Accidental poisoning of livestock is statistically most likely to occur from ingestion of plants in pasture or hay, but poisoning can also occur from ingestion of ornamental plants. One common ornamental in our area – rhododendron – is toxic to livestock and can be fatal.
Rhododendron is a genus of more than 1,000 species of woody plants that are common in the Appalachian region. Rhododendrons are small shrubs that may be evergreen or deciduous and bloom with attractive flowers from late winter through early summer. Flowers are typically white, pink, red, yellow, or orange. Azaleas are part of the rhododendron genus.
Rhododendrons and azaleas contain toxins called grayanotoxins. These toxins bind to the receptor sites on cell membranes that are responsible for activation and inactivation of the cell and inhibit normal function. The same toxins are found in mountain laurel, which is also toxic. Rhododendron and mountain laurel are both part of the heather family (Ericeceae).
The first signs of poisoning include gastroenteric signs such as salivation, vomiting (in capable species), diarrhea, colic, and bloat. If a larger quantity of the plant was consumed, additional signs may include abnormal heart rate and rhythm, loss of coordination, and/or convulsions, followed by coma and death.
The toxic dose is 0.2% body weight of green leaves, which equates to about 2 pounds for a 1,000 pound horse or cow, 0.3 pounds for a 150 pound sheep or goat, and less than a quarter ounce for a 5 pound chicken. Rhododendron is also toxic to dogs and cats although they are less likely to consume it than livestock. All parts of the plant are toxic, but the leaves contain a higher percentage of the toxic. Dried plant parts are also toxic. Clinical signs are usually observed within a few minutes to three hours of ingestion.
The severity of the poisoning depends on the amount of plant tissue consumed. There is no antidote, so prompt veterinary care to provide supportive care is paramount. Some animals that consumed only a small amount will recover without treatment.
As is the case with most toxic plants, prevention is key. If you have rhododendrons or azaleas growing on your property, take precautions to ensure animals cannot access them. Hungry or curious animals may be able to reach ornamental plants near barns or fence lines. Plant residues or yard trimmings dumped into animal pastures can be problematic if they include toxic ornamentals. Free ranging animals – especially small ones like chickens – can easily be poisoned by consuming toxic ornamentals in your yard or garden. Horseback riders with mounts who like to nibble should also be aware as mountain laurel is commonly found along trails.
Grayanotoxins can also be present in honey made from rhododendron and azalea nectar. Affected honey is known as “mad honey” and causes “mad honey disease” in humans with symptoms ranging from low blood pressure and blurred vision to hallucinations and seizures. Most known cases of mad honey disease are traced to the Black Sea region or from honey that originated there, as rhododendrons are very prolific in that area. However, local beekeepers should be aware of the potential and take care in placing beehives.