Through experience, many people know that they are susceptible to
poisoning by poison-ivy, poison-oak, or poison sumac, while some others
either have escaped contamination or have a certain degree of immunity.
The extent of immunity appears only relative, and absolute immunity to
the toxic principle apparently does not exist. Persons who have shown
a degree of immunity upon repeated contact with the plants may develop
poisoning on subsequent exposure.
The skin irritant of poison-ivy, poison-oak, and poison sumac is the same
toxic agent. It is a nonvolatile phenolic substance called urushiol and
is found in all parts of the plant, including roots and fruit. It occurs
in great abundance in the plant sap. The danger of poisoning is greatest
in spring and summer, when the sap is abundant, and least late in fall or
in winter.
Poisoning is usually caused by contact with some part of the plant. A
very small quantity of the poisonous substance is capable of producing
severe inflammation of the skin and can easily be transferred from one
object to another. Clothing may become contaminated and is often a source
of such prolonged infection that it is likely to be judged as a case of
poisoning difficult to cure. Dogs and cats frequently touch the plants
and transmit the poison to unsuspecting persons. The poison may remain on
the fur of animals for a considerable period after they have walked or
run through poison-ivy plants. Smoke from burning plants will carry the
toxin and has been reported to cause severe cases of poisoning. Cases of
poisoning of children from eating the fruit have been reported. A local
belief that eating a few leaves of these plants will develop immunity in
the individual is unfounded. It never should be attempted. No part of the
plant should ever be taken internally, as it is a violent irritant and
poisonous to man.
Cattle, horses, sheep, hogs, and other livestock apparently do not suffer
from skin irritation caused by these plants, although they have been
observed to graze upon the foliage occasionally. Bees collect honey from
the flowers, though no reports are on record of any ill effects from the
use of the honey.
The time between contamination of the skin and the first symptoms
varies greatly with individuals and probably with conditions. The first
symptoms of itching or burning sensation may develop in a few hours
or after 5 days or even more. The delay in development of symptoms
is often confusing in attempting to determine the time or location
when contamination occurred. The itching sensation and subsequent
inflammation, which usually develops into water blisters under the skin,
may continue for several days from a single contamination. Persistence
of symptoms over a long period is likely to be due to new contacts with
plants or with previously contaminated clothing or animals. Severe
infection may produce more serious symptoms, which result in much pain
through abscesses, enlarged glands, fever, or complicated constitutional
malfunction. Secondary infections are always a possibility in any break
in the skin, such as is produced by breaking vesicles that have formed as
large water blisters.