Shiitake mushroom dermatitis (flagellate mushroom dermatitis) is a rare cutaneous phenomenon that can occur after consuming raw or undercooked shiitake mushrooms.

Shiitake mushrooms (Lentinula edodes) are the second most commonly cultivated mushrooms worldwide. These mushrooms are particularly popular in Japan, where they are consumed as normal cuisine. In traditional Asian medicine, shiitake mushrooms are also used as adjunct therapy in the treatment of hypertension, hypercholesterolemia, and various cancers. Partly due to these proposed health benefits, shiitake mushrooms are becoming increasingly available and utilized in the Western world.

Although the pathophysiology remains unclear, the current leading hypothesis is that flagellate mushroom dermatitis is caused by a thermolabile polysaccharide called lentinan, which is found in shiitake mushrooms. Lentinan is thought to activate leukocytes and cause vasodilation. Given the linearly streaked nature of the lesions, an additional trauma-induced mechanism via the Koebner phenomenon has been suggested but remains controversial.

Most cases have been described in Japan, but recently, cases have also been reported in Europe and North America. In all cases, the dermatitis was observed in individuals who consumed raw or undercooked shiitake mushrooms, with the onset of lesions ranging from 12 hours to 5 days after ingestion. In its classic presentation, there is a rapid eruption of erythematous papules and/or vesicles grouped in multiple linear streaks, coalescing in a flagellate pattern, with accompanying pruritus.

Without continued consumption of raw or undercooked shiitake mushrooms, this condition is self-limited, with spontaneous resolution in 1-8 weeks.