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.
Milkweeds 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 deflexed. Extending upward from the base of each petal is a club-shaped or hooded lobe.
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. 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.
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.