The two most definitive traits of poison ivy or poison oak are
1) 3 leaflets on each compund leaf and 2) a red coloration at the apex
where the three leaves connect. Poison ivy can climb trees as a vine (usually
attaching close to the trunk), look like a shrub, or be a single, simple
plant. All parts of a poison ivy plant--leaves, stem, fruits, and roots--produce
an oil that can cause skin irritation in some people. Any contact of the
body with the oils can cause a problem, whether from patting a dog that
has just walked through poison ivy or touching clothes that have come
in contact with the plant. You can even get a case of poison ivy internally
by inhaling oil droplets that become airborne in smoke when the plants
are burned. Also, you can get poison ivy in the winter simply by touching
the stem, even though the leaves are gone.
Poison ivy has more facts, myths, and disagreements about its properties than most plants. You do not spread poison ivy by scratching where it itches, despite what some people say. New blisters and irritated areas can appear more than a week after exposure to the oils, but these merely represent the normal lag time that can occur after initial contact. Also, you cannot give poison ivy to others, except by bringing them into contact with the oils that are on your own body or clothes after encountering the plant.
Many forms of wildlife can eat poison ivy without being adversely
affected. Dozens of kinds of birds including bobwhite quail eat the fruits,
which are clusters of smooth, white berries that appear in late summer.
One serious case of poison ivy was contracted by a wildlife student who
sorted through the stomach contents of a recently killed deer. Among the
data he recorded for his research on the diet of deer was that they sometimes
eat a lot of poison ivy leaves.
The skin irritation--blisters, burning, itching--normally occurs 24 to 48 hours after contact with any part of a poison ivy plant, and expression of the ailment follows a bell-shaped curve. The most severe symptoms occur midway between a 2- to 24-day period. One treatment for relief of the symptoms of a severe case is a steroid, such as predisone. The steroid masks the symptoms even though the body's response to the irritation continues. Some dermatologists caution that some doctors treat with steroids for too short a period. Thus if your reaction is following a 24-day cycle and you take steroids for only seven days, the symptoms could reappear before the peak of irritation has been reached.
Botanical field guides distinguish between poison ivy and poison oak. Subtle differences are noted, ones that do not really matter for recognition of the plants. Both have three leaves and produce the oils that make you itch. Some biologists claim that poison oak is more virulent than poison ivy.