Common Name
Botanic Name
Rhododendron L.
Plant Family
Ericaceae (Heath family)
There are at least 250 species of rhododendrons found mostly in the acidic soils of western and eastern North America.
Animals Affected
Cattle, sheep, goats, horses, humans
Toxic Principle
All parts of the plant including the nectar contain grayanotoxins. Grayanotoxins can also be found in honey using nectar collected from rhododendrons. Most poisoning occurs in the winter months because the leaves are generally evergreen and are attractive to animals when other forages are scarce. Animals eating approximately 0.2 percent of their body weight of leaves are likely to develop signs of poisoning. Grayanotoxins can bind and alter the function of voltage-gated sodium channels.
Generally large shrubs or open trees growing to heights of 30 feet or more. The leaves are alternate, simple, leathery, lanceolate, and often evergreen. The flowers are produced in large, showy, terminal clusters, ranging in color from white to purple, to red. The fruits are elongated capsules that split into five sections to release small, scalelike seeds. Many hybrids have been developed for their showy flowers. Azaleas are considered by some to be a subgenus of rhododendron. Azaleas are generally deciduous and have been extensively hybridized to produce showy garden and house plants in a wide spectrum of colors.
Animals poisoned by rhododendrons initially have clinical signs of digestive disturbances characterized by anorexia, excessive salivation, vomiting, colic, and frequent defecation.
Oral and intravenous fluids should be given as necessary to counteract the effects of vomiting and diarrhea. Call your veterinarian.
Cardiovascular system
In severe cases, muscle weakness, bradycardia, cardiac arrhythmia, weakness, paralysis, and coma may precede death.
Respiratory System
Regurgitation of rumen contents may result in inhalation pneumonia.
Reproductive System
Fetal mummification has been reported in goats following severe Japanese pieris poisoning.
The clinical signs and evidence that the plant has been consumed are highly suggestive of rhododendron poisoning. Postmortem findings are not specific and generally consist of multiple hemorrhages on internal organs. The detection of grayanotoxins in the rumen contents is also possible and is a means of confirming rhododendron poisoning.
Special Notes
Depression, vomiting, slow erratic heart rate, painful neck, and weakness are reported in people who have consumed "mad honey" made by bees feeding on rhododendrons or who have consumed tea made from the leaves of rhododendrons. Rhododendrons are not appropriate shrubs to plant around or in horse or livestock enclosures where the animals can gain access to the plants.