Wood’s lamp

One of the quickest, easiest, and oldest diagnostic tests for fungal identification is the use of the Wood’s light or Wood’s lamp. This is a light emitting ultraviolet rays (320- to 400-nm wavelength) and is a useful initial test for certain dermatophytes. However, the Wood’s lamp is not definitive because only about one-half of all M. canis infections will show fluorescence, and other animal-infecting dermatophytes do not fluoresce at all. The characteristic “apple-green” or “Jimi Hendrix–green” fluorescence results from metabolites of the fungal organism that grow only on the hair and never on scale or claw material. In early infections, the whole hair shaft will fluoresce. As the dermatophyte infection clears, only the more distal portion of the hair glows as the proximal portion of the hair shaft is no longer infected. In later stages of infection, only the very tips of hair shafts fluoresce (Figure 17-1).

Figure 17-1 Dermatophytosis. Positive Wood’s lamp examination of a cat with M. canis. Note the apple-green glow.

(From Medleau L, Hnilica K: Small animal dermatology—a color atlas and therapeutic guide, ed 2, St Louis, 2006, Saunders/Elsevier.)

Direct hair examination (microscopic)

Direct microscopic examination of suspected fungally infected hairs can be time consuming and not very cost effective.

Fungal culture

Fungal culture of affected material (scale and hairs) is the most reliable method to diagnose dermatophytosis and the only way to identify specific fungal pathogens. However, both false-positive and false-negative results are possible. Proper specimen collection techniques are critical for successful diagnosis.

Specimens are best collected by plucking hair near the lesion and active inflammation. Hair can be plucked with a hemostat or forceps or through use of a “brushing” method. This involves a brand-new (mycologically sterile) human toothbrush vigorously combed over suspect areas for 2 to 3 minutes. The bristles of the toothbrush (and their attached hairs) are lightly and repeatedly pressed into the surface of the culture medium.

In kittens, brushing both the face and the hair inside the bell of the ear is extremely important because early lesions often start at these sites. In the case of suspect claw or nail bed infection, special culture techniques may be necessary.

Dermatophyte test medium can be easily obtained for in-house procedures and is best bought as a plate for the toothbrush technique (Figure 17-2). Dermatophyte medium containers are best incubated loosely covered or capped at room temperature protected from ultraviolet light and desiccation, and covered in a container holding a damp paper towel. Colonies of dermatophytes can appear within 5 to 7 days of inoculation. Nevertheless, plates must be maintained for at least 3 weeks before determining a culture negative. Culture plates must be examined daily for medium color change to red. Growth of a white to buff-colored, powdery to cottony mycelium also will be observed. Color change must occur simultaneously as the first colony is visible, never later. All fungi, including nonpathogen fungal growth, will produce a red color change after colonies have growth from several days to a week. Colonies of infectious dermatophytes are never black, brown, gray, or green. Often M. canis will fail to grow or sporulate on dermatophyte medium. Recently studies have demonstrated that increased sporulation and growth can be obtained when plates were incubated at higher temperatures of 70° to 73° F (21° to 23.8° C).

Figure 17-2 Dermatophytosis. M. canis growing on several commercially available dermatophyte media. The white colony and red color change of the culture media are suggestive of a dermatophyte.

(From Medleau L, Hnilica K: Small animal dermatology—a color atlas and therapeutic guide, ed 2, St Louis, 2006, Saunders.)

Most colonies will produce spores after 7 to 10 days of growth, which will allow specific identification on microscopy. Spore collection is best accomplished by brushing clear cellophane tape lightly over the colony surface. Place the tape sticky side down onto a drop of lactophenol cotton blue stain on a glass slide. Then add another drop of stain on top of the tape. After a coverslip is placed, the prep can be examined at 100× magnification. Among the hypha strands will be microconidia (spores) that will have characteristic shapes according to their species. If no spores are visible, wait another 4 to 7 days. Certain colonies may not sporulate until they are old and established. The presence of an infectious dermatophyte can only be definitively confirmed by culture results, appearance of the colonies, and microscopic confirmation of fungal elements.

Biopsy—histopathological examination

Demonstration of the organism in a biopsy sample is definitive proof of a true fungal infection.

Hair clipping

Clipping of the entire haircoat is optimal in all cases of dermatophytosis because it removes infected, fragile hairs that will spill spores back onto the haircoat and into the animal’s environment. Clipping allows for penetration of topical medications and significantly reduces both the amount of medication needed and the duration of the therapy. Short-haired animals with fewer than five focal lesions need not be clipped.

Topical treatment

Localized or spot treatments of dermatophyte infections in companion animals have almost no evidence for their efficacy. More effective than topical “spot” treatment are whole-body shampoos, rinses, or dipping with topical antifungal medications. The most effective topical antifungal whole-body baths, rinses, and dips are lime sulfur, enilconazole, and miconazole. Captan, povidone-iodine, and chlorhexidine are ineffective against dermatophytes when used as whole-body topical treatments.

Lime sulfur has shown superior antifungal activity at 8 oz per gallon of water (a 1:16 dilution). Lime sulfur applied twice weekly at 4 oz per gallon has demonstrated to be effective against dermatophytes if used together with whole-body clipping and aggressive environmental treatment methods. Lime sulfur is virtually nontoxic if applied properly and can even be safely applied to newborn puppies and kittens.

Enilconazole is effective in treating dermatophytosis when used as a sole, whole-body therapy (dipping) following whole-body hair clipping. Enilconazole is generally well tolerated. Side effects can include hypersalivation, anorexia, weight loss, and generalized muscle weakness. There have been anecdotal accounts of severe toxicity in cats after ingestion of enilconazole. It is believed these cases result from cats ingesting the medication by grooming after application of the antifungal agent. It appears that enilconazole is safe for cats if they are fitted with Elizabethan collars for a few hours after treatment and grooming is prevented until the animal is dry. Enilconazole is an effective treatment as a topical medication (10% solution or 100 mg/ml), but it is licensed only for dogs and horses.

Miconazole can be used as a sole therapeutic agent or used together in combination with chlorhexidine. Generally it is used twice weekly as an adjunct topical to systemic therapy rather than as a sole treatment method. As with all medicated shampoos and dips, for optimal therapeutic effects a skin contact time of 10 minutes is recommended for miconazole. Synergism between miconazole and chlorhexidine has been confirmed, and shampoos with this combination have been shown to hasten mycological cure. A list of topical antifungal therapy is included in Box 17-1.

Systemic treatment

Systemic therapy of dermatophytosis is used to hasten resolution of infection. Several drugs have been shown effective in the treatment of dermatophytosis. The appropriate choice for treatment is based on fungal species present, patient species involved, age and size of the patient involved, possible adverse effects and toxicity, and cost. Dogs and cats with multifocal lesions, all long-haired animals, and those in multianimal situations are candidates for systemic antifungal treatment. Animals nonresponsive following 2 to 4 weeks of topical therapy should also be considered for systemic treatment. See Box 17-2 for dermatophyte treatment summary. Itraconazole is currently the systemic treatment of choice for dermatophytosis in dogs and cats. Griseofulvin and ketoconazole both have significant issues of safety and efficacy. Box 17-3 and Table 17-2 list systemic fungal medications and antifungal drug dosages.

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