Clinical update: Decoding dermatophytes

The itch from the red, inflamed rash on my stomach was intense. Really intense. I knew I shouldn’t scratch it but I just couldn’t stop myself.

I blamed a contact dermatitis to my belt buckle or thought that my eczema had recurred in an unusual place – but changing the belt and my usual dermatitis treatments didn’t work. As the circular rash emerged, I realised I had ringworm. An antifungal cream cured the infection. 

Dermatophytoses, superficial infections of the skin, hair and nails by fungi that feed on keratin, can develop on almost any part of the body including the scalp (tinea capitis), beard (tinea barbae), body (tinea corporis), hand (tinea manuum), groin (tinea cruris), feet (tinea pedis) and nails (onychomycosis; also called tinea unguium).¹ As in my case, ringworm (tinea of the body other than nails) is usually easily treated – but do not underestimate dermatophytoses.

Patients with toenail onychomycosis, for example, may find walking painful and buying comfortable shoes difficult.² Onychomycosis, which accounts for about 50 per cent of nail disorders and approximately 30 per cent of superficial fungal infections, may also allow secondary bacterial infections to develop, which may increase the risk of foot ulcers in people with diabetes.³ Indeed, before the launch of modern antifungals, one dermatologist called onychomycosis “that condition we do not wish on our worst enemies”.⁴ 

Fungal foes

Numerous dermatophytes picked up from other people, soil and animals can cause tinea, including species of Trichophyton (responsible for infections of skin, hair and nails), Epidermophyton (skin) and Microsporum (skin and hair).¹ Trichophyton rubrum and Trichophyton mentagrophytes cause 60-70 per cent of dermatophyte nail infections. Other causes of onychomycosis include non-dermatophyte moulds, such as Scopulariopsis brevicaulis and Aspergillus species. 

Yeast onychomycosis, usually caused by species of Candida, is more likely in fingernails, especially in people who frequently immerse their hands in water.⁵ Some onychomycosis patients have mixed infections.³

Certain dermatophytes are species-specific: T. rubrum “rarely, if ever,” causes disease in non-human animals.⁶ Other dermatophytes are more promiscuous. Microsporum canis can infect mammalian species as diverse as gibbons, goats and guinea pigs. T. mentagrophytes also infects numerous mammalian species and even chickens. Both can infect humans⁶, so hands should always be washed after contact with soil or animals. Suggest that people check their skin if they have been in contact with a person or animal with ringworm and take animals to a vet if they have signs of ringworm, such as patches of missing fur.⁷ 

Nailing the diagnosis

Once a dermatophyte spore sticks to skin or nails, the fungus releases enzymes that digest keratin. In response, keratinocytes (the most common cell in the epidermis) release a cocktail of cytokines to destroy the dermophyte¹ but the fungal infection can overwhelm these defences. 

The cytokines trigger inflammation. The ringworm rash is, typically, red or silver. A ringworm rash may appear darker than the surrounding skin on brown and black skin making it harder to see. The rash may be scaly, dry, swollen or itchy.⁷ Sweating, friction and an alkaline environment mean that intertriginous areas (e.g. groin, axilla and spaces between toes) are particularly susceptible to ringworm.¹

Dermatologists recognise several types of onychomycosis. Distal subungual onychomycosis accounts for about 90 per cent of toenail infections.⁵ Typically, onychomycosis patients show subungual hyperkeratosis – i.e. keratinocytes accumulate under the nail plate, which may lift and detach (onycholysis). Unless treated, the nail plate thickens and crumbles.²  

More severe onychomycosis may cause ridging and ingrown nails.5 Occasionally, patients show haemorrhages and may lose the entire nail.² 

The infection pattern can offer a clue. In older people, toenail onychomycosis is four times more common than infection of fingernails.³ Unless the patient has experienced trauma or is immunosuppressed, onychomycosis of more than one fingernail is unusual without concomitant toenail infection.⁵ 

If a patient presents with tinea, ask if they have onychomycosis or ringworm elsewhere on their body. For instance, many people with onychomycosis (about a quarter of older patients) also have tinea pedis.³

Some people are at particularly high risk. Older men are twice as likely to develop toenail onychomycosis as women³, while older patients with diabetes are between two and three times more likely to develop onychomycosis than people without diabetes.³ Immunosuppressed people (e.g. chemotherapy, steroids or diabetes) should be referred to their GP.⁷ 

Onychomycosis is responsible for about half of nail dystrophies.3 Several conditions can, however, mimic onychomycosis including lichen planus, psoriasis, contact dermatitis, paronychia, trauma (e.g. friction from footwear), Bowen’s disease, squamous cell carcinoma and melanoma.^2,3 Any cases that are not clearly tinea or that do not respond to treatment should be referred.