Definition
Ringworm (dermatophytosis) is a skin disease caused by infection with fungi that invade the superficial keratin layers of the claw, hair shaft, and epidermis.Dermatophytes may be found on animals without skin disease resulting. People may potentially develop infection after handling infected animals (zoonotic risk).
Cause
Most disease in dogs is caused by the following dermatophytes:
- Microsporum canis, a species normally found on animals
- Microsporum gypseum, a species that normally inhabits soil
- Trichophyton mentagrophytes, a species normally found on animals
These dermatophytes are able to utilise keratin to survive in the superficial layers of skin, hair and claws.
The incidence of ringworm varies around the world particularly with climate: warm, humid conditions increase the prevalence of infection. Which species is causing disease also varies with geography, season and access to rural environments.
Transmission occurs through direct contact with infected animals or through contact with infected hairs or scale on surfaces such as grooming equipment and bedding or in the infected animal's environment. Some animals may act as asymptomatic carriers.
Factors influencing likelihood of disease and type of disease displayed include skin trauma, ability to self-groom, and ability to form an inflammatory reaction. Skin inflammation results from a reaction to substances produced by the fungi. The reaction is described as similar to a contact dermatitis.
Signalment and history
- Lesions are more commonly seen in dogs < 1 year old
- Dogs with compromised immune systems, such as those on long-term immunosuppressive medications (for example glucocorticoids or chemotherapy) or those with systemic disease (for example neoplasia or hyperadrenocorticism) are also at higher risk of developing dermatophytosis
Zoonosis
Ringworm lesions, most commonly due to Microsporum canis, have been reported in humans who have handled infected animals. Humans considered at greatest risk of zoonosis include:
- Children
- People not previously exposed to Microsporum canis
- Immunosuppressed individuals
Clinical signs
- Asymptomatic carriers may occur
- Lesions are generally follicular, resulting in circular patches of alopecia with variable scale
- Signs vary significantly between individuals. Pruritus is usually minimal or absent, except in rare cases (Trichophyton is more likely to be pruritic)
- Dermatophytosis should be considered when any annular, papular or pustular lesions are observed
Focal lesions
- Classical lesions have an annular appearance with healing tissue in the centre and an expanding periphery of alopecia, scale, crusting and follicular papules and pustules
- Area of folliculitis or furunculosis affecting one paw or leg (for example Trichophyton spp.)
- Nodular exudative furunculosis with multiple draining tracts ('kerion')
- Rarely depigmentation of the nose
- Nail bed lesions (onychomycoses) are rare, generally asymmetric and result in inflammation of skin around the nail base or nail malformation
Auto-immune disease-like lesions
- T. Mentagrophytes in particular may cause symmetrical nasal or facial folliculitis and furunculosis
Generalised lesions
- Seborrhoeic eruptions with greasy scale
- Widespread collarettes without involvement of the hair follicles
Lesions often start on the face and forelegs.
Diagnosis
Differentials
- Most commonly other follicular diseases such as Staphylococcal dermatitis and demodicosis
- Other skin disease such as hypersensitivities (fleas, atopic dermatitis)
- Auto-immune disease with severe crusting lesions
- Kerion may resemble granulomas, foreign body reactions, acral lick granuloma or neoplasia
Fungal tests
- Examination under Wood's lamp: a useful screening test. Some species or strains will fluoresce green under UV light (for example approximately 50% of M.canis infections fluoresce). A positive result should be confirmed with fungal culture
- Microscopy: not as sensitive as fungal culture. Trichogram or superficial skin scrape may show fungal hyphae and arthrospheres
- Fungal culture of affected hair and scales: the diagnostic test of choice
Treatment and management
Treat with an appropriate antifungal medication
- In some instances this may involve extra-label use of medication
- Consider treatment of in-contact animals
- Treatment combines 3 different approaches
- Topical antifungal medications
- Creams and lotions are used to treat localised lesions. Hair is gently trimmed (avoiding trauma to skin). Treatment is applied over lesions and up to 6 cm of surrounding skin
- Shampoos or rinses are used to kill infective material before it enters the environment. Rinses have some residual component. Antifungal shampoos may include chlorhexidine and/or azoles
- Skin trauma from clipping or maceration (for example stripping shampoos) has the potential to spread infection
- Topical agents are used until 2-3 fungal cultures 1 week apart are negative
- Systemic antifungal therapy
- Generally used with multi-focal lesions, long-haired animals, multi-animal households
- Consider if lesion is not responding to topical therapy after 2-4 weeks
- Examples include azole medications
- Environmental management
- Clean housing, bedding, brushes etc to reduce spread of infection or re-infection
- Treatment targets potentially infectious hair shafts and scales shed from infected animals
- Examples include chlorine bleach (1:10 - 1:100 dilution)
