Some 17,000 lichen species are distributed from the equator to the Arctic and Antarctic and from sea level to mountains. A lichen plant body is composed of an alga and a fungus, usually an ascomycete. A classification of the fungi is provided by Ainsworth (1971) and of the algae by Ahmadjian (1967).
Chemistry is recorded by Asahina & Shibata (1954) and Culberson (1969). The unique chemicals generally known as "lichen substances" e.g. usnic acid are derived from the fungi of lichens. Fungal isolates retain a capacity to produce them in culture (Ahmadjian & Reynolds 1961).
[Summary yet to be added. This monograph is unrevised. A recent comprehensive review is provided by Schmidt RJ (1996) Allergic contact dermatitis to liverworts, lichens and mosses. Seminars in Dermatology 15(2): 95-102.]
A patient who was contact sensitive to Evernia could not tolerate perfumes. Evernia enters into the composition of various perfumes (oak moss, Cuir de Russie, Chypre type perfumes) (Favennec M.-J and Favennec, F. 1976).
Two dealers in firewood developed contact dermatitis from the spores of Parmelia caperata on firewood logs (Leonardi 1954). Laria rufimana was implicated by Ciambellotti (1927, 1928).
Resinoid Mousse de Chene produced positive patch test reactions in 2/10 patients who had perfume dermatitis (Novak 1974).
Spillmann (1921) described dermatitis on the ear and side of the neck in a man who had carried freshly cut oak logs on his shoulder. He suggested that fungi on the bark might be responsible. Tommasi (1929) reported similar cases from Italy and suspected an idiosyncrasy because attacks often followed returning to work with wood. Dubreuilh (1931) described six cases of "woodcutter's eczema" and emphasized the role of the bark. Schulmann & Détouillon (1932) distinguished between chronic lichenified eczema of the hands, perhaps due to resin, and acute "sylvestro-sensibilization" of face, hands and genitals. Conjunctivitis was often associated with the dermatitis (Lagrange 1923).
Tenchio (1948) then obtained positive patch tests with Parmelia caperata, found growing on the bark of firs (Abies), larches (Larix) and chestnuts (Castanea).
Ghillini (1954) reported dermatitis from the same lichen growing on oak (Quercus) and Ilex. Le Coulant & Lopes (1956) found that other lichens and mosses also contained the allergen, but much the strongest reactions were obtained with Frullania and other liverworts of the Jungermannales. These liverworts were found on the bark of most deciduous trees in south-western France, and on undergrowth and rocks even among conifers, on the bark of which lichens prevail. In their review Le Coulant et al. (1966) concluded that liverworts were the main cause of woodcutter's eczema, and were in fact responsible for 2-4% of all contact dermatitis in that part of France. According to Philippe et al. (1965) they also cause rhinitis, asthma and urticaria, with positive intradermal tests. Knoche et al. (1969) showed that sesquiterpene lactones similar to those widespread in the Compositae were the sensitisers in Frullania tamarisci.
Mitchell (1965) investigated 'cedar-poisoning' in British Columbia lumbermen, obtaining positive patch tests with bark lichens and the usnic acid they contain, but negative results with the wood itself (Mitchell and Armitage 1965). Champion (1965) found similar usnic acid sensitivity in an English woodcutter's wife who had only indirect contact with ash bark through her husband's working clothes. Since then he has seen patients with contact sensitivity to lichens who did not react to usnic acid. Several atopic patients had exacerbations of eczema as well as rhinitis and asthma after contact with lichens and algae, especially on apple (Malus) trees. Three of them had positive patch tests with lichens (mainly Lecanora spp.), but algae gave only positive prick tests (Champion 1971). Mitchell et al. (1969) now consider that in America as in Europe Frullania is a more frequent cause of dermatitis than lichens, and have studied cross-sensitivity between liverworts and Compositae (Mitchell et al. 1970, 1972).
Alectoria, Cetraria, Cladonia, Lecanora, Parmelia, Physica, Usnea and Xanthoria species have been reported allergenic.
So-called "cedar-poisoning" in Canada and "pine-poisoning" in the north-west USA are principally due to occupational contact with epiphytes, lichens and Frullania. The dermatitis affects principally the exposed skin surfaces. Lichen substances, which were tested, were found to have no phototoxic activity (Mitchell 1966a). Less obvious distribution patterns are found when lichens fall inside an open shirt and lodge at the waist-line or when the clothes are wetted by rain. Usnic acid is insoluble in water but is carried down from trees by rainwater and is present in soil until degradation occurs.
Dermatitis occurring in woodworkers in Canada was found to be due to usnic acid present in Usnea spp. and other lichens present on trees (Mitchell 1965). d-Usnic acid was allergenic but l-usnic acid was inactive (Mitchell 1966b). Some depsides of lichens viz. atranorine, evernic and perlatolic acids were also allergenic (Mitchell and Shibata 1969). Some individuals who are contact sensitive to lichens show negative patch test reactions to usnic acid (Champion 1971).
Contact dermatitis from lichens occurring in forest-workers is often accompanied by multiple specific sensitivity to Frullania (liverwort). The algae of lichens can produce Type I hypersensitivity reactions (Champion 1965).
Opportunity for contact with lichens was discussed by Mitchell and Armitage (1965). Lichens are susceptible to air-pollution and tend to disappear from urban areas (Le Blanc 1961, Gilbert 1965) probably because they can accumulate extraneous chemicals from the environment (Smith 1962, Anon 1963). Lichenized fungal spores and portions of the lichen body (thallus) can be air-borne (Petterrson 1940, Garrett 1971). Evernia prunastri (oak moss) is used in pertumery (Arctander 1960) and various lichens have been used for dyeing cloth (Bulletin 1935). Usnic acid is marketed as an antibiotic ointment (Mitchell 1966b).
Immediate hypersensitivity to lichens was reported by (Philippe et al. 1965).