Introduction

Dermatophytosis is a superficial infection of keratinised tissues including hair, nails/claws and stratum corneum most commonly caused by Microsporum canis in the cat, and occasionally by M. gypseum, M. persicolor and Trichophyton mentagrophytes species. The prevalence of disease is not known with accuracy, but probably accounts for approximately 2% of skin infections in companion animals; it is more common in warm subtropical to tropical climates and where there are large numbers of feral animals. Isolation of dermatophytes from cats in the Pacific coastal USA in 2000 revealed an overall prevalence of 5.5%, but other studies have shown up to 35% of normal cats at a UK cat show and even 88% in a Brazilian report. Long-haired breeds appear to be predisposed and severity of clinical signs is worse in animals with underlying disease(s) affecting immune competence. In fact, the pivotal work of Moriello and DeBoer at the University of Wisconsin, Madison, has shown that it is very difficult to establish infection in fit, healthy, short-haired cats that are permitted to groom normally; indeed in such healthy, immunocompetent animals, self-cure is the norm, in 60-100 days.

Nonetheless, dermatophytosis in a multicat household or cattery situation presents a huge therapeutic challenge to the attending veterinarian, hampered particularly by the persistence of infective spores in the environment for 12-24 months. The zoonotic implications and risks to in-contact family members must also be of concern and clients should be appropriately warned and advised.

Clinical Signs

Classical dermatophytosis causes a folliculocentric alopecia, with breakage of infected hair shafts giving rise to the typical annular patches of alopecia with variable scaling and often little evidence externally of inflammation, although an inflammatory folliculitis is seen histologically. In some cases, with some strains of dermatophyte, the infection is primarily of the stratum corneum rather than hair shafts.

Sometimes cases of dermatophytosis may be confused both clinically and histologically with pemphigus foliaceus, although this is more common in dogs than cats. Occasionally, particularly in Persian cats, deeper inflammatory lesions known as kerions are seen.

Diagnosis

Wood's lamp examination can be helpful in cases of infection with strains of dermatophyte that fluoresce in ultraviolet light, but many strains of dermatophytes do not fluoresce. False-negative results may be obtained with lamps that have not been adequately warmed or are malfunctioning or in a room inadequately darkened. False-positive fluorescence may result with certain topical products.

Microscopic examination of hair plucks and scale from lesional areas may demonstrate fungal hyphae and spores. Samples for fungal culture can be taken from the edge of lesions or collected by Mackenzie toothbrush technique. Whilst in-house dermatophyte test medium (DTM) systems are available and can be useful, they require regular, daily examination to detect dermatophyte growth. False-positive results are not uncommon since saprophytic organisms will cause a colour change in the medium, but later than pathogenic fungi. Another problem with DTM isolates is that fungi tend not to form macroconidia as well as they do on Sabauraud's agar, making it difficult to identify species with certainty. For this reason it is recommended that a reputable commercial laboratory is used.

Clinical Management

Over the past 10-15 years there have been several studies examining the efficacy of various topical and systemic treatments for feline dermatophytosis, including in vitro studies using infected hairs or spores for topical agents, in vivo topical therapy studies and studies of systemic drugs and vaccine studies.

Clipping of Affected Animals

It has been demonstrated that clipping cats with lesions of dermatophytosis can exacerbate the clinical lesions and so, for short-haired cats with limited lesions, complete clipping of the coat is probably not indicated. Obvious lesions should be clipped with scissors to limit contagion. Cats with generalised lesions and long-haired cats should be entirely clipped to limit the spread of infected spores and contagion.

Topical Therapy

Topical therapy has long been held as an important part of the management of dermatophytosis, but studies have raised doubts about its efficacy as the sole treatment. In fact, over-vigorous shampooing actually worsened lesions, perhaps because of microtrauma to the epidermis.

In vivo studies have confirmed the inefficacy of chlorhexidine, whereas enilconazole 0.2% dip twice weekly can achieve fungal cure in 4-5 weeks. Use of 2% miconazole + 2% chlorhexidine shampoo twice weekly in conjunction with oral griseofulvin resulted in more rapid improvement in clinical lesions, but no difference in the time to mycological cure. Experimentally infected cats treated with griseofluvin alone or with twice weekly bathing with miconazole-chlorhexidine shampoo showed combination-treated cats cured after 9 weeks of therapy compared with 11 weeks for the systemic therapy alone group and 12 weeks for untreated controls.

Systemic Antifungal Therapy

Griseofulvin is a fungistatic agent that inhibits nucleic acid synthesis and interferes with spindle microtubule function in metaphase of cell mitosis. It is better absorbed with a fatty meal and in divided doses. Side effects may include vomiting, diarrhoea, anorexia and, rarely, bone marrow suppression, although severe neutropenia has been reported in feline immunodeficiency virus (FIV)-positive cats. The drug is teratogenic and contraindicated in pregnant animals. At a daily dose rate of 50 mg/kg it has been shown to be achieve mycological cure in cats in 63-70 days. In an experimental study where griseofulvin 50 mg/kg once daily was compared with itraconazole and untreated controls, negative fungal cultures were obtained from 3-4 weeks into therapy and the griseofulvin group was cured after 70 days of therapy, whereas none of the untreated controls was cured at the end of the 100-day study period. There is no longer a griseofulvin product licensed in the UK for use in cats or dogs.

Itraconazole is a triazole derivative that binds to cytochrome P450 and inhibits ergosterol synthesis and thus alters fungal cell membrane permeability. At low doses it is fungistatic and at higher doses fungicidal. It is better tolerated than ketoconazole in cats and is less hepatotoxic, although salivation, vomiting, diarrhoea, anorexia, depression and apathy may occur. Toxicity studies resulted in reversible adaptive liver changes with increased liver enzymes and bilirubin. It is not recommended for pregnant and lactating animals since at high doses it is embryotoxic and teratogenic. Adverse drug interactions with other agents affecting the cytochrome P450 system may occur with this and other members of the azole group of drugs. A veterinary licensed preparation of itraconazole is now available in 16 European countries as a 10 mg/ml oral solution for treatment of dermatophytosis in cats caused by M. canis. The data sheet instructs daily administration of solution at a dose rate of 5 mg/kg for 7 days, followed by 7 days without treatment for three cycles, preferably given between meals. Where positive culture is not obtained 4 weeks after the end of administration it is recommended that the same three cycles of treatment are repeated. Repeated treatment is recommended in immunosuppressed animals and the underlying disease should be addressed, as well as disinfection of the environment being recommended.

Terbinafine suppresses the synthesis of ergosterol by inhibition of the fungal enzyme squaline epoxidase and is considered to be fungicidal to dermatophytes. Several studies have used this drug at various dose rates and have demonstrated efficacy, after varying periods of time. The drug does not have a veterinary licence in the UK.

Lufenuron disrupts chitin synthesis, a component of fungal cell walls, and, subsequent to a retrospective study suggesting that recovery from fungal infection was associated with receiving the drug for routine flea control, various studies have looked at the use of the drug in dermatophytosis. Evidence for benefit is lacking.

Vaccine Studies

A killed dermatophyte cell-wall vaccine is licensed for the treatment and prevention of lesions due to M. canis in cats in the USA. In direct challenge and cohabitant exposure challenge the vaccine was not protective against infection

Environmental Decontamination

In experimental studies of a number of commonly used chemicals and products, only concentrated bleach and 1% formalin were completely effective at killing spores, with 1:10 dilution of bleach the next most effective product. 24 hours after treatment 22% of 1:10 bleach treated rooms and 33% of enilconazole treated rooms were culture positive.

Clients should be advised to vacuum all surfaces thoroughly preferably on a daily basis with careful disposal of vacuum bag contents, and daily cleaning of all non-porous surfaces with 1:10 bleach solution, which is cheap and readily available.

Summary of Recommendations for Clinical Use

 Clip hair around lesions

 Clip out cats with generalised lesions and long hair

 Twice weekly topical antifungal therapy

 Lime sulphur dip

 Enilconazole dip

 Miconazole-chlorhexidine shampoos bathing with 10 minutes' contact time

 Concurrent systemic therapy

 Itraconazole is drug of choice, 5 mg/kg daily as per data sheet, or alternative off-label regime if necessary

 Alternative drugs--griseofulvin, 50 mg/kg daily in single or divided doses, and terbinafine at 30-40 mg/kg daily

 Environmental decontamination

 Fungal culture monitoring every 2-4 weeks until mycological cure is established (two to three consecutive negative cultures)

Control of Dermatophytosis in Catteries or Multicat Households

In an ideal world the recommendations for attempting to eliminate dermatophytosis in a cattery situation are as follows:

 Test all cats and put into groups of lesional cats, non-lesional but culture-positive cats and culture-negative cats and keep isolated from each other.

 Topical treatment for cats in all groups, with the culture-positive animals also treated systemically

 Monthly clinical examinations and fungal cultures.

 Once lesions resolve, cats should be moved to the lesion-free group, but continue treatment until at least two consecutive negative cultures obtained.

 Mycologically cured cats can stop systemic treatment and move to the culture-negative group but ongoing topical therapy is advisable until the premises are considered dermatophyte free.

 All breeding and movement of cats in and out of the premises must be suspended for the duration of the treatment period and subsequently new arrivals and cats leaving for shows or breeding purposes should be quarantined on return and not reintroduced until a negative fungal culture has been obtained.

A treatment regime such as this requires considerable client commitment and a not inconsiderable financial investment.