Rhododendron, a member of the Ericaceae family along with azaleas, contains the same toxic agent (grayanotoxin) as our previous ‘poisonous plant of the month’, Pieris japonica, and causes similar signs.
All parts of the plant contain the grayanotoxin. Poisoning usually occurs when hungry animals break into woodlands or gardens where rhododendrons are grown, or when clippings are dumped in fields. The lush evergreen leaves can be particularly attractive when there is little else to eat, e.g. in times of drought or snow.
Rhododendron is famous for being one of the few things to cause true vomiting in cattle, sheep and goats, and the results can be spectacular. Other signs include excess salivation and intense abdominal pain, followed in severe cases by weakness, cardiac arrhythmia, coma and death. Horses can also be affected, although they cannot physically vomit and so their initial signs will be of colic.
There is no specific antidote, and treatment is symptomatic only. Activated charcoal can be useful in the first few hours to bind to the toxin and reduce the amount absorbed, and if an animal is caught in the act of eating large amounts, a rumenotomy can be performed to attempt to remove the rhododendron form the rumen before it has time to be absorbed. A 500kg cow that eats about 1kg of leaves is likely to develop signs of poisoning.
A note of caution for the rhododendron-loving bee keepers among you – honey made from the pollen and nectar of rhododendrons can also contain the grayanotoxins, and is known as ‘mad honey’. People consuming mad honey can suffer low blood pressure and cardiac arrhythmias that bring on nausea, numbness, fainting, hallucinations, and very occasionally seizures and death. If you’re interested in history, ask me when you next see me about how mad honey was used as a weapon of war against the Romans!