# Flagellate Dermatitis Induced by Shiitake Mushrooms—Clinical Features of a Rare but Characteristic Entity

## TL;DR

A rare skin condition called flagellate dermatitis can occur after eating undercooked shiitake mushrooms and is caused by a reaction to a substance called lentinan.

## Contribution

This case report highlights flagellate dermatitis as a rare but diagnosable condition linked to shiitake mushroom consumption and emphasizes the importance of proper cooking.

## Key findings

- The patient's rash was diagnosed as flagellate dermatitis after consuming undercooked shiitake mushrooms.
- The rash resolved with corticosteroids and antihistamines, and the patient was advised to cook shiitake mushrooms thoroughly.
- Flagellate dermatitis is a self-limiting hypersensitivity reaction to lentinan, a heat-labile polysaccharide in shiitake mushrooms.

## Abstract

We present the case of a 25-year-old male who attended a dermatological online consultation due to a whiplash-shaped, pruritic rash. The lesions in the form of well-demarcated linear erythematous papules, located mainly on the trunk and arms, had first appeared four days prior to the consultation. Chronic disease history was negative and similar symptoms never appeared previously. After reviewing the clinical images and excluding dermatographism, flagellate dermatitis was suspected. The diagnosis was subsequently confirmed when the patient reported having consumed undercooked shiitake mushrooms two days prior to the onset of the lesions. Topical corticosteroids and oral antihistamines were recommended, and the patient was informed about the necessity of high-temperature preparation of shiitake mushrooms in the future. Flagellate dermatitis is a rare entity speculated to be a hypersensitivity reaction to lentinan, a heat-labile polysaccharide found in shiitake mushrooms (Lentinus edodes). Symptoms include characteristic papular or vesicular lesions erupting in a linear pattern resembling whiplash marks, usually on the trunk and extremities. While the condition is self-limiting, the awareness of its manifestation is important in order to prevent unnecessary biopsies. Patients should be educated to avoid further exposure to lentinan, as instances of severe reactions following repeated contact have been described.