Why You Must Cook Shiitake Mushroom Properly—Or Risk Painful Rash That Sent Man to the ER

Photo byNew England Journal of Medicine

When you think of mushrooms, particularly the common shiitake variety, the last thing that comes to mind is a painful skin condition. But for one 72-year-old man in Switzerland, a simple meal of undercooked shiitake mushrooms left him with an agonizing rash that covered his back in whip-like streaks, landing him in the emergency room (ER) in 2023.

Agonizing Rash that Landed Man in Emergency Room (ER)

The unusual case, published in The New England Journal of Medicine, details how the man developed an intense rash two days after consuming the mushrooms. The itching became so unbearable that he was sent to the ER.

Doctors were greeted with an alarming sight: inflamed streaks across his back and upper buttocks that resembled whip marks. These distinctive lines baffled the medical team at first, as they ruled out common causes like dermographism, a condition where raised marks appear on the skin after it’s scratched, and infections that typically present with swollen lymph nodes.

It was the man's recent meal of shiitake mushrooms that provided a crucial clue. After considering his diet, the doctors diagnosed him with shiitake dermatitis, a relatively rare but well-documented skin reaction characterized by linear, whip-like rashes.

What is Shiitake Dermatitis?

First documented in Japan in 1977, shiitake dermatitis occurs primarily in Asia, where shiitake mushrooms (Lentinula edodes) are a dietary staple. But cases have been reported across Europe, North America, and South America, particularly among people who eat raw or undercooked shiitake mushrooms.

This skin condition is caused by a compound found in the mushrooms called lentinan, a complex carbohydrate that can trigger an inflammatory response in the body. Lentinan sets off a chain reaction of chemical messengers, such as interleukin-1, which cause blood vessels to dilate, leading to the characteristic streaky rash.

Interestingly, this reaction only happens when the mushrooms are eaten raw or undercooked. Cooking the mushrooms at temperatures above 145°C (293°F) breaks down lentinan, preventing the inflammatory response.

Dr. Eglė Janušonytė, a dermatologist involved in the case report from Swiss Geneva University Hospitals, emphasized the importance of thoroughly cooking shiitake mushrooms to avoid this condition.

Similarities to Other Conditions

The rash caused by shiitake dermatitis bears a striking resemblance to skin reactions seen in other medical conditions.

For example, patients undergoing treatment with the chemotherapy drug bleomycin sometimes develop similar linear streaks on their skin. It also mimics the rashes tied to inflammatory diseases like dermatomyositis, which is known for causing both muscle weakness and skin inflammation, and adult-onset Still's disease, a form of arthritis.

Despite the rash’s alarming appearance, shiitake dermatitis is usually harmless and resolves on its own, although the inital rash can be unberable and painful. Most patients don't require treatment beyond anti-inflammatory medications to alleviate the discomfort.

In this particular case, the man was prescribed topical steroids and oral antihistamines to reduce the itching and swelling. Two weeks later, his symptoms had largely subsided, although he was left with lingering darker patches of skin, a common side effect after inflammation.

So, next time you're preparing shiitake mushrooms, make sure they’re fully cooked—your skin will thank you!