Grain mites. A small epidemic caused by Pyemotes species

Abstract

Pyemotes mites, of which the grain itch mite is the most important for human medicine, are ectoparasites of many insect species. Human infestation occurs only accidentally and temporally. The source materials are most often grain, straw and plant-based foods. Bites produce an intensely itching urticarial wheal capped by pin-point vesicles, which often are destroyed by scratching. Pyemotes dermatitis favours the back, the abdomen and the flexor side of the forearms. The mites are barely visible to the naked eye and reside only briefly on humans and thus are not found by physicians during clinical examination. When itching starts, the parasites have usually left their hosts. Therefore it is important to investigate the patient's environment and to identify the infested materials. We describe an outbreak of Pyemotes dermatitis in at least 16 persons, most of them warehouse workers in a hobby mail-order house. Infested decorative wheat was the source of the problem. Successful eradication was achieved by fumigation of the storage facilities with the insecticide methylbromide. Our cases demonstrate the importance of examination of environmental specimens for ectoparasites in cases of unexplained dermatoses. Biological agriculture avoiding insecticides may cause reappearance of this almost forgotten skin disease.