Human Toxicity

Ingestion of plants containing cardioactive steroids have resulted in detectable digoxin concentrations, toxicity, and even death. Although cases of human milkweed exposure are rare, a 2013 case reported a 42-year-old man experiencing cardiac symptoms and nausea after consuming fried milkweed pods from a recipe he found online. In the ED, he was found to have a heart rate in the 40s, and a digoxin level of 1.0 ng/mL (cross reactivity of the assay) and a potassium of 4.2 mEq/L. He was observed for several hours and eventually discharged with minimal intervention.⁵ Symptoms of milkweed poisoning may include abdominal discomfort, nausea, vomiting, diarrhea, weakness, lethargy, and confusion, progressing to seizures, heart rhythm changes, and bradycardia.

Corneal injury and defects have been documented from direct contact of milkweed sap to the eye and any exposure should be immediately treated with copious water irrigation. Yang, in 2021, reported local milkweed toxicity resulting in a large corneal epithelial defect and diffuse stromal edema in a 13-year-old boy who presented to the emergency department after direct corneal exposure to milkweed latex. He was treated with antibiotic and steroid eye drops and had complete resolution in 18 days from injury.

Milkweed sap exposure to the skin can cause a significant contact dermatitis and poses a hazard for gardeners. Sap exposed areas should be cleaned soon after exposure with soap and copious water.

Antidote

There is no systemic antidote. It is unknown if digoxin specific antibody antidotal therapy would have any effect. Would likely recommend supportive care and cardiac observation for symptomatic patients.

Traditional Use

While common milkweed is well known to be toxic to humans, various Native American communities have consumed it for centuries (including the Tsalagi, Anishinaabe, Haudenoshonee, Lakota, Menominee, and Myaamia people) when prepared in a specific way. These culinary practices are very specific and result in minimal to no toxic exposure—only young plants are harvested and they are repeatedly boiled with several changes of water ridding the desired plant material of the water-soluble toxins.⁶

Animal World

The results of the milkweed species containing toxic cardenolides has far reaching effects into animal world. Plant strategies for survival are often a key factor of plant-insect co-evolution by herbivorous insects evolving offensive strategies by making use of the plant defenses for their own benefit.

IMAGE 5. Click to enlarge.

One classic example are Monarch butterflies (Danaus plexippus) (image 5). They have no natural defenses—no fangs, claws, armor, or electric zaps. They also do not construct toxins from material they ingest. However, they do have one trick not to get eaten. They drink the sap of the milkweed while in their caterpillar stage and concentrate the cardioactive toxins from the plant in their bodies. These accumulated toxins do not affect them, as they have developed through time substitutions within the alpha subunit of their Na+/K+ ATPase, rendering the pump insensitive to cardiac glycosides otherwise known as ‘target-site-insensitivity.’⁵ The concentrated toxins persist in their tissues through their metamorphosis into butterflies and cause predators, primarily blue jays, to vomit and avoid eating them in the future. This technique, of making themselves unpalatable and very easy to be seen with their bright aposematic coloring, figures prominently into several fascinating defense mechanisms for an otherwise defenseless animal—including conditioned food aversion (it made me ill, I won’t eat that again), observational learning (others of your species see what happened to you and avoid the source), social transmission of preference (I see what you eat and follow your lead), neophobia (fear of new things, as in, I’m not going to eat that new food source), innate avoidance of bitter tastes (many animals don’t eat things that are bitter!).⁷