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Dermatophytosis (ringworm) is a common infectious agent of the dog and cat. Its incidence is related to geographic region, climate and animal husbandry techniques. It is more common in young, stray, sick or debilitated animals. It is of increasing significance because of its zoonotic potential, especially in humans in an immuno-compromised state. M.canis causes 70% of dermatophyte infections in dogs and 99% of infections in cats. 5% of dogs are considered carriers, and various studies from cats have set the carrier rate at anywhere between 4 - 20% of healthy stray cats being positive for M.canis.

Infection is aided if the skin surface has been abraded, such as occurs when animals are groomed or clipped. Dermatophytosis is often a self-limiting infection with an incubation period of 1 - 6 weeks. Young animals are considered more susceptive because of the immaturity of their immune system. M.canis is considered highly contagious.


Question and Answer

What is dermatophytosis?

Dermatophytosis is the medical term for a fungal infection of the superficial layer of the skin, nails, or hair. The infections are caused by specific fungal species known as Microsporum or Trichophyton. This disease commonly is referred to as "ringworm," although the disease itself is not caused by a worm. Dermatophytosis affects dogs, cats, and people.

What causes dermatophytosis?

Dermatophytosis is caused by specific types of fungus (Microsporum or Trichophyton) that grow in the surface of the skin, nails and hair. Dermatophytosis tends to occur in animals that are immunocompromised, where their immune system is unable to fight off the fungal infection. Conditions that impair the function of the immune system include poor nutrition and severe, generalized infections. The incubation period from the time of exposure to the fungus and signs of disease ranges from 1 to 4 weeks. Some animals may be carriers of the fungus; in other words, they have the fungus on their body but they do not exhibit the disease. These carriers may infect other animals that are exposed to them.

What are the signs of dermatophytosis?

The most common clinical sign associated with dermatophytosis is a circular area of hair loss (alopecia). Other clinical signs include scaling, redness (erythema), darkening of the skin (hyperpigmentation), and itchiness.

How is dermatophytosis diagnosed?

Dermatophytosis is diagnosed by a good medical history and an examination of the skin. Some strains of fungus will glow (fluoresce) under a special black light (Wood's ultraviolet lamp); however, the absence of fluorescence does not rule out "ringworm." A small number of hairs are plucked from the edge of a hairless area. Some of the hairs may be examined with a microscope for the presence of fungal structures, such as spores. Dermatophytosis is diagnosed definitively using a fungal culture. The remaining hairs will be placed into a container with special fungal growth media. The sample then is monitored daily for characteristic changes associated with growth of "ringworm" fungi. Although blood and urine tests will not diagnose "ringworm" infection, they may identify an underlying disease that has allowed "ringworm" to develop.

Diagnosing animals that are carrying the fungus without having signs of disease is more difficult. A technique, known as the "toothbrush" technique can be used in the diagnosis. The toothbrush technique involves taking a new, sterile toothbrush and gently brushing throughout the animal's hair coat. The toothbrush will pick up individual hairs as the brushing proceeds over the body. A fungal culture is done using these hairs. The culture is done in the same manner as a culture for an animal with evidence of "ringworm.

How is dermatophytosis treated?

The treatment of dermatophytosis involves the use of systemic or topical antifungal medications for several weeks. Many animals will improve visibly; however, they may still have fungus on their body and test positive on fungal culture. Treatment should be continued until at least one fungal culture yields no growth of "ringworm" fungus (that is, the result is negative). The affected animal should be quarantined in the household to prevent spread of infection. Other animals that have been exposed may be treated with antifungal medications as a precautionary measure. Some antifungal medications have toxic side effects so the animals will need periodic monitoring. Recheck physical examinations, blood work, and repeated fungal cultures may be required to assess the response to treatment.

Dermatophytosis is a zoonotic disease. Zoonotic diseases are diseases that can be spread from animals to humans. Immunocompromised individuals should avoid handling infected animals. People should discuss their treatment with a physician.

What is the prognosis for animals with dermatophytosis?

The prognosis (outcome) for animals with dermatophytosis generally is good, as most animals will clear the infection over a few months. Treatment helps to speed the resolution of the disease. Unfortunately, some infections may remain persistent, particularly in longhaired cats or multi-animal households.

The information on this page was obtained from the site www.vetmedcenter.com