By far the most common causes of dermatitis in agricultural workers are members of the the Anacardiaceae family of the Toxicodendron genus: poison ivy, poison oak, and sumac poison collectively, followed by pesticides. The phytochemical substance common to the three related plants, not members of the ivy, oak, or sumac families, is a light, nearly colorless oily substance, urushiol. These plants have very fragile leaves, which allows for the oily urushiol to escape from the resin canals onto the surface of the leaves following minor contact. The difference among the substances in these three plants is in the number of carbons and saturation of the side chains, owing to varying levels of potency (35,38,42).
Poison ivy, T. radicans and T. rydbergii, is related to the cross-reacting species of cashew (Anacardium occidentale), which causes dermatitis as a result of a brown oily substance in the cashew nut shell; and ginkgo (Ginkgo biloba, Grevillea), which contains ginkgolic acid (similar to urushiol). Poison oak, T. diversilobum and T. toxicarium, is related to the Indian marking nut (Semecarpus anacardium), which contains a black juice used to mark clothing; and Japanese lacquer (Rhus verniciflua), which contains a thick substance used to lacquer furniture and other items. Poison sumac, T. vernix, most commonly affects peat farmers and is related to the mango (Mangidera indica), which causes dermatitis when the fruit is eaten with the skin intact. The entire genus Toxicodendron is related to African poison ivy (Smodingium argutum), which is native to South Africa as a tree or shrub and is generally very similar to the American poison ivy. There is great debate over the classification of these three plants. Many want them to be considered from the genus Rhus, with Toxicodenron as a subgenus. This is in large part a result of the great variety among these plants. Therefore, many will refer to "Rhus dermatitis" when discussing poison ivy, oak, or sumac exposure, but this is no longer considered correct (34).
Although pollen from Compositae family, Ambrosia genus, like ragweed is well known to induce rhinitis, skin contact is required for dermatitis (with the exception of feverfew which can cause dermatitis by pollen or plant material). The common members of this family causing dermatitis include: short, low, or common ragweed and Roman wormwood (A. artmisiaefolia) (the most ubiquitous, and a high sensitizer), western ragweed (A. coronopifolia), great, tall, or high ragweed (A. aptera), lance-leaved ragweed (A. bidentata), false ragweed (A. acanthicarpa), or Hooker's gaertneria (Franseria acanthicarpa). Dermatitis caused by ragweed can be seen throughout the growing season (spring through fall). It causes a widespread sensitivity reaction on exposed skin surfaces, mimicking photodermatitis. The allergen can be contacted directly from the plant, fomites, or airborne (most common). Other plants within this highly allergenic family include lichens (symbiotic algae and fungi) that are usually found growing flat on rocks or trees in moist areas such as the Pacific Northwest; the lady slipper (Cypripedium and Paphiopedilum), an orchid; tulips (Tulipa), which cause "tulip finger" (hyperkeratosis and Assuring in the fingertips) among frequent handlers of the bulbs. A plant that is allergenically similar to the Compositae family and cosmetically similar to the lichens but a member of the Jubulaceae family is the liverwort (Frulania) which incites an allergic response to sesquiterpene lactones (34,38,41,43).
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