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MILKWEED - Asclepias species

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Distinguishing features

Milkweed is a native perennial herb with milky sap and leaves opposite or whorled, simple and entire; the flowers are in umbels, purple to greenish white; the fruit is a follicle, with numerous seeds, each with a tuft of silky hairs. 

Description.  Plants of the "milkweed" family exude a white, milky juice from broken or cut surfaces.  Both narrow-leafed (whorled) and broad-leafed species exist; the narrow-leafed variety is most toxic.  The fruit is a follicle (i.e., a capsule filled with numerous seeds); a silky tuft aids spread of seeds by the wind.  The flower is very distinctive: each flower has five sepals and petals which are strongly reflexed.  Extending upward from the base of each petal is a club-shaped or hooded lobe.
Geographic range.  Milkweeds are found throughout the United States in open, sunny areas that may be either dry or swampy.
Toxic principle.  The primary toxic principle, galitoxin, is of the resinoid class.  Galitoxin is found in all vegetative parts of the plant.  In addition, a group of toxicants known as cardenolides may be responsible for digitalis-like signs that cause or contribute to death.  In general, it appears that the broad-leaved species produce cardiotoxic and GI effects while the narrow-leaved species are more commonly neurotoxic.
Toxicity.  Dosages of whorled milkweed as low as 0.1 % - 0.5% of the animal's body weight may cause toxicosis and, possibly, death.  Cattle, sheep and horses are most susceptible.  Toxicity is not lost when the plant is dried.  Therefore, contaminated hay is potentially toxic.

Diagnosis

Clinical signs include profuse salivation, incoordination, violent seizures, bloating in ruminants and colic in horses.  Early signs are followed by bradycardia or tachycardia, arrhythmias, hypotension and hypothermia.  Death may occur from 1-3 days after ingestion of the milkweed.

Laboratory diagnosis. Non-specific.

Lesions are few and unreliable for diagnosis.

Treatment includes gastrointestinal detoxification, sedation or tranquilization and medical treatment to counteract cardiac glycoside effects (arrhythmias).

As early as the mid 1800s, naturalists observed that birds avoided eating certain butterflies (Monarch) whose larvae fed primarily on milkweed.  It was later shown that the feeding larvae accumulated emetic cardiac glycosides which were retained and even concentrated in adult butterflies.  Birds eating the butterflies containing these glycosides vomited shortly after ingestion thus learning to avoid certain butterflies.

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Copyright 2002

University of Pennsylvania
Created by:    Alexander Chan (2003), Daphne Downs (2002), Chris Tsai (2001), Brett Begley (2000), Janet Triplett (1997)
Faculty Advisor:  Dr. Robert Poppenga