Oleander (Nerium oleander) is a widely cultivated ornamental evergreen shrub, prized for its showy, fragrant flowers and drought tolerance. However, from a botanical perspective, it is a quintessential example of a plant that has evolved potent chemical defenses, making all parts of the plant extremely dangerous if ingested by humans or animals.
The entire oleander plant—leaves, stems, flowers, roots, and even the sap and dried plant matter—contains a powerful cocktail of toxic compounds. The primary toxins are cardiac glycosides, specifically oleandrin and nerioside. These compounds are secondary metabolites, meaning they are not essential for the plant's primary growth processes like photosynthesis but are instead produced as a defense mechanism against herbivores, insects, and pathogens. From an evolutionary standpoint, these chemicals effectively deter browsing animals, giving the plant a significant survival advantage.
Ingestion of any part of the oleander plant can lead to severe poisoning. The onset of symptoms typically occurs within a few hours. The cardiac glycosides work by disrupting the electrolyte balance in heart muscle cells, which directly affects the heart's function. Key symptoms include severe gastrointestinal distress (nausea, vomiting, abdominal pain, and diarrhea), which may be bloody. This is often followed by cardiovascular effects such as an irregular heartbeat (arrhythmia), slowed heart rate (bradycardia), and a drop in blood pressure. Neurological symptoms like dizziness, drowsiness, tremors, and even seizures can also occur. In severe cases, poisoning can lead to coma and death from heart failure.
Animals, particularly grazing livestock like cattle, sheep, and horses, as well as household pets like dogs and cats, are highly susceptible to oleander poisoning. The symptoms are similar to those in humans but are often observed after an animal has browsed on clippings or leaves. Signs include colic, diarrhea (often bloody), sweating, weakness, and muscle tremors. The most critical effects are cardiac, leading to abnormal heart rhythms, heart failure, and sudden death. Even a small amount of ingested leaf material can be fatal; for instance, consumption of just 0.005% of an animal's body weight in dry leaves can be lethal to horses and cattle.
The toxicity of oleander's cardiac glycosides is persistent. They remain active even in dried leaves and are not destroyed by composting or burning. Inhaling smoke from burning oleander trimmings can cause severe respiratory irritation and systemic toxicity. Furthermore, the plant's sap can cause significant skin irritation and dermatitis in sensitive individuals upon contact. The mechanism of action involves the inhibition of the sodium-potassium pump (Na+/K+-ATPase) in cell membranes, crucial for maintaining the electrical potential of nerve and muscle cells. This disruption is what leads to the severe cardiac and neurological symptoms observed.
