Shiitake dermatitis is a big price to pay for the joy of shiitake mushrooms.  The dermatitis consists of intense itching that occurs shortly after ingesting raw or undercooked shiitake mushrooms.  Characteristic of the rash are raised, red streaks which is also called “flagellate erythema” because it literally looks like the sequela of being whipped.  Other than this characteristic appearance, the dietary history is the cue to diagnosis.    

It responds quickly to oral antihistamines and/or steroids.  It is prevented by adequate cooking of the mushrooms.  But some Eastern health supplements may also contain raw shiitake. 

The shiitake mushroom contains lentinan, a sugar molecule called 1,3 beta-glucan.  In mice experiments when its given intravenously it has anti-cancer and anti-viral benefits.  For cancer it disrupts harmful intra-cellular signaling that helps cancer spread.  For viruses it inhibits tissue cells from absorbing the harmful virus.  Given orally to mice it has no benefit.  It also increases the production of an immune molecule called interleukin-1 (IL-1).  It’s the IL-1 that causes vasodilation (dilation of skin capillaries) and rash.