Two forest workers affected with allergic contact dermatitis, which occurred only during work in forest areas, showed positive patch test reactions to lichens containing usnic acid and to isolated usnic acid.
Lichens are plants composed of fungi living in symbiosis with algae. Usnic acid, one of the lichenic acids which accumulates in lichenized fungi, is a monobasic acid with dibenzofuran structure and antibiotic properties. Dibenzofuran is chemically related to furocoumarans.
Lichens are plentiful in temperate zone forests and allergy to usnic acid represents some part of the "cedar-poisoning" problem in British Columbia. Geographical distribution of lichens containing usnic acid suggests that allergy to usnic acid will be found to be more common than presently recognized.
MITCHELL JC. Allergy to Lichens: Allergic Contact Dermatitis From Usnic Acid Produced by Lichenized Fungi. Arch Dermatol. 1965;92(2):142–146. doi:10.1001/archderm.1965.01600140030006
Coronavirus Resource Center
Customize your JAMA Network experience by selecting one or more topics from the list below.