Oleander (Nerium oleander)
is an attractive, hardy, ornamental, evergreen shrub that is a major cause of animal poisonings worldwide. Oleander is found in tropical and subtropical regions. It has dark gray-green, leathery leaves. It's showy clusters of flowers bloom from early summer until mid-autumn.
Oleander is a highly toxic plant. It contains several cardiac glycosides--the most toxic chemicals being oleandrin and neriine. Cardiac glycoside poisoning
acts by inhibiting Na+/K+ ATPase. Horse's don't need to ingest very much to have fatal consequences.
As little as 0.005% of it's body weight in dry oleander leaves can be fatal (which equals roughly 10 to 20 leaves for an average-sized adult horse). The most frequent source for oleander exposure in livestock is yard/garden clippings. There have also been incidences where horses consumed hay that was contaminated with oleander leaves. Bored horses may also chew on plants that grow near their paddocks. Oftentimes, horses are found suddenly dead, or present with rapidly developing nonspecific signs that may resemble colic.
If signs do appear, there is usually a 2-5 hour delay from ingestion of the plant to apparent symptoms. Signs of poisoning may begin with diarrhea, excessive salivation, depression and loss of appetite and progresses to the development of cardiac signs such as bradycardiaor tachycardia, weak and irregular pulse, heart blocks, and arrhythmias.