Flagellate Mushroom Dermatitis (Shiitake Dermatitis, Toxicoderma)

Are You Confident of the Diagnosis?

Flagellate mushroom dermatitis, or shiitake dermatitis, was first described in Japan by Nakamura in 1977. Most cases have been reported from Asia, but recently, scattered cases have been reported from the United Kingdom, Germany, and France. Three cases have been reported recently from the United States.

Characteristic findings on physical examination

A characteristic rash typically occurs 24-48 hours after consuming either raw or cooked shiitake mushrooms (Lentinus edodes). Characteristic skin lesions appear as linear, streaky, urticarial papules, mostly noted on the trunk and extremities (Figure 1, Figure 2). The rash is typically very pruritic. In Japan, photosensitivity reactions have been reported. Mucous membrane involvement is not seen.

Figure 1.

Acute flagellate erythema, occurring 48 hours after ingesting a dish with shiitake mushrooms.

Figure 2.

Acute flagellate erythema, occurring 48 hours after ingesting a dish with shiitake mushrooms.

The skin lesions generally cannot be reproduced by scratching. Unlike dermatographism, the lesions are persistent, but the rash does eventually resolve on its own within 1-3 weeks.

Expected results of diagnostic studies

Histologic findings are nonspecific; most cases demonstrate epidermal spongiosis, acanthosis, and a nonspecific perivascular infiltrate.

Laboratory studies are usually within normal limits. However, eosinophilia, leukocytosis, leukopenia, elevated transaminase levels, and increased serum lactate dehydrogenase levels have been reported.

Patch testing to the mushroom is negative and is not useful.

Diagnosis confirmation

The differential diagnosis would also include flagellate erythema following the use of the drug bleomycin. Unlike shiitake dermatitis, flagellate erythema from bleomycin results in linear hyperpigmentation. Flagellate erythema can also occur after use of the drugs docetaxel and peplomycin, as well as in adult Still’s disease, breast cancer, and dermatomyositis. The cause of flagellate erythema in breast cancer and dermatomyositis is unclear, but it is not related to drug use in these conditions. In dermatomyositis, flagellate erythema may be the presenting sign of the disease.

Treatment is not necessary, as the condition resolves on its own. Oral antihistamines and topical steroids have been used for symptomatic relief. Systemic corticosteroids should rarely be needed.

Who is at Risk for Developing this Disease?

Shiitake dermatitis typically occurs 24-48 hours after eating raw or partially cooked mushrooms. Cases have also been reported from eating fully cooked mushrooms. The condition has also occurred after consumption of a health drink containing dried shiitake mushroom extract. Patients who develop shiitake mushroom dermatitis may be unaware of having consumed the mushrooms, as mushrooms can be concealed in many dishes in Asian cuisine. Therefore, a detailed history of the patient’s recent diet is of utmost importance.

In mushroom workers, reactions to shiitake mushrooms can present with allergic alveolitis or bronchitis, contact dermatitis, and contact urticaria. Patch testing in these patients may be positive, but not in individuals who only consume the mushrooms.

What is the Cause of the Disease?

It is likely that the reaction is a toxic reaction to the mushroom polysaccharide lentinan, which is thought to have anti-tumor properties and has been used as a chemotherapeutic agent in Japan. When used as a chemotherapeutic agent, it may cause cutaneous reactions similar to shiitake dermatitis. Lentinan and bleomycin are completely different structurally, and work by different mechanisms. It seems likely, therefore, that they cause flagellate erythema by different mechanisms.


Shiitake dermatitis is thought not to be an allergic reaction, as these patients have negative prick and patch tests. In addition, rechallenge to shiitake mushrooms does not always produce a cutaneous reaction.

Shiitake mushrooms are the second most commonly consumed mushroom, and are widely eaten all over the world. It is, therefore, very curious why so few cases have been reported outside Asia. More cases of shiitake dermatitis are likely to appear as dermatologists become more aware of this diagnosis.

Systemic Implications and Complications

There are no known systemic complications from shiitake mushroom dermatitis. Although allergic alveolitis (mushroom worker’s disease) has been reported in individuals who cultivate shiitake mushrooms, this condition has not been seen in those who only consume the mushrooms.

Treatment Options

Treatment is symptomatic only. The condition resolves on its own in several days to weeks.

Topical steroids, and antihistamines such as hydroxyzine 25mg daily, may be helpful. For severe cases, prednisone at a dose of 1mg/kg, tapered over 2-3 weeks, would be recommended.

Optimal Therapeutic Approach for this Disease

The most important factor is prompt diagnosis and recognition of the offending agent—the shiitake mushroom. The history and clinical picture of flagellate erythema from shiitake mushrooms is quite characteristic and would be unlikely to be confused with any other disease. Patients should be reassured that the condition will resolve on its own, without any permanent sequelae. Treatment is symptomatic.

Patient Management

Most cases of shiitake dermatitis reportedly have resulted from eating raw or partially cooked mushrooms. Therefore, it is important to fully cook the mushrooms before consuming. However, cases have also been reported from eating fully cooked or boiled mushrooms, and such patients should avoid eating shiitakes in the future. In addition, when eating in restaurants, patients should always inquire about whether there are shiitake mushrooms used in the food preparation, as it may not be obvious.

If patients with a history of shiitake dermatitis require chemotherapy with bleomycin, it would unlikely be a concern, as the active component of shiitakes, lentinan, is unrelated to bleomycin.

Unusual Clinical Scenarios to Consider in Patient Management

Although allergic alveolitis and respiratory symptoms have been reported in mushroom workers, these symptoms have not been reported from simply eating shiitake mushrooms.

What is the Evidence?

Nakamura, T. “Toxicoderma caused by shiitake (Lentinus edodes)”. Jpn J Clin Dermatol. vol. 31. 1977. pp. 65-8. (This was the first reported case history of the characteristic whip-like rash seen from ingesting shiitake mushrooms.)

Nakamura, T. “Shiitake (Lentinus edodes) dermatitis”. Contact Dermatitis. vol. 27. 1992. pp. 65-70. (Article describes fifty-one cases of shiitake dermatitis. Most occurred after eating half-baked or raw shiitake mushrooms. The polysaccharide lentinan is proposed as an etiologic factor.)

Hanada, K, Hashimoto, I. “Flagellate mushroom (shiitake) dermatitis and photosensitivity”. Dermatology. 1998. pp. 255-7. (A case of a 44-year-old man with shiitake dermatitis is presented with a photodistribution. The authors review ninety-seven cases of shiitake dermatitis, of which 47% developed involvement of sun-exposed areas. They proposed that the toxin may be a photosensitizer.)

Tarvainen, K, Salonen, JP, Kanerva, L, Estlander, T, Keskinen, H, Rantanen, T. “Allergy and toxicodermia from shiitake mushrooms”. J Am Acad Dermatol. vol. 24. 1991. pp. 64-6. (A woman presents with pulmonary symptoms and allergic alveolitis after cultivating shiitake mushrooms. Patch tests to shiitake were positive. The patient did not ingest the mushrooms, and a rash was not seen. A second patient was presented who ate raw shiitake mushrooms and developed the characteristic rash.)

Lee, JH. “Five cases of shiitake dermatitis”. Korean J Dermatol. vol. 36. 1998. pp. 477-81. (Five patients with shiitake dermatitis are presented, 2-3 days after eating raw or fully cooked mushrooms. The rash cleared within 7 days in all patients. These were the first reported cases in the Korean literature.)

Chu, E, Anand, D, Dawn, A, Elenitsas, R, Adler, D. “Shiitake flagellate dermatitis: A report of 3 cases”. Cutis, submitted for publication. (Three cases of shiitake dermatitis are described—the first cases reported in the United States. Two patients ate fully cooked mushrooms after eating at an Asian restaurant, but were not aware they had consumed the mushrooms. Only after contacting the restaurant kitchen was it confirmed that shiitake mushrooms were in the dishes they had consumed.)

Mak, R, Wakelin, S. “Shiitake dermatitis: the first case reported from a European country”. Br J Dermatol. vol. 154. 2006. pp. 800-1. (A 37-year-old man is presented with a recurrent pruritic rash in a flagellate pattern. He had eaten large amounts of raw shiitake mushrooms 2 days prior to the onset of the rash. This is the first reported case from a European country.)