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Author: Dr Maneka Gnanasegaram, Dermatology Registrar, Greenlane Hospital, Auckland, New Zealand, 2012.
A kerion is an abscess caused by fungal infection. It most often occurs on the scalp (tinea capitis), but it may also arise on any site exposed to the fungus such as face (tinea faciei) and upper limbs (tinea corporis). It is often misdiagnosed as bacterial infection.
A kerion presents as a boggy pus-filled lump, often several centimetres in diameter. It is characterised by marked inflammation. Hairs within the kerion are loose and fall out, often resulting in a bald area (localised alopecia).
Enlargement of the regional lymph nodes can occur, and some people become systemically unwell with fevers and malaise. It may be followed by a widespread itchy eczema-like rash (dermatophytide).
A kerion is caused by dramatic immune response to a dermatophyte fungal infection (tinea). The most common fungi found in kerion are:
Trichophyton rubrum is not a common cause of kerion.
Suspicion is raised due to the typical appearance of a kerion. Examination using a Wood lamp emitting long wavelength UVA may reveal yellow-green fluorescence if kerion is due to Microsporum canis, but is often negative even when this is the responsible organism because the inflammation obscures the presence of the fungus.
To confirm the diagnosis, scrapings and hair samples may be taken from the affected area for microscopy and fungal culture (laboratory tests for fungal infection). A bacterial swab should be considered as secondary infection with bacteria is common.
Yes, fungal infection may be transmitted by kerion to other members of the household if they are in close contact, especially if sharing bedding and towels. Combs and hairbrushes should be disinfected or discarded to prevent transmission of infection or re-infection.
As the fungus causing the kerion (e.g. M canis) is often residing on a household pet (e.g. kitten), family members may also be infected by direct contact with the animal.
Kerion should be treated by oral antifungal agents. A course of 6-8 weeks of treatment is normally prescribed at minimum. Topical antifungal agents are not effective due to deep invasion of fungus into the hair follicle.
The hair normally grows back after the infection is treated, but sometimes hair loss is permanent, especially if the infection has been longstanding.
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