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Veterinary Forum June 2009 (Vol 26, No 6)

Clinical Report — Most time- and cost-efficient way to find dermatophytes

by Sophia Yin, DVM, MS (Animal Science)

    When pets present with skin lesions or small patches of hair loss, every veterinarian knows to include ringworm on the differentials list, but sometimes it can be difficult to decide the best way to reach a definitive diagnosis.

    The first test is usually direct visualization of hair using a Wood's lamp. "A Wood's lamp is relatively simple to use," says Karen Moriello, DVM, DACVD, professor of dermatology at the University of Wisconsin"Madison School of Veterinary Medicine, "but is only time- and cost-effective when screening cats, particularly kittens, with suspected lesions."

    This test is limited because the only species of dermatophyte that fluoresces is Microsporum canis, but not even all strains of M. canis fluoresce. Consequently, false-negative results are common.

    There also are false-positive results. Scales and medications can fluoresce, although they are a lighter or different color. For instance, tetracycline-based medications consistently give a false-positive fluorescence, and some carpet fibers will glow a perfect apple green.

    Marcia Schwassmann, DVM, DACVD, who practices at the Veterinary Dermatology Center in Maitland, Fla., offers a different kind of advice: "Don't use a battery-operated Wood's lamp. It isn't strong enough. And allow the lamp to warm up for 15 minutes so it emits the correct wave length."

    Because both positive and negative results of a Wood's lamp examination need to be confirmed, a backup culture is done, Moriello adds. "You can skip directly to a culture and initiate topical therapy with lime sulfur dip or Malaseb rinse [miconazole"chlorhexidine, DVM Pharmaceuticals] until you have the results back."

    If practitioners opt to perform a direct examination of hairs that glow on Wood's lamp examination or hairs at the edge of a suspected ringworm lesion, Moriello offers this tip: "Contrary to popular belief, clearing agents, such as KOH [potassium hydroxide], are not needed to find infective hairs or spores. Mineral oil works well."

    Because it is sometimes difficult to find glowing hairs in a mass of plucked hair, Moriello uses the Wood's lamp again. "Hold the lamp over the hairs in mineral oil or a clearing agent for several minutes and they will glow, allowing you to find them on the microscope. Once you find the hairs, focus your examination on the hair bulb and examine the slide under the Wood's lamp," she says, adding that infected hairs have cuffs of spores around the shaft.

    Schwassmann finds that this microscopic examination can be challenging unless it is done frequently, so she doesn't expect practitioners to use this test as the only diagnostic tool for dermatophytosis.

    Both Moriello and Schwassmann recommend fungal culture for all suspected cases. Moriello even suggests adding a toothbrush fungal culture to new pet diagnostic screening protocols. "More pets are being adopted from shelters and rescue organizations. The cost of fungal culture is significantly less than [the cost for] treatment."

    The toothbrush method can be used for pets with suspected lesions, as well as those that might be asymptomatic carriers. Brush from the tip of the nose to the tip of the tail and then gently embed the bristles onto a dermatophyte test medium (DTM) plate (Bacti-Lab). Using this plate is preferred over tubes because it has a wider surface area for growth, making it easy to inoculate and take samples. The plate contains cycloheximide and antibiotics to minimize saprophytic fungal growth and growth of potential bacterial contaminants. The hairs also can be plucked from the periphery of lesions and placed gently on the test medium, the experts explain.

    When culturing plates in-house, let them sit at an ambient temperature of 75°F to 80°F. "We use a reptile tank with a heating lamp," Schwassmann says, adding that the plates do not need to be in the dark.

    "Identification of suspect colonies is not difficult," Moriello says. "First, darkly pigmented colonies are not pathogens. Ignore them. Second, focus your daily examination on pale to white colonies that become red as they are growing in the DTM. Third, remember it only suggests a possible pathogen, it is not diagnostic. Definitive identification requires microscopic confirmation."

    Finally, it is important to keep in mind that dermatophytes use the protein in the medium first, thereby causing the pH to become alkaline, which turns the medium red. A color change well after the colonies have developed does not indicate the presence of dermatophytes, only that the rest of the nutrients have been used.

    Microscopic examination can be done with lactophenol cotton blue stain or new methylene blue stain. Moriello recommends taking samples from the end of a growing colony using clear acetate tape. If a colony cannot be definitively identified, the sample can be sent to a diagnostic laboratory, but this is rarely necessary, as the most common pathogens — M. canis, M. gypseum and Trichophyton spp — are not difficult to identify.

    Dermatophytes should grow within 7 to 14 days, but it is advisable to keep the plates for 21 days because sometimes medications can slow the growth of pathogens. By 30 days, however, the plate dries up.

    After dermatophytes have been confirmed, both the pet and the environment need to be treated. Lime sulfur dip works the best but has an objectionable odor and temporarily stains white hair coats yellow. Schwassmann suggests starting with 2 oz/gal for the first treatment to determine whether the pet's skin will become irritated and then doing subsequent dips at 4 oz/gal. Some clinicians recommend 8 oz/gal.

    Apply the solution to the pet once a week using a rose sprayer so that the dip reaches the skin. The dip is safe, she adds.

    For long-haired cats, Moriello uses blunt-tipped children's scissors to clip around the lesions and then throws the scissors away. Cats are best treated with a combination of topical and systemic therapy (fluconazole, itraconazole or terbinafine) but can be treated using lime sulfur alone. She advises twice-weekly application if lime sulfur is the primary treatment. An Elizabethan collar can be used until the hair coat is dry, she adds.

    Infective spores can remain viable for up to a year, but it is unknown what "infective dose" poses a risk for people and pets. Contaminated environments pose a major problem because the spores can be mechanically carried on the animal's hair coat, giving false-positive culture results while the animal is being monitored for dermatophytes.

    "It's easiest to confine the pet to one room that is cleaned daily to minimize the spread of spores," says Moriello. "Rooms need to be swept or vacuumed daily and washed several times a week. The key to keeping contamination to a minimum is similar to flea control in an infested home. Treat the animal with an effective topical antifungal, and keep the home clean." Moriello recommends cleaning with the Easy Trap Duster (3M) because its sticky surface picks up hair and dust.

    Carpets should be thoroughly vacuumed and then steam-cleaned at the end of the treatment period. It is important to mechanically remove organic debris, scrub surfaces with a detergent numerous times and selectively use disinfectants, such as bleach 1:100. "To determine whether an area is contaminated, the client can sample the target area using one-quarter sheet of a duster cloth, double bag it and bring it to the clinic, where you can press the soiled surface to a DTM plate," says Moriello. "If spores are in the environment, this will detect them."

    Moriello recommends weekly fungal cultures to monitor treatment. Continue therapy until two or three negative weekly fungal cultures have been obtained.

    For more information:

    Special issue: Dermatophytes and dermatophytoses. Mycopathologia 2008; 166(5-6). This entire issue is devoted to state-of-the-art reviews on current topics in dermatophytosis.

    Visual Veterinary Dermatology. Dermatophytosis — scheduled for publication in June 2009.

    www.doctorfungus.org. This is a great site for information and pictures on dermatophytosis.

    NEXT: Dental Dilemma — Can Shadow's Jaw Be Saved?


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