Shiitake Dermatitis (Flagellate Rash from Raw Shiitake)
{{Shiitake dermatitis}} is an itchy, streaky 'flagellate' rash caused by eating raw or undercooked {{shiitake}} mushrooms, attributed to the polysaccharide {{lentinan}}. It appears about a day after eating, resolves on its own in a few weeks, and is prevented by thoroughly cooking the mushrooms.
Shiitake dermatitis (also called flagellate dermatitis or shiitake toxicoderma) is a distinctive skin reaction caused by eating raw or undercooked shiitake mushrooms (Lentinula edodes). **Appearance.** The hallmark is an erythematous, intensely itchy, micro-papular rash arranged in linear, whip-like ('flagellate') streaks — as if the skin had been lashed — that can appear all over the body, including the face and scalp. It typically emerges about 24 hours after eating the mushrooms and resolves on its own over roughly 3 to 21 days. **Cause.** The reaction is attributed to lentinan, a heat-sensitive beta-glucan polysaccharide in shiitake. It is generally considered a toxic/irritant reaction rather than a classic allergy, since prior sensitization is not required. The condition was historically most reported in East Asia where raw shiitake is more commonly eaten, but reports have risen elsewhere as shiitake consumption spread. **Prevention.** Thorough cooking denatures lentinan and eliminates the risk, which is why shiitake is recommended to be eaten well cooked. The same reasoning is one reason raw shiitake is discouraged for dogs, where cooked shiitake is otherwise considered non-toxic.