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Recent advances in the investigation and therapy of dermatophytosis

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01 January 2014, at 12:00am

Sue Paterson reviews the diagnostic and therapeutic challenge of dermatophytosis when identified in companion animals and examines the latest developments in investigation

DERMATOPHYTOSIS can be a diagnostic and therapeutic challenge when it is identified in companion animals. Whilst it is a self-resolving disease, it is a zoonosis, so responsibly it should be identified and treated wherever possible.

Because it is often recognised in puppies and kittens and children’s pets such as guinea pigs and rabbits, expense can often be an important factor when considering diagnosis and therapy.

In the dog and cat, the classical clinical signs of Microsporum canis – the well circumscribed areas of alopecia with the “cigarette ash” type of scale – makes diagnosis easy. In rabbits and rodents there is overlap between the clinical manifestations of fungal and ectoparasitic disease, so that clinical signs alone are less useful.

Whilst a Wood’s lamp will only identify 50 -60% of hairs infected with Microsporum canis, the apple green colour that can be seen on plucked hairs can be a useful guide. Trichophyton mentagrophytes (TM) on the other hand, which is most commonly seen in rabbits and rodents, will never fluoresce. Identification of dermatophytes can be achieved by plucking hairs from the periphery of lesions and examining them in potassium hydroxide or lactophenol cotton blue.

The typical ectothrix spores can be seen clustered on the hair shafts obscuring its otherwise sharp contours. Although the collection of scale and hair to grow and identify dermatophytes seems like a great concept, often the vigour with which samples are collected is lost when it comes to putting up and then monitoring the samples.

Dermatophytes can be easily grown on Sabouraud’s dextrose agar (SDA) and also on Dermatophytes test medium (DTM). Both media have advantages and disadvantages. SDA gives the best results for fungal culture. It allows the development of good colony morphology and the typical pigment changes which are important in identification of the different dermatophytes (Table 1).

The colony appearance of all of the common dermatophytes from small animals in the UK is white or brownish white so that anything that grows on the plate that is a marked variant from this will be a contaminant.

DTM is essentially SDA with the addition of cycloheximide, gentamicin and chlortetracycline as anti-fungal and anti-bacterial agents. Phenol red is added as an indicator.

Dermatophytes metabolise protein before they metabolise carbohydrate producing an alkaline metabolite that changes the medium from yellow to red. Most other fungi metabolise carbohydrate before protein so the colour change generally occurs after 10-14 days.

Therefore, after incubation of samples it is essential to check the plates daily, certainly for the first seven days. DTM has the advantage over SDA that is an excellent early indicator of the presence of pathogens but will mask colony morphology and reverse pigment formation.

Another dermatophyte culture medium is enhanced sporulation agar (ESA). This contains peptones, dextrose, agar, chloramphenicol, gentamicin and cycloheximide to inhibit the growth of contaminants and bromothymol as a pH indicator which turns the medium blue at an alkaline pH.

The ideal combination, therefore, for diagnosis and identification of dermatophytes is to use a combination of DTM and ESA. Double plates with both DTM and ESA offer the advantages of a speedy diagnosis with the ability to identify the species involved by the colony colour (Figures 1 and 2) and by identifying the macroconidia (Table 2).

The most recent evidence-based review on the therapy of dermatophytosis in dogs and cats (Moriello, 2004) suggests that many cases of dermatophytosis will respond to topical therapy without having to add in systemic drugs. The review suggests it is appropriate to attempt to treat all cases with topical medication for 2-4 weeks before adding in systemic therapy.

Where systemic therapy was used, itraconazole, terbinafine and lufenuron were evaluated. Itraconazole was reported to cure infected animals in 56-70 days. In the UK the licensed drug for cats is itraconazole which can be given at a dose of 5mg/kg po daily for seven- day cycles, a week on and a week off. This is also useful for dogs and small children’s pets (see Table 3).

Various doses of terbinafine (5-40mg/kg) were reportedly used to treat dogs or cats. The higher doses of terbinafine (> 20mg/kg) were required to achieve a mycological cure; the number of treatment days to cure varied from 21 to > 126 days. Lufenuron was reported anecdotally to be an effective cure; however, this was not substantiated in controlled studies.

In many cases, however, superficial fungal infection with dermatophytes can be resolved using topical therapy.

Shampoo therapy is useful and products containing such actives as chlorhexidine 2-4%, ketoconazole 1- 2%, miconazole 2% or acetic acid/boric acid 2% may be beneficial in resolving infection.

Moriello’s review suggests, based upon in vitro studies using isolated infected hairs and controlled or field in vivo studies, that lime sulfur (1:16), 0.2% enilconazole rinses, and a combined 2% miconazole/ chlorhexidine shampoo were the best products to treat infection topically.

Animals or hairs were either bathed or rinsed once or twice weekly in the studies reviewed. A more recent study in 2007 looked at a combination of lime sulphur used twice weekly as a leave on dip with itraconazole to treat shelter cats with dermatophytosis. The study found the combination was effective and very safe, producing a mean time to mycological cure of 18.4 days (range 10-49 days); the more severe cats requiring longer treatment. 

References and further reading 

Jackson, H. and Marsella, R. (eds) (2012) BSAVA Manual of Canine and Feline Dermatology, 3rd edition: Core investigation and diagnostic techniques, Littlewood, J. D. pp24-30; BSAVA.

Miller, W. H., Griffin, C. E. and Campbell, K. L. (2013) Muller and Kirk’s Small Animal Dermatology, 7th edition, chapter 2: Diagnostic techniques, pp89-90; 

Elsevier. Miller, W. H., Griffin, C. E. and Campbell, K. L. (2013) Muller and Kirk’s Small Animal Dermatology, 7th edition, chapter 5: Fungal and Algal Skin Diseases, pp225-230; 

Elsevier. Moriello, K. A. (2004) Treatment of dermatophytosis in dogs and cats: review of published studies. Vet Derm 15 (2): 99-107. Newbury, S., Moriello, K., Verbrugge, M. and Thomas, C. (2007) Use of lime sulphur and itraconazole to treat shelter cats naturally infected with Microsporum canis in an annex facility: an open field trial. Vet Derm 18 (5): 324-331.