Feline Dermatophytosis

Kitten with dermatophytosis. Courtesy Dr. Karen Moriello

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Dermatophytosis of cats is a superficial fungal infection of the skin, hair, and claws that is most commonly caused by Microsporum canis.^1,2 Infection commonly spreads directly from cat to cat, but transmission through fomites, such as infected hairs, can also occur.² In cats, M. canis most often presents as single or multiple, irregular to round alopecic areas  that can be scaly and are occasionally pruritic. However, signs are variable and atypical manifestations can occur. Because of the potential for rapid spread and zoonosis, treatment of dermatophyte infections of domestic animals should include a combination of topical and systemic antifungal medications as well as environmental decontamination.^1,3

The Wood’s ultraviolet lamp identifies pteridine, a metabolic product of M. canis, which fluoresces a characteristic apple-green color. Neither Microsporum gypseum nor most Trichophyton species, with the exception of T. schoenleinii, fluoresce.² Correct use of the Wood’s lamp takes practice and is aided by using a lamp with a magnifier. Under a Wood’s lamp, scaling, crusts, soap residue, and topical medications can also fluoresce but lack the apple-green color.^1,2 The lamp can also be used to monitor treatment progress. As the infection resolves, the base of the hair shafts will stop fluorescing, though the tips may continue to glow.^2,3

*Colony forming units (cfu)

Culture using dermatophyte test medium (DTM) is a diagnostic and monitoring tool for dermatophytosis. Cultures are taken by brushing the coat using a sterile toothbrush, then pressing the brush onto a flat DTM plate several times.³ Cytology of DTM colonies is essential for speciation and to rule out contaminant fungi, which can also eventually cause change of media color. A decreasing pathogen (P) score indicates that an infection is clearing (Table).^2,3 Mycological cure is defined as 2 to 3 negative cultures taken two weeks apart.²

Other diagnostic methods include microscopic examination of plucked hairs to look for dermatophyte arthroconidia infecting the hair shafts, lesion biopsy (which is recommended in cases of suspected fungal nodules and skin lesions with no attributable cause), and real-time polymerase chain reaction.²

The only available FDA-approved systemic treatment to eliminate active fungus from the hair follicle is Itrafungol^TM (itraconazole oral solution), a 10 mg/mL cherry-caramel flavored formulation.⁴ Itraconazole, a triazole antifungal, acts on the fungal cytochrome P450 enzyme 14 α-demethylase, blocking ergosterol synthesis in the fungal cell membrane and leading to structural degeneration of the fungal organism.²  Itraconazole is highly lipophilic, so the medication accumulates at high levels in sebum for distribution across the skin and to the hair.^2,5 It has been found to persist in the skin for up to 4 weeks in humans, allowing weekly pulse dosing.^2,5 

Prior to the introduction of Itrafungol, compounded itraconazole was available for veterinary use. However, compounded formulations of itraconazole have inappropriately low bioavailability in some species, leading to non-therapeutic plasma concentrations, and are not recommended in veterinary patients.^6,7 Itrafungol is administered (5 mg/kg once daily on alternating weeks for 3 treatment cycles) using the dosing syringe included in the package.⁸ 

Itrafungol effectiveness was demonstrated in a placebo-controlled laboratory study of 80 experimentally infected cats that did not receive any topical treatment. Within 1 week after the start of treatment, lesions had improved in the Itrafungol-treated cats. After 3 weeks, over 90% of cats in the treatment group reached clinical, but not culture, resolution. By the end of the 9-week study, 90% of treated cats had a least one negative fungal culture.⁸

Topical antifungal therapy is recommended in order to disinfect the patient’s hair coat, minimize environmental contamination, and shorten the total treatment time as compared to systemic antifungal treatment alone. Common topical products include lime sulfur leave-on rinse and chlorhexidine/miconazole formulations.^1,2 Environmental decontamination is also important to minimize false-positive culture results. Effective cleansers for non-porous surfaces include diluted household bleach (1:10 to 1:100 concentration), accelerated hydrogen peroxide, and one-step cleaners with labeled effectiveness against Trichophyton spp. Carpets can be disinfected with vacuuming to remove infective hairs, followed by 2 washes with a beater brush carpet shampooer or hot water extraction. Laundry can be disinfected through 2 washing cycles at any temperature, as long as the washer is not overly full; bleach is not necessary.²

Please examine references 2 and 3 for further discussion of dermatophytosis diagnosis and treatment.

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