis is considered to be the most common cause of ringworm infections in cats. The ringworm fungus is most prevalent in hot, humid climates, but interestingly enough, most cases occur in the fall and winter. The fungus is most commonly found either on an infected animal or in the living quarters of infected animals. Spores from infected animals can be shed into the environment and live for over 18 months. Cats can also be asymptomatic carriers and harbor and shed the organism without showing signs of infection. Animals thought to be chronic carriers can be routinely cultured to determine if they are actual carriers.

How do cats get ringworm?

Ringworm is transmitted through direct contact of infected spores to a healthy animal. These fungal spores can be found on an infected animal, on infected grooming equipment or brushes, in a contaminated boarding facility or kennel, or in the environment where an infected animal has visited. As you can see, because of their ability to survive for long periods in the environment, your cat can contract ringworm just about anywhere other dogs or cats have been. Fortunately, most healthy adult cats have some resistance to ringworm and never develop symptoms from the fungus. Young cats under a year old are most often infected. Cats with a suppressed immune system from diseases or overuse of steroids are also more susceptible to contracting the disease.

What are the symptoms?

Cats with ringworm may have lesions, which appear different depending on where they occur and how long they have been present. The classic symptom is a small round lesion that is devoid of hair. The lesion will often have scaly skin in the center. Small pustules are often found in the lesion. The lesion may start as a small spot and continue to grow in size. The lesion may or may not be irritated and itchy. The lesions are most common on the head, ears, and tail. In some infections, the fungus will not be in a circle and can spread across the face or nose and look like an autoimmune disease or other generalized skin disease. Occasionally, the infection will occur over the entire body and create a generalized scaly or greasy skin condition. Chronic ear infections are also occasionally caused by ringworm.

How is it diagnosed?

Ringworm can be diagnosed through several different methods. A popular but not completely accurate way to diagnose the disease is through the use of a specialized black light called a Wood's lamp. Several species of the ringworm fungus will glow a fluorescent color when exposed to a Wood's lamp. However, it is estimated that up to half of the most common species of M. canis do not fluoresce under a Wood's lamp. In addition, a healthy animal may have spores on her coat but may not have an active infection.

Another method for identifying ringworm is to pluck and examine hairs on the periphery of the lesion under the microscope. Between 40% and 70% of the infections can be diagnosed this way.

The best and most accurate way to identify a ringworm infection is by collecting scales and crust from the skin and coat and performing a culture. There are special culture mediums designed specifically for identifying ringworm infections. Your local veterinarian can easily perform this routine culture.

How is ringworm treated?

In healthy shorthaired kittens and cats with small isolated lesions, the ringworm infection will often resolve without treatment in about four months. The goal with these animals is to treat any underlying conditions, provide good nutrition, and prevent the spread to other animals. Remission of the disease without treatment is also possible in longhaired cats although it might take from 1 to 4 years to resolve. Longhaired cats especially Persians and Himalayans in breeding colonies can be very difficult to treat.

In more severe cases, several different treatments are used. For isolated lesions, the area around the lesion should be thoroughly clipped down close to the skin. Care should be taken when clipping not to irritate the skin, as this may make the infection spread. Also realize that the clipped hair, clippers, and any grooming instruments that come into contact with an infected animal will harbor the spores and must be heat or chemically sterilized before being used on any other animal. The lesions can then be treated topically twice a day with a topical antifungal medication. Several well known dermatologists insist that all longhaired cats must be shaved completely if the owner wants to have any success with treating ringworm. Popular topical treatments include miconazole cream, Lotrimin cream, or 1% chlorhexidine ointment. Because of the risk of a cat becoming an asymptomatic carrier, a cat should also be shampooed or dipped with an antifungal product. A 0.5% chlorhexidine shampoo , 2% miconazole shampoo, ketoconazole shampoo, lime sulfur dip, or a 2% chlorhexidine solution that is applied every 2 to 4 days have been used effectively, however, the lime sulfur dip is the most highly recommended. Topical treatment must be continued for 6-10 weeks, or for at least 2 weeks after the lesions have resolved.

Another treatment option is to use oral antifungal agents. Historically, griseofulvin was the drug of choice. Ketoconazole, and most recently itraconazole, have been used successfully. These products all have to be given for several months, and because of their potential toxicity, must only be used under close direct veterinary supervision.

There is an injectable ringworm vaccine available for cats. The product is FEL-O-VAX® MC-K from Fort Dodge. The vaccine has been used successfully in some ringworm eradication and control programs, however, due to the resistant nature of the ringworm fungus, there are cases where the vaccine fails to control or completely cure a ringworm infection. The recommendation by the manufacturer is to only use this product in cats over four months of age. After an initial dose is administered, a second dose is given 12-16 days later. A third dose is given 26-30 days after the second dose. This product is only effective against M. canis in cats, so for best results, a culture and treatment plan from a local veterinarian should be obtained.

Because the ringworm fungus can survive for such long periods in the environment, it is critical that an effective cleaning plan be used in all infections. The spores are very resistant to most cleaners, however, bleach diluted to 1:10 with water or chlorhexidine will kill most of the organisms. All grooming tools, bedding, and kennels should be thoroughly cleaned and disinfected. Carpets should be steam cleaned and disinfected. Heating and cooling ducts and furnaces should be professionally vacuumed and filters replaced. Housing units that contain wood or rusty metal should be re-painted.

How can ringworm be controlled in catteries?

Some of the most difficult cases of ringworm come from catteries or small breeders, particularly those with Persian or Himalayans. Eliminating ringworm from these units can take months to years of diligent treatment. Remember that it will take a complete commitment to properly eliminate ringworm from a cattery. The owner must be willing to devote a large amount of time and make some difficult decisions if success in treatment is to be achieved.

There are several different approaches to controlling and eliminating ringworm in a cattery. The first approach includes total depopulation of the cattery, decontamination of the facility, and repopulating with only animals that test negative on three consecutive cultures performed at two-week intervals. The second approach is to treat the entire colony and facilities with appropriate topical medications, systemic therapy, and environmental cleanup. The colony is isolated and breeding and showing are interrupted. The third option would be to treat only infected kittens. This third option is only suitable for a breeder that produces kittens for the pet cat market and usually is not recommended for most breeders.

Cultures should be performed on new cats coming into the cattery or returning from a show or a breeding. They should be quarantined and dipped once with lime sulfur. Since dogs and humans can carry ringworm into a cattery, both human and canine visitors should be kept at a minimum.

How is ringworm prevented?

Ringworm is a disease where an ounce of prevention is truly worth more than a pound of cure. If you have cats in your home, be very careful about bringing a new kitten into your household. Cat shows, kennels, and grooming facilities can also be a source of infection and caution should be used when exposing your cat to these places. Breeders of Persians and Himalayans need to be especially cautious about bringing any new animal that has not been cultured into their facility. If any sign of ringworm is seen, make sure you isolate the infected cat and seek prompt veterinary attention.

Is ringworm transmissible to people?

Yes. Ringworm can be transmitted between cats and people. Persons with suppressed immune systems, such as those with HIV infections or AIDS, and those undergoing chemotherapy may be especially vulnerable. Persons should wear gloves when handling affected animals and wash hands well afterwards.

References and Further Reading

Ackerman. Skin and Haircoat Problems in Dogs. Alpine Publications; 1994.

DeBoer, DJ; Moriello, KA; Cairns, R. Clinical update on feline dermatophytosis - part II. The Compendium on Continuing Education for the Practicing Veterinarian. 1995; 17(12): 1471-1480.

Griffin, C; Kwochka, K; Macdonald, J. Current Veterinary Dermatology. Mosby Publications. Linn, MO; 1993.

Scott, D; Miller, W; Griffin, C. Muller and Kirk's Small Animal Dermatology. W.B. Saunders Co. Philadelphia, PA; 1995.

© 2004 Drs. Foster & Smith, Inc.
Reprinted as a courtesy and with permission from PetEducation.com
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