DermHelpCentral

Poison Ivy Overview

Jane Brody had a great overview of "poison ivy dermatitis" in the June 17, 2014 NY Times. There are two factual errors, but overall this is an extremely useful overview.

Brody states: "Myths and misconceptions abound about these three plants and the reactions they can cause. Knowing the facts can help to spare you and your family considerable distress."  Her fine piece will enable you to avoid some of these. See: "Steering Clear of Poison Ivy"

The errors:

1) Brody states: "Even people repeatedly exposed who appear to be immune may react to high concentrations of the toxin."   Immunity is a state of sensitization or reactivity.  The correct term is tolerant, or "exhibit immune tolerance."  The immune system has a mechanism of turning off "immune reactivity."  Therefore, the sentence should read, "Even people repeatedly exposed who appear to be tolerant may become sensitized."

2. When she states "Nor do you become immune to urushiol through repeated exposures to small amounts. Quite the opposite. There is no way to desensitize a person to urushiol as there is with pollen and peanut allergies.  Eating mangoes or cashews will not work." This is not true!  In a natural "experiment' with millions of persons, it has been shown that individuals who are reared in areas where mangos grow are most often "tolerant" to the poison ivy resin and related urushiols.  It appears that early exposure (especially via the oral route) induces specific immune tolerance.  This is difficult, and often impossible, to induce once immunity has been established.

References:
1. The Baleful Weed by Wayne Winterrowd.  The author describes how he was desensitized, as a young child, to poison ivy.
2. Exploring the mango-poison ivy connection: the riddle of discriminative plant dermatitis.  Hershko K, Weinberg I, Ingber A. Contact Dermatitis. 2005 Jan;52(1):3-5.
Abstract: Our observations suggest that individuals with known history of poison ivy/oak allergy, or those residing in area where these plants are common, may develop allergic contact dermatitis from mango on first exposure. We hypothesize that previous oral exposure to urushiol in the local Israeli population might establish immune tolerance to these plants.

 

Plants That Cause Rashes

Dermnet, a database from New Zealand, is the brainchild of New Zealand dermatologist, Amanda Oakley.  It has a great section on plants that cause skin rashes.  You can find most of the key players there.

Cow Parsnip Dermatitis (aka Hogweed)

This is a very common weed which grows along the roads and fields in New England. 

DermNet has a good section:

"Botanical name:     Heracleum spondylium

Family:     It belongs to the Apiaceae family (or Umbelliferae), which has over 2500 species in 275 genera. These include common herbs such as giant hogweed, anise, carrot, celery, coriander, dill, fennel, parsley, and parsnip, as well as the highly toxic hemlocks.

Origin:     Native to the British Isles.

Description:     The Apiaceae family are characterised by alternate leaves, which widen at the base into a sheath that clasps the stem. The stems are often furrowed with the flowers usually compound, almost always concentrated in flat-topped umbels. The flowers have 5 petals, usually uneven, and 5 stamens. Seeds and fruit form below where the petals and stamen originate Some parts of the plant usually have a strong aroma, due primarily to various oils produced by the plant.

(Photos taken in Williamastown, MA)

 

Uses:     Cow parsnip, often grows in moist meadows, and along the borders of fields, and near ditches. Traditionally the young leaf stalks and stems were eaten like stewed celery. The leaves may be dried, burned and the ashes used as a salt substitute. Native Americans of the northern US ate the peeled stalks raw or cooked. The blossoms were steeped in oil and rubbed on the body to keep off flies and mosquitoes.

Allergens:     Furocoumarins (psoralens) including bergapten, isobergapten, sphondin, isopimpinellin and pimpinellin.

Allergy:   The umbilliferae contain furano-coumarins which, when exposed to sunlight, cause significant photo-toxic reactions (photosensitivity). One of the more common skin reactions to Cow parsnip/hogweed, is the ‘weed wackers’ or ‘string trimmers’ dermatitis. When string trimmers are used to clear long grass, components of weeds are usually mulched and scattered in all directions, often coating the legs and arms of the operator. The sap of the plants then gets on the skin, and, when exposed to sunlight, gives rise to a very characteristic rash which may persist for many months."

Reference:  Oakley AM, Ive FA, Harrison MA. String trimmer's dermatitis. J Soc Occup Med. 1986 Winter;36(4):143-4.