Case Presentation:

A 60–year–old man presents to the hospital with 4 days of rash accompanied by severe pain and pruritus. The rash began on the patient’s hands bilaterally, then, began to spread centrally. He denied any exposure to wooded areas or bug bites. He had no fevers, chills or constitutional symptoms. His past history was significant for PTSD, Bipolar disorder, and Alcohol abuse. He had been on stable doses of divalproex sodium and bupropion. Family history was unremarkable. Review of systems was notable only for change in diet, as the patient had increased his intake of vegetables (including shiitake mushrooms) while trying to lose weight. He was afebrile and hemodynamically stable. His physical exam revealed erythematous and purpuric linear plaques on his upper arms, trunk, and upper legs. Nonpalpable petechiae were noted on his fingers and dorsal aspect of his feet. Mucosal surfaces were spared. Otherwise physical exam was unremarkable. A dermatology consult was obtained for skin biopsy. Skin biopsy from the abdomen showed perivascular lymphocytic infiltrate with eosinophils and slight spongiosis. A diagnosis of flagellate dermatitis secondary to shiitake mushrooms was made. The patient was discharged home with topical steroids, hydroxyzine for pruritis, and oxycodone–acetamenophen for pain.

Discussion:

Flagellate dermatitis is a toxicoderma caused by eating undercooked shiitake mushrooms. The toxic reaction is thought to be secondary to the polysaccharide lentinan contained by the mushrooms, which is a thermophile – explaining why the dermatitis occurs after eating raw or half–cooked mushrooms. The mechanism is not completely understood, but thought to be related to IL–1 secretion caused by lentinan. The rash occurs 24–48 hours after ingestion in afflicted patients, and is in a flagellate type pattern. It is intensely pruritic and can involve any part of the body, although the trunk is most common site. There is no mucosal involvement. It is important to rule out other causes of rashes that may present in a similar fashion, including tick borne illnesses, viral exanthems, bacteriologic infections, and dermatographism. Pathology shows spongiosis, dilated capillaries and lymphocytic infiltrate. Treatment is withdrawal of the offending agent, topical steroids, and anti–histamines.

Conclusions:

The purpose of reporting this case is to increase awareness of this uncommon dermatologic phenomenon, as it can go undiagnosed if consideration of shiitake mushroom exposure is overlooked.

Figure 1Flagellate dermatitis from eating raw Shitake mushrooms.