Chemicals in rhododendron leaves, flowers and nectar make the plant toxic. It can poison many species, from humans to horses, birds and insects that ingest it. For that reason, it can’t be controlled by grazing, as is often the case with other aggressive shrubs. Most cases of human poisoning come through eating honey made with rhododendron nectar, resulting in a condition sometimes known as mad honey disease or honey intoxication. The shrubs are sometimes known as “lambkill” or “calfkill” because young animals have not learned the toxicity of the plant and eat it in large quantities.
Cases of rhododendron poisoning among humans who have eaten toxic honey date as far back as 400 B.C. This is the first written account of “honey intoxication,” in which 10,000 Greek soldiers were affected by eating honey made from rhododendron nectar. In rhododendron ponticum’s original home, the Black Sea area of Turkey, cases of poisoning have been reported since that century as well. Outbreaks among humans in the 1980s were confirmed in Turkey and Austria, as well as in goats in Britain. In 2008, relief workers in North Korea documented poisoning among hungry children who would eat azalea flowers, which resulted in at least nine deaths.
The toxicity found in varieties of rhododendron is not uniform across all the plants' species, although it is a characteristic of Rhododendron ponticum, one of the most popular varieties of the shrub. It is also common to relatives of rhododendron, such as other members of the azalea, or Ericacae, family. Because of the chemicals’ presence in nectar, placement of beehives near rhododendron is unwise, as the honey they make may turn out to be toxic in turn. As little as three milligrams of nectar consumed per kilogram of body weight can prove fatal. Leaves are less poisonous than nectar, as a range from 100 to 200 grams of leaves would be required to seriously poison a child weighing 55 pounds.
The toxicity comes from free phenols and diterpenes, also known as grayanotoxins, in the plant tissues. The production of these is basically a defense mechanism by the plant against being consumed by herbivores such as grazing animals. The phenols are present in more concentrated amounts in younger rhododendron tissues, before they toughen up. The sticky sap that comes from young rhododendron buds is full of these toxic compounds and can irritate skin and eyes. Many animal poisonings happen in the early spring and late fall, since rhododendron remains green longer than other plant species, and presents a more attractive foraging draw.
Rhododendron toxicity is rarely fatal to humans, and symptoms usually last no more than 24 hours. It results in short-term gastrointestinal and cardiac problems, and the severity depends on the amount of honey or nectar ingested. Symptoms include low blood pressure, lowered heart rate and shock, nausea, increased salivation and vomiting, accompanied by dizziness, loss of balance and difficulty breathing. Serious illness and death is more common in animals who eat rhododendron, again depending on how much is eaten. Horses and elephants are two herbivores known to respond severely to rhododendron poisoning.
Detoxification remedies are used to clear rhododendron toxicity. These include vomit-inducing emetics when appropriate for the patient, and repeated use of activated charcoal on the first day symptoms are noted. Since the poisoning dehydrates many patients, fluid replacement is recommended, and other supportive actions such as respiratory and cardiac support may be needed. Atropine can be prescribed to stimulate the pulmonary muscles in case of a severely slowed heart rate.