Andrew H. Sparkes, BVetMed, PhD, DECVIM-CA, MRCVS
The dermatophytes are a highly specialised group of related, filamentous, pathogenic fungi that share the unusual ability to digest and derive nutrition from keratin. The term dermatophyte is restricted to those members of the three genera of Microsporum, Trichophyton or Epidermophyton that cause disease by parasitising the keratinised tissues (skin, hair and nails) of man and animals.
Microsporum canis is the most common cause of feline dermatophytosis and cats are considered to be the natural reservoir for this organism. Other dermatophytes that may be isolated from cats are generally thought to be transmitted from rodents during hunting (Trichophyton mentagrophytes, M. persicolor) or less commonly from the soil (M. gypseum, M. fulvum, T. terrestre). While no sex predisposition has been found for feline dermatophytosis, there is a marked age predisposition with young cats (less than a year of age) being the highest risk group. This may reflect a lack of acquired immunity in these young cats and/or differences in the microclimate of the skin which predispose to development of the disease in young cats. Disease is more common in multicat households where M. canis can spread rapidly to susceptible cats, and although no specific breed susceptibilities are recognised, there is also good evidence that disease is more common in long-haired cats.
Although direct contact between cats is probably the most efficient means of transmission of infection, the widespread dissemination of infected material from affected cats into the environment, and the prolonged survival of arthrospores in the environment means that infected hair and skin scales shed into the environment are also be an important source of infection.
Particularly with dermatophytes well adapted to infecting certain species (e.g., M. canis with cats), some infections may remain chronic and localised and thus an affected animal may not necessarily show obvious lesions. However, these chronic localised infections should be distinguished from passive carriage of dermatophyte spores in the hair-coat. Due to the keratinophilic nature of arthrospores, if cats are exposed to a heavily contaminated environment, cultures of hair-brushings frequently yield dermatophytes due to passive contamination of the hair-coat. In general, the infection to inoculation ratio for dermatophytes is considered to be low, so passive asymptomatic carriers should not present a major risk either to other animals or to man, although if clinical disease occurs as a result of their carriage, they may then become a significant source of infectious material.
Treatment and control
In feline dermatophytosis, topical therapy with azoles, combined with clipping hair around the lesion, may be adequate for small focal lesions, but if a rapid response is not achieved systemic therapy is indicated. For generalised or recalcitrant localised dermatophytosis a combination of systemic and topical treatment is usually indicated. Total body clipping of hair has been advocated in long or medium haired animals to facilitate topical treatment and reduce the environmental contamination of spores, but this procedure may increase the severity of the lesions, presumably through trauma to the skin facilitating spread of the disease. It is advisable therefore to clip only after systemic therapy has been instituted for 2-4 weeks at which time the procedure is most effective.
The only systemic agent currently licensed for use in animals is griseofulvin. On an empirical basis, the initial dose of this drug administered to dogs and cats is usually around 50 mg/kg daily, but this is doubled if response is poor. As in humans, infections recalcitrant to griseofulvin are encountered, and treatment during pregnancy is contraindicated due to the teratogenic nature of the drug. Other serious side-effects occasionally reported with griseofulvin use in cats include ataxia, anaemia, neutropenia, bone marrow hypoplasia and hepatopathy.
Lipophilic azoles and allylamines may prove much more efficacious in the treatment of feline dermatophytosis, and certainly based on experience in human medicine, such drugs can be expected to perform much better than griseofulvin. Itraconazole at 10 mg/kg daily or every other day is efficacious in the treatment of feline dermatophytosis and side-effects are rarely seen (although monitoring of liver enzymes is prudent). As the drug accumulates and persists in the stratum corneum, reducing from daily to every other day therapy after the first 2-4 weeks therapy is likely to maintain efficacy. Successful treatment with oral terbinafine has also been documented, and this again appears to be a safe drug in cats. Both these drugs are likely to be much more effective than griseofulvin, and have the advantage that they persist in the stratum corneum for 2-4 weeks after cessation of therapy.
The use of environmental disinfectants will have a limited role where the cat is an indoor pet, but wherever feasible the environment and fomites should be treated and this should be combined with thorough and regular vacuum cleaning of carpets and furnishings to reduce the burden of infection.
Recent advances in ringworm therapy
Systemic therapy is generally regarded as the treatment of choice for feline dermatophytosis although topical therapy with creams, ointments, shampoos or dips has been widely recommended as an adjunct therapy. In one study, a shampoo containing 2% chlorhexidine and 2% miconazole was assessed as an adjunctive therapy in feline M canis infection under controlled conditions. For the study, M canis infection was induced in 21 healthy cats. After 4 weeks, once lesions were established, the cats were divided into three groups of 7. The control cats (C) received no therapy, one group (G) received oral griseofulvin at 50mg/kg once daily, and the final group (M) received the same dose of griseofulvin, but in addition these cats were shampooed twice weekly with shampoo, using 10ml shampoo on each occasion and allowing a 10 minute contact period before the cats were rinsed. The results of the study demonstrated a clear and beneficial effect from the addition of twice weekly topical shampoo to systemic griseofulvin alone. There was a major difference in the results of mycological monitoring of the cats and the environment lending strong support to the clinical use of an antifungal shampoo as adjunctive therapy in treating dermatophytosis.
Treatment in multicat households/catteries
Several approaches may be taken to this problem, but success is most likely to be achieved by 'blanket' treatment of the whole colony: Once dermatophytosis is confirmed, with this approach all cats in the colony are treated in the same manner using a combination of systemic treatment and topical therapy as described above. Rigorous environmental control should also be undertaken. Cats should be examined regularly and also examined with a Wood's lamp to monitor progress of the disease. When clinical signs have resolved, eradication of disease may be confirmed by negative fungal cultures from coat brushings of the cats, but it is recommended that three negative cultures should be obtained from samples collected at two-week intervals to be sure disease is eradicated.
Lufenuron and dermatophytosis
There has been much interest recently in the possible use of lufenuron for the treatment of dermatophytosis in cats. Lufenuron has been used for many years as a flea control product, being a chitin synthetase inhibitor-chitin makes up 25-50% of the dry weight of fleas. Fungal organisms are also partially composed of chitin, which forms part of the fungal cell wall, and because of this there has been interest in the potential use of lufenuron to treat fungal infections. This was highlighted by a publication in 2000 in JAVMA where the authors described the use of lufenuron to treat 129 dogs and 159 cats with dermatophytosis and they suggested that both cats and dogs treated in this way benefited remarkably from the use of lufenuron. The results appeared to show a remarkable effect of lufenuron on dermatophytosis, and this, along with other anecdotal reports of its successful use, has lead to widespread use of lufenuron to treat dermatophytosis.
Although there is little doubt that lufenuron is a very safe product, serious questions have been raised about its supposed efficacy in treating dermatophytosis and some inconsistencies, along with anecdotal reports of poor responses to the use of lufenuron, have led to questions over whether lufenuron is of real benefit in treatment of dermatophytosis. Recent studies reported at the American Academy of Veterinary Dermatology meeting in 2002 by DeBoer and Moriello showed little or no efficacy for lufenuron when used under carefully controlled conditions, with various doses of lufenuron neither preventing infection, nor resulted in more rapid resolution of disease (although there was a suggestion of an effect in potentially delaying the onset/establishment of infection).
Further work is clearly necessary to fully evaluate the potential role of lufenuron in treating feline dermatophytosis but current work suggests that while the drug may potentially have some effect on the course of disease, this should not be relied upon for treating dermatophytosis. At best, therefore, lufenuron should be regarded as an adjuvant therapy in dermatophytosis treatment rather than being relied on as a first-line treatment.