I read a sad story last week. A horse owner found her young, apparently healthy, horse dead in the field when she went to bring him in that evening. He had been fine in the morning.

One of the possible causes of death: ingesting leaves from a Yew tree. The leaves of the Yew are highly toxic to horses. It takes just a mouthful to kill a 1,000 pound horse. The clinical signs are sudden death, often within 2-3 hours of ingestion. Apparently there was a yew tree on the way to the pasture and it’s possible he grabbed a mouthful.

It turns out there are a quite a few plants that are toxic to horses — many of which surprised me. Most horses tend to avoid toxic plants, but sometimes they don’t recognize them or they are especially hungry, or they accidentally ingest them.

Horses under stress can be more susceptible to relatively low levels of toxicity.  In addition, some toxins are stored in the horse for life, the level increasing each time the horse ingests the toxic plants and the effects only becoming visible when the horse has finally accumulated a lethal level of the toxic agent. .

Take a look at these plants and make sure you don’t have any growing near your horses!

Rhododendron/azalea/laurel bushes are moderately toxic to horse. Symptoms include acute colic, diarrhea, excessive salivation,depression, in-coordination, stupor, and heart irregularity. Most horses will avoid Rhododendron bushes if they have access to good forage. Symptoms can occur within six hours of ingestion.

Treatment: Supportive care.

Oleander is highly toxic to horses. If it’s ingested, the symptoms include colic, sweating, bloody diarrhea, difficult breathing and heart arrhythmia. Symptoms usually appear 6-8 hours after ingestion.

Treatment: Administering laxatives to purge the oleander from the horse and treatment of the symptoms can sometimes save the horse.

Yew is highly toxic to horses. It is the most common cause of animal poisoning which often occurs when grass clippings from an area where Yew trees grow, are eaten by horses. Sudden death is the most typical symptom of Yew poisoning, but other indicators include heart irregularity, nervousness, difficult breathing, in-coordination, and convulsions.

Treatment: There is not treatment other than supportive care. Prevention is your best defense.

Black Locust is moderately toxic to horses with the new growth being the most dangerous. It affects the gastrointestinal tract and the nervous system. If a 1,000 pound horse eats even just half a pound of leaves or bark it can be fatal. Horses start exhibiting symptoms within a few hours of ingesting. These can include colic, depression, lost of appetite, and kidney failure.

Treatment: Detoxification and supportive care.

Braken Fern inhibits the absorption of thiamine, which is vitamin B1, which is necessary to enable to nerve function,. Deficiencies can lead to neurological impairment. Although bracken fern only mildly toxic (a horse would need to consume hundreds of pounds to experience poisoning). However, it’s unique among toxic plants because horses can develop a taste for it and seek it out.

Symptoms result from vitamin B1 deficiency and can include depression, incoordination and blindness.

Treatment: Large doses of thiamin over one to two weeks.

Red Maple can be highly toxic to horses. Fresh, growing leaves are fine; wilted leaves are very dangerous. The toxins in wilted red maple leaves cause the red blood cells to break down so that the blood can no longer carry oxygen; the kidneys, liver and other organs may also be damaged. As little as a pound or two of leaves can be fatal. Depending on how many leaves were eaten, signs can appear within a few hours or as long as four or five days after consumption. Symptoms  include lethargy; refusal to eat; dark red-brown or black urine; pale yellowish gums and mucous membranes at first, advancing to dark muddy brown; increased respiratory rate; rapid heart rate; dehydration.

Treatment is mainly  to give large amounts of intravenous fluids and possibly blood transfusions. Recovery depends on how many leaves were consumed and how promptly the horse receives care.