Poison ivy and poison oak (Rhus spp.) are major causes of allergic dermatitis in the USA, but rare in western Europe, although occasional outbreaks are attributed to the Japanese lacquer tree (Rhus verniciflua), planted as an ornamental. This family is an important cause of dermatitis worldwide, including mango (Mangifera indica) in the tropics, cashew nut tree (Anacardium occidentale) and marking nut tree (Semecarpus anacardium) in India and Smodingium argutum in Southern Africa. Allergic reactions present typically as a florid vesiculobullous dermatitis, often in streaks, affecting the face, limbs and sometimes genitalia. Black specks of oxidised urushiol may be found on skin or clothes. Patch testing with these plants or their allergens (pentadecylcatechols) carries a high risk of active sensitisation and should be avoided.

Members of this huge family (over 20,000 species) are the commonest causes of plant dermatitis reports worldwide. It includes ornamental plants such as the florist’s chrysanthemum (Dendranthema cvs.), vegetables and herbs and native and introduced weeds, such as Parthenium hysterophorus in India and Africa. Dermatitis may initially affect the hands, but an airborne pattern is characteristic, mimicking photosensitivity but affecting skin folds and areas shielded from sunlight (Fig. 22.6). Photoallergy to Asteraceae rarely, if ever, occurs; however, secondary photosensitivity (chronic actinic dermatitis) may follow repeated episodes of airborne antigenic challenge from material shed from the surface of the plants.

Together with Anacardiaceae, Asteraceae may cause systemic contact dermatitis after ingestion of material to which the individual has been previously sensitised by contact. Feverfew, taken orally for migraine, is an example. This resembles the “baboon syndrome” or Systemic Drug-Related Intertriginous and Flexural Exanthema (SDRIFE) caused by drugs [10].

There are over 5,000 sesquiterpene lactones, the terpenoids responsible for Asteraceae contact dermatitis. The “sesquiterpene lactone mix”, comprising three pure chemicals, costunolide, dehydrocostus lactone and alantolactone, has traditionally been used as a screen for Asteraceae allergy [11]. Active sensitisation is rare with this mix, but it only detects around 30–35 % of allergic patients [12]. Additional patch testing with parthenolide, with careful addition of extracts of locally important Asteraceae, increases the sensitivity [13]. The compositae mix [14] is comprised of ether extracts of five wild plants (Table 22.4); it detects more patients but carries a higher risk of active sensitisation and can give false-positive irritant reactions in higher concentrations. Furthermore, the potential variability of chemical constituents in wild populations of these plants can affect the reproducibility of such a mix. The 6 % mix devised by Hausen elicits more positive reactions than the 5 % mix currently available commercially [13]. Recently a modified sesquiterpene lactone mix has been proposed, which better reflects the structural range of different sesquiterpene lactones and may prove to be a more sensitive testing material [15].

The classical streaky eruption on the forearms from handling Primula obconica is now rare, since the development of “hypoallergenic” cultivars which contain less primin [16]. Primin, a quinone, is the principal allergen, although miconidin can also sensitise [17]. In common with dermatitis induced by other quinones, primin may induce an erythema multiforme – like eruption. Primin appears not to be responsible for the dermatitis induced by hardy Primula species in alpine gardeners [18].