Toxicodendron species: Poison ivy

Family

Anacardiaceae

Common Names

Poison ivy (Toxicodendron radicans), eastern poison ivy (T. pubescens), western poison ivy (T. rydbergii), poison oak (T. diversilobium or T. quercifolium), and poison sumac, poison ash (T. vernix).

Plant Description

There are 6 - 10 species of Toxicodendron native to North America and parts of Asia, 5 species of which are variously named and found in North America. Depending upon the area and growing conditions, there are close similarities between the species making their identification difficult at times. The leaves are glossy, hairless, alternate, always three-parted, and may be entire, toothed, or lobed. The climbing stems produce aerial roots that anchor the plant to the bark of trees on which it is climbing. Species growing in the drier western States tend to be more shrub-like rather than climbers. The flowers are small, whitish, hanging clusters, and the fruits are white and berry-like with longitudinal grooves. The leaves turn a bright red in the Fall (Fig. 391 - Fig. 395).

Figure 391. Toxicodendron radicans.

Figure 392. Toxicodendron pubescens (Poison ivy).

Figure 393. Toxicodendron diversilobium (Poison oak).

Figure 394. Toxicodendron vernix (Poison sumac).

Figure 395. Toxicodendron radicans - fall color.

Toxic Principle and Mechanism of Action

Poison ivy, poison oak, and poison sumac all contain highly irritating allergenic phenolic compounds collectively called urushiol (oleoresin) [1]. The most toxic being the resin 3-n-pentadecylcatechol [2].

All parts of the plant, green or dried contain urushiol. The oily resin is not volatile or soluble in water but is soluble in alcohol. Smoke from burning the plants can contain droplets of the toxin and will affect people who are highly allergic to the toxin. Urushiol coming into contact with human skin, penetrates and binds to cell proteins that then induces an immune response, the severity of which depends on the sensitivity of the individual. The cell-mediated immune response may take 1 - 3 days to develop in people who have had previous exposure to urushiol. Sensitization to urushiol can be induced by plants other than Toxicodendron species that contain the urushiol e.g.: mango (Mango indica), cashew (Anacardium occidentale), Brazilian pepper tree (Schinus terebinthifolius), and skunkbush sumac (Rhus trilobata) [3].

Risk Assessment

Poison ivy and its related species are common indigenous plants that are often present in gardens especially those adjacent to forested areas and open spaces where the plants can become established. Recognition and awareness of poison ivy and its look-alikes is important in order to avoid human exposure. The rule of thumb "leaves of three, leave it be" is a good one, however, there are several harmless plants that have three leaves that mimic poison ivy that are worth differentiating. These include boxelder (Acer negundo), white ash seedlings (Fraxinus americana), Virginia creeper (Parthenocissus quinquefolia), and Boston ivy (Pathenocissus tricuspidata) [4].

Clinical Signs

Skin reddening, swelling, and intense itching where ever the skin is in contact with the urushiol. In some individuals fluid filled vesicles form, rupture, and form scabs. Contrary to popular rumor the fluid from the vesicles does not contain urushiol and does not spread the rash. In highly sensitized individuals, systemic signs of vomiting, diarrhea, and abdominal pain may develop. Without therapy the dermatitis may persist for several weeks.

Treatment

As soon as there is known contact with poison ivy, the skin should be liberally washed with water. A mild soap that does not remove all the protective skin oils may be helpful in removing the urushiol. Once urushiol has bound to the cell membranes (about 10 minutes of contact time), it cannot be removed easily [5]. Once dermatitis and itching have developed, soothing topical drying agents such as calamine may help reduce the severity of signs [6,7]. Numerous "remedies" have been recommended for treating poison ivy dermatitis including such things as applying the juice of jewel weed (Impatiens spp.), but none have been proven to be universally effective [8].

In severe cases, of poison ivy dermatitis, a person should see a physician because the most effective treatment may be the use of the more potent systemic corticosteroids [5].