Tackling feline ear disease, dermatophytosis - DVM
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Tackling feline ear disease, dermatophytosis

DVM Best Practices

Photo 1: Microsporum canis dermatophytosis lesion on the bridge of the nose of a cat.
The most common clinical presentation is one or more circular patches of scaly alopecia with broken frayed hairs, most common on the face, head, ears and/or forelimbs (see Photo 1, and Photo 5). Other presentations of dermatophytosis include: pruritic miliary dermatitis, non-inflammatory pruritus and alopecia, chin acne, localized or generalized pruritic dermatitis, onychomycosis, and kerion formation (granulomatous firm ulcerated draining nodules). Kerion formation is most common in Persian cats. All cats with generalized disease or kerion formation should be screened for immunosuppressive diseases such as FeLV and FIV. Diagnosis of dermatophytosis is best made by fungal culture. Only 50 percent of M. canis isolates fluoresce with the use of a Wood's lamp, thus lack of fluorescence does not rule-out dermatophytosis, and a culture should always be obtained. A positive fungal culture isolate shows a red color change as the dermatophyte utilizes the protein in the dermatophyte test media first, with a simultaneous white cottony colony growth. The colonies should be identified microscopically to rule-out saprophytes. Cats with suspected kerion formation should be diagnosed via skin biopsy and histopathology. Asymptomatic cats in a household suspected of being dermatophyte carriers should be cultured using the McKenzie toothbrush technique. In this method, the cat is vigorously brushed with a new individually wrapped sterile toothbrush for two minutes over the entire body. The bristles are then impregnated gently into the dermatophyte test media, along with any hair or scales accumulated in the bristles.

Photo 5: Dermatophytosis on the head and ears of a Persian cat showing alopecia and grayish crusts.
Treatment of dermatophytosis requires long-term topical and systemic therapy, as well as environmental treatment in all cases. It is important to explain to clients that this treatment will be time-consuming, long-term and expensive, if a good result is to be obtained. Treatment often takes two to six months. The patient should be isolated in one room or in a large crate from children, elderly and immunocompromised people and non-infected (culture negative) animals. No new pets should be introduced to the household during this period of time. All culture positive animals must be treated. Most cats with localized lesions eventually develop generalized disease, thus both topical and systemic therapy is recommended to decrease shedding of infected spores into the environment. All infected cats should be bathed with an antifungal shampoo such as Dermazole®, Malaseb® or Ketochlor® (avoid the eyes to prevent corneal ulcers which is possible with chlorhexidine-containing products) then dipped with 2 percent lime sulfur twice at five to seven day intervals before gently and atraumatically clipping the entire body, including the whiskers. All clipped hair must be disposed of in a biohazard sealed container. Clippers should be sterilized after using. Clipping the haircoat may temporarily worsen and spread lesions, but is needed to reduce environmental contamination. All infected cats should continue having weekly lime sulfur dips for the duration of their therapy. Spot treating localized lesions has limited value but lesions can be treated with anti-fungal lotions or creams containing miconazole or clotrimazole. Systemic therapy is recommended for most cases of dermatophytosis and in all cases of generalized disease. Micronized griseofulvin is the initial treatment of choice in most cases with the exception of cats with kerion formation, for which it is ineffective. Micronized griseofulvin is dosed at 50mg/kg/day given with a fatty meal. Side effects are uncommon and include gastrointestinal upset, teratogenicity (should not be used in pregnant queens), and irreversible idiosyncratic bone marrow suppression (most common in FIV positive cats, also may be more common in Persian, Himalayan, Siamese and Abyssinian cats), so should not be used in these cases. Regular monitoring with a complete blood count performed every two weeks is recommended on cats on griseofulvin. In cats that cannot tolerate griseofulvin, or in those with a kerion, itraconazole is the drug of choice.

Itraconazole is dosed at 5mg/kg/day with a fatty meal. Itraconazole needs to be compounded, is expensive, but is usually well-tolerated. Side-effects in sensitive animals include decreased appetite and increased liver enzymes. If these side-effects are seen, the drug is discontinued until the cat is eating well and liver enzymes are normal, then reinstituted at a lower daily dose or every other day. In my experience this drug is very helpful in Persian cats with griseofulvin-resistant generalized dermatophytosis. Recent evidence suggests that pulse-dosing itraconazole may be as effective as daily dosing. Anecdotally reported effective dosing protocols include: 28 days of daily therapy followed by one week on, one week off therapy for four to six more weeks or until cured, or two weeks of daily therapy followed by two days on, five days off until cured. Terbinafine, at 10-30mg/kg/day (use lower end of the dose range for kittens) has anecdotally been reported to be effective with no reported side effects by a few dermatologists, however, no long-term studies on its safety and efficacy in cats have been published. Recently leufeneron, a chitin-synthesis inhibitor used for monthly flea larvicidal/ovicidal control has received attention as a possible treatment for dermatophytosis. A dose of 70-100mg/kg (higher dose range for multi-cat households or infected catteries) given orally once and repeated in one month is reportedly effective. Anecdotal reports on efficacy vary from complete cure to ineffective. In some mild cases, this treatment may be helpful, however, studies need to be published to evaluate its true efficacy when used alone to treat dermatophytosis in larger numbers of cats. As this drug is very safe, I recommend it to be used as adjunctive therapy with other topical and systemic therapies at this time. A vaccine against M. canis was introduced in 1994. No long-term controlled studies on its efficacy have been published to date. Anecdotal reports of efficacy have been disappointing and it has been associated with sterile abscesses in some cases. Therefore, use of the vaccine to prevent or treat dermatophytosis is not recommended.

After one month of therapy, weekly fungal cultures should be obtained. Treatment must be continued for at least eight weeks, or until three consecutive weekly negative fungal cultures are obtained. Treatment of fungal kerions is difficult, and requires surgical removal followed by long-term (10-18 months) treatment with itraconazole.

Environmental control is extremely important in the eradication of dermatophytosis, since infected spores can remain viable in the environment for months or years. An extremely thorough household cleaning, continued until the cat is cured, is the only way to ensure the successful treatment of dermatophytosis without relapse. As long as there are infected spores in the house, cats can continue to carry the organism or be re-infected and transmit it back to people and other pets. The first step is a thorough vacuuming of all carpets and furniture. This is repeated twice daily if possible to remove infected hair. The vacuumed bags should be discarded after each use. Heating and air conditioning vents should be professionally cleaned and vacuumed if possible and furnace filters should be changed weekly. Carpets should be steam-cleaned to kill fungal spores. Draperies should be dry or steam-cleaned and not replaced until the infection is eradicated. Hard surfaces such as floors, baseboards, window sills, lamps, and counter tops as well as litter boxes, food/water bowels and heating/cooling vents must be disinfected at least once a week with a 1:10 solution of bleach (mopped, sponged, washed or sprayed on surfaces). Brushes, bedding, combs and toys should be disinfected with a 1:10 solution of bleach or discarded if this is not possible. If pets ride in the car, this too, must be vacuumed and disinfected. Surfaces should be completely dry and bowls rinsed well before allowing pets to contact them.

All this hard work and effort will pay off by decreasing the risk of pet and human re-infection with fungal spores, and helping to ensure successful therapy for dermatophytosis.


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