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Plicatic Acid

Dateline: 01/11/99

By Alan Bruzel

Plants use clandestine means to protect themselves. Stationary, they are beset by the voracity of all living creatures, excepting the most confirmed carnivores. In addition, they are preyed upon by their own kind, including such parasitic plants as dodder (Cuscuta and Grammica) and mistletoe (Viscum). Under this assault, plants would have long ago perished if not for their clever deployments of physical and chemical defenses.

Physical defenses of the mature plant may take the appearance of the spines of cactus (Cactaceae). There are seeds, such as coconuts (Cocos nucifera), armored with an impervious coat. These physical barriers, however, are of little protection against the machete, and man has wonderfully used his appetite to stimulate his tool-making ingenuity.

But there are certain plants that will be pointedly avoided by even the most heavily armed vegetarian. A soufflé prepared with Amanita bisporigera may well be delicious, but this Death Angel mushroom has not been named in vain. Thus, we find evolution has given us the brain to construct plant-gathering tools, but not the ability to desensitize or reconstruct at will our delicate biochemical machinery. We may find ways to circumvent the plant's physical defenses of husk and spine, but a plant's metabolic poison is a deadlier foe.

The chemical villain of this article is plicatic acid, produced as the levorotatory form, (–)-plicatic acid, in the heartwood of western red cedar (Thuja plicata). It is present in lesser concentrations in eastern white cedar (Thuja occidentalis) and Japanese cedar (Cryptomeria japonica). This compound is responsible for the occupational disease western red cedar asthma occurring in sawmill workers.

Plicatic acid

Plicatic acid is of medical interest as it elicits both the rapid type-I hypersensitivity reaction mediated by immunoglobulin E, as well as the delayed type-IV hypersensitivity reaction. Regrettably, the respiratory effects of plicatic acid are long-lived; the asthmatic condition persists in about one-half of occupational plicatic acid-induced asthma cases long after exposure to the chemical ceases.

Not only can plicatic acid induce new asthma or exacerbate a pre-existing condition, but it also has been implicated in other inflammatory responses such as conjunctivitis and rhinitis. Possibly, an equivalent irritant action repels clothes moths from apparel stored in cedar chests and evicts bloodsucking arthropods from animal cages lined with cedar chip bedding. Discontinuing this practice of including cedar shavings in animal bedding may curb the respiratory distress caused by plicatic acid in humans and pets.

Recommended Web resources for additional information:

Allergic Reactions to Workplace Allergens
By David I. Bernstein. From Primer on Allergic and Immunologic Diseases, Fourth Edition.

Botanical Dermatology Database
Plant families manufacturing toxic or irritating chemicals.

Ferret FAQ
Inadvisability of using cedar wood chips as bedding for animals.

Plicatic Acid
Information about plicatic acid and other asthma-inducing chemicals. From James Jarvis, University of Edinburgh.

Respiratory Toxicity of Cedar and Pine Wood
Review article written by Jeff Johnston, University of North Carolina at Chapel Hill.

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