Summary
Dermatophytosis is the term applied to an infection caused by one of several species of organisms referred to as dermatophytes. This common infection is notorious for its ring-like hairless lesions that can be readily transmitted between dogs, cats and humans who enjoy skin-to-skin contact with one another. Hence, the term "ringworm" typically used to describe it.
Despite the unappetizing moniker, dermatophytes are not even remotely worm-like. The organisms that cause it are fungi, which are known for their ability to live in moist areas such as soil as well as the superficial layers of the skin, hair and claws of animals.
Several fungi may be responsible for dermatophytosis but this itchy, unsightly infection is most often caused by a fungus called Microsporum canis. Although two other species of fungus can also cause ringworm infections, they tend to do so less frequently.
Young animals and those with compromised immune systems are considered most at risk.
Symptoms and Identification
In pets, the fungal infection causes the hair to become brittle and break off, resulting in hairless patches of skin. These occur most commonly on the face, ears, and legs. Within these hairless patches, the skin may be crusty or mildly inflamed, especially around the edges (hence, the "ring"-like appearance). Claws can also be affected. If so they may become deformed as they grow, just as in human with fungal infections of their nails.
Note: In some cases, ringworm lesions do not take on the characteristic ring-like appearance. That's why any dermatitis whose cause is not readily attributable to a particular cause should remain suspicious for dermatophytosis.
Typically, the infection itself is not itchy, though secondary bacterial infections (pyoderma) may elicit significant pruritus (itchiness). Some animals may show no signs but may be sources of infection nonetheless, shedding fungal spores into the environment and serving as a reservoir for infection.
Ringworm is typically spread by contact with an infected animal. Because animals can shed fungal spores and infected hairs into the environment, touching objects the infected animal has been in contact with, including bedding and brushes, can also lead to infection. Organisms that are shed into the environment can remain infectious for months.
The best way to diagnose ringworm infection in an animal is by fungal culture. The veterinarian will pluck a few hairs from several lesions and place them on a culture medium where the organism can grow. Because it takes time for fungi to grow, results may not be available for two weeks or more. Preliminary results, however, can often be obtained within five days.
Veterinarians might also examine skin lesions under an ultraviolet light. In some cases--but not all--the organism may fluoresce an apple-green color. Because this test is not always accurate, a fungal culture is still the preferred method of diagnosis.
In cases in which people are diagnosed with ringworm, all animals in the household should be tested because some animals may be infected but show no signs. The same goes for multi-pet households in which one pet has been diagnosed with ringworm. Other pets should be tested and treated (if positive) in order to eliminate sources infection.
Affected Breeds
Any breed of dog or cat is susceptible to the effects of this infection. Breeds of dogs or cats that suffer from diseases that affect the normal workings of the immune system are more likely to suffer dermatophytosis.
Treatment
The good news is that for healthy animals, infections are usually self-limiting. That means that it'll eventually resolve without any treatment whatever -- typically within three months. However, treatment can help resolve the issue more quickly so as to limit the spread of infection.
Pets can be treated with the following approaches:
topical ointments, lotions, shampoos or dips (lime and sulfur)
oral antifungal medications (itraconazole and griseofulvin, for example)
or both
Clipping the infected area is often recommended to help the topical treatments reach their intended target.
It's important to recognize that antifungal drugs can have side effects, which is why topical treatments are often preferred for infections that are more localized and less severe. In fcat, administration of griseofulvin requires periodic blood monitoring tests because of its potentially toxic effects on the bone marrow. Itraconazole's fewer side effects explains why it's rapidly becoming the preferred oral treatment for cats, in particular.
Treatment is typically required for six weeks or more. Once skin lesions have resolved, fungal cultures should be performed again. Treatment shouldn't be discontinued until fungal cultures are negative.
Meticulously cleaning the home environment is important to prevent recurrence and spread of the infection to pets and people. The following are commonly recommended courses of action:
Clipping affected areas
Confining infected pets to one area of the home
Thoroughly vacuuming any of the pet's preferred areas
Washing all bedding and toys in hot water
Disposing of any carpets or bedding, if possible
Cleaning all kennels, surfaces and floors with a dilute bleach solution or an anti-fungal spray
These steps should be repeated monthly until the infection is completely resolved.
Veterinary Cost
For all the annoyance it's known to cause, ringworm isn't considered a particularly expensive condition. In fact, for those with single, localized lesions that respond readily to treatment, the cost of diagnosis and treatment can come in well under $100.
But for owners whose pets suffer from many lesions -- particularly if they're recalcitrant to treatment -- can find themselves in a financially uncomfortable position, especially if they have many pets in the household who may or may serve as reservoirs for future infection.
Indeed, in some cases, long-term treatment of dermatophytosis can add up. $500 per patient is not uncommon for some tougher-to-treat cases.
Prevention
Preventing exposure to dermatophytes isn't always possible for pets who spend a significant amount of time out of doors. After all, these organisms are ubiquitous in most garden settings. Preventing exposure to other pets who are known to currently harbor organisms, however, is a good approach to prevention. Nonetheless, this may prove difficult within households.
Since dermatophytosis can also be transmitted via inanimate objects such as grooming tools, carriers and bedding so care must be taken to clean objects properly -- especially since dermatophyte spores can persist in the environment for up to 18 months!
References
Colombo S, Cornegliani L, Vercelli A. Efficacy of intraconazole as a combined continuous/pulse therapy in feline dermatophytosis: preliminary results in nine cases. Veterinary Dermatology (12): 2001; 347-50.
Feldman E, Nelson R. Canine and feline endocrinology and reproduction. 3rd ed. Baltimore, Md: Elsevier-Saunders, 2003.
Guaguere E, Prelaud P (eds): A Practical Guide to Feline Dermatology. Merial; 2000.
Griffin CE, Kwochka KW, MacDonald JM (eds): Current Veterinary Dermatology. St. Louis: Mosby Year Book; 1993.
Medleau L, Hnilica KA. Small Animal Dermatology: A Color Atlas and Therapeutic Guide. Philadelphia: WB Saunders; 2001.
Rosenfeld AJ. Veterinary medical team handbook. Ames, IA: Wiley-Blackwell, 2007.
Scott D, Miller W, Griffin C. Muller and Kirk's small animal dermatology. Baltimore, Md: Elsevier-Saunders, 2000.