Milkweeds contain various toxic cardenolides (cardiac glycosides) that have cardiotoxic effects. The cardenolides act by inhibiting Na+-K+-ATPase thereby affecting myocardial conduction and contractility. In addition to the cardiotoxic effects of the cardenolides common to most milkweeds, other glycosides and resinoids identified in milkweeds have direct effects on the respiratory, digestive and nervous systems causing dyspnea, colic and diarrhea, muscle tremors, seizures and head pressing. Milkweeds are most toxic during rapid growth, and retain their toxicity even when dried in hay. Toxicity varies with the species and growing conditions, however all milkweeds should be considered potentially poisonous, especially the narrow-leafed species.
Some broad-leafed species that contain high levels of cardenolides include Asclepias asperula, A. labriformis, A. eriocarpa, and A. curassavica.
The verticillate or narrow leafed species such as the whorled milkweed (A. subverticillata), the eastern whorled milkweed (A. verticillata) and the plains milkweed (A. pumilla) are neurotoxic, although they may have some affect on the cardiovascular system.
Milkweeds are erect perennial herbs which have either 6-12 cm broad, veined leaves or narrow linear leaves seldom more than 2-4 cm wide, arranged either alternately or in whorls. Most species (except A. tuberosa) contain a milky sap or latex. The flowers are produced in terminal or axillary umbels consisting of two, 5-parted whorls of petals, the inner one being modified into a characteristic horn-like projection. The color of the flowers varies amongst species from greenish-white to yellow-red. The characteristic follicle or pod contains many seeds each with a tuft of silky white hairs that aids in its wind born dispersion.
Abdominal pain, colic, bloat and diarrhea due to gastrocuteritis.
Muscle tremors, weakness and recumbency.
There is no specific treatment for milkweed poisoning. Those animals that have not consumed a lethal dose of the plants recover over several days. Affected animals should be moved from the source of the plants and given fresh water, good quality hay and shade. Sedatives, laxatives and supportive intravenous fluid therapy.
Slow irregular heart rate. Heart block may cause sudden death of the animal eating the milkweed.
Labored and slow respiratory rate.
Animals consuming the narrow-leafed species of milkweed develop severe colic, muscle tremors, incoordination, seizures and respiratory failure prior to death
Since there are no specific post mortem signs in animals poisoned by milkweeds, diagnosis is based on identifying the plants in the animals stomach and food source.
Milkweeds remain toxic when dry, and therefore can be a problem when incorporated in hay.