Dermatophytosis

What it is

Dermatophytosis ("ringworm") is an infection of keratinised structures (hair, stratum corneum, claws) by dermatophyte fungi. In the cat the great majority of cases are caused by Microsporum canis; less often Trichophyton mentagrophytes (from rodents) and Nannizzia gypsea (formerly Microsporum gypseum, from soil) are involved. The cat is the main reservoir of M. canis. The spores (arthroconidia) spread by direct contact with infected animals or with hairs and scales in the environment, where they can survive for a long time, and need a small skin injury to establish infection. The fungi invade the follicle and the hair shaft, which becomes fragile and breaks.

It is a zoonosis: people, especially children, are easily infected and develop circular skin lesions.

Who it affects

Kittens, long-haired cats (the Persian is strongly predisposed), cats in shelters, catteries and colonies, and immunosuppressed cats. In healthy adults the infection is usually mild and may resolve on its own within months, although the cat remains contagious meanwhile. Some cats are mechanical carriers without lesions.

Anatomy involved

• Hair follicles and hair shafts: the fungus forms spores around and inside the hair, causing it to break and resulting in alopecia.
• Skin: scaling, crusting and sometimes erythema; inflammation is usually mild.
• Claws and claw folds: onychomycosis, uncommon.
• In Persians, pseudomycetomas may form: deep dermal or subcutaneous nodules caused by M. canis.

Clinical signs

Highly variable. Typical lesions are circular or irregular patches of alopecia with scaling and broken hairs, mainly on the head, ears, muzzle and forelimbs, although lesions can be generalised. Pruritus is variable, usually mild. Other presentations: miliary dermatitis, acne-like chin lesions, granulomas or diffuse scaling. People in the household may develop red, circular lesions with a scaly border.

Diagnosis

• Wood's lamp: many M. canis isolates make infected hairs fluoresce apple green; a negative result does not rule out infection, and fluorescence of scale or medications can mislead.
• Trichogram: hairs with spores and hyphae under the microscope.
• Fungal culture, the reference test: the whole coat is brushed with a sterile toothbrush (McKenzie technique) and inoculated onto dermatophyte medium; it identifies the species and is used to monitor treatment.
• PCR: fast and sensitive, but it can detect DNA from dead fungi, so on its own it cannot confirm cure.
• Biopsy for pseudomycetomas and atypical presentations.

Treatment

Although it can resolve on its own in healthy cats, it is always treated because of the risk of spread to animals and people:

• Systemic treatment: itraconazole (licensed for cats in some countries) or terbinafine. Griseofulvin is used less because of its adverse effects: it is teratogenic and can cause bone marrow suppression, especially in FIV-positive cats.
• Whole-body topical treatment twice weekly: lime sulphur dips, enilconazole rinses or miconazole and chlorhexidine shampoo, which reduce coat contamination. Clipping can help in long-haired cats, taking care not to injure the skin.
• Environmental decontamination: vacuuming and cleaning surfaces, washing bedding and disinfecting with sporicidal products; confining the cat to an easily cleaned area during treatment.

Treatment continues until mycological cure: resolution of lesions and negative cultures. Shelters and catteries need a control plan for the whole population.

Key studies

• WAVD consensus (Moriello et al., 2017): the reference guideline for dogs and cats. No single test is the gold standard; effective treatment combines an oral antifungal with topical treatment of the whole coat, and physical cleaning is the most important step in decontaminating the environment. Serious complications from transmission to people are exceedingly rare.

Diagnosis and cure

• DeTar et al. (2019), two years of records from a US animal shelter. The prevalence of M. canis was 1.8% and kittens were 8 times more likely to be infected than adults. Compared with culture, the Wood's lamp had a sensitivity of 66.8% and a specificity of 74.8% in this shelter. All diagnostic cultures from infected cats showed growth within 7 days (median 4).
• Moriello and Leutenegger (2018), 52 shelter cats with lesions. Quantitative PCR agreed with culture in 49 of 52 cats and detected 45 of 46 M. canis infections. It was less useful for confirming cure: after two negative cultures, the M. canis-specific PCR correctly classified 84.8% of cats and the Microsporum spp. PCR only 65.2%, because it remained positive in culture-negative samples.
• Stuntebeck and Moriello (2020), 371 shelter cats with weekly cultures during treatment. In 90.3% the first negative culture already indicated mycological cure. The exceptions were sampling errors early in treatment and cats with other illnesses, which took longer to cure (mean 11 weeks). In healthy, well-treated cats two negative cultures may not be necessary. It is retrospective and from a single programme.

Topical treatment

• DeTar et al. (2025), a randomised trial of 76 cats from four shelters, all treated with oral itraconazole. With lime sulphur all cats were cured within 7 weeks (mean 27 days); with miconazole-chlorhexidine shampoo 37.5% were not cured in that time (mean 37 days) and with hydrogen peroxide shampoo 21.7% (mean 36 days). The formal inferiority test was inconclusive. Younger cats took longer to cure, and staff noticed no difference in how the cats tolerated each product.
• Newbury et al. (2011), an open, non-randomised trial of 90 shelter cats with severe dermatophytosis, all given oral itraconazole for 21 days. Median time to cure was 30 days with lime sulphur, 34 with an odour-masked lime sulphur. With a miconazole-chlorhexidine rinse, 13 of 32 cats needed repeat treatment and the remaining 19 took a median of 48 days to cure.