By: Dr. Gina Tranquillo

In this edition we will focus on discussing the remaining five poisonous plants. Top ten poisonous plants to horses in the United States as compiled by the American Association of Equine Practitioners (AAEP) –

  • Bracken fern
  • Hemlock
  • Water Hemlock
  • Tansy Ragwort
  • Johnson Grass/Sudan Grass
  • Locoweed
  • Oleander
  • Red Maple Trees
  • Yellow Star Thistle
  • Yew

Locoweed – This plant is often found in the West and Southwest in dry, sandy soil. The toxin found in the plant is called swainsonine. This is an alkaloid which is necessary for the metabolism of complex sugars in cells. Swainsonine can cause swelling of the nervous system if ingested over the course of two weeks.  Cell function is disrupted and this is usually most noticeably in the brain. Locoweed does have palatability to the horse and so horses that consume locoweed may develop a taste for it for this reason. Clinical signs you may observe include strange behavior, similar to the horse going crazy or “loco.” A horse may acquire exaggerated, high stepping gaits, stagger, aimless wandering, bob their heads, or even fall down. Other signs may include – 2 abortion, birth defects, stallion infertility, or vision impairment. Unfortunately if the horse is suspected to be poisoned by locoweed, there is no treatment in advanced poisoning. The effects are irreversible. If the horse has less severe poisoning then they may recover by prompt removal of the weed and supportive veterinary care.

Oleander – Unfortunately, this pretty flowering plant is dangerous to your horse. In fact, all parts of the plant contain toxins, which disrupt the beating of the heart. It is important to know that the leaves are even toxic once they are dried and only a small amount of leaves (30‐40 leaves) can be deadly to a horse. Clinical signs include – colic, recumbency/down and unable to rise, irregular heart rate, difficulty breathing, tremors, or even death. Effects are seen several hours after ingestion and can last more than 24 hours in duration. Supportive care from your veterinary team is imperative if spotted early. Many times poisonings occur to horses when trimmings of the plant are thrown into the pastures. It is recommended to avoid planting Oleander in or around your horse pastures and initiate prompt removal and disposal after pruning.

Red Maple Trees – You may have heard of this one before. Red Maple Trees are planted all over the country so be sure to search your property. If leaves are ingested fresh they seem to do little or no harm. However, ingestion of wilted leaves is extremely toxic to horses. Horses gain access to wilted leaves most commonly after storms or windy condition when branches fall or when autumn leaves begin to fall to the ground and wilt. If horses graze wilted leaves the toxins cause their red blood cells to lyse (break down); therefore, inhibiting the transportation of oxygen to the tissues. As a result of the lack of oxygenation, organ damage occurs. As little as 1‐2 lbs of wilted leaves can be fatal to your horse. Clinical signs can be apparent within a few hours after ingestion or even up to 4‐5 days post consumption. Clinical signs include – increased respiratory and heart rate, dehydration, lethargy, anorexia, or dark red/brown urine. Involve your veterinarian right away if you suspect ingestion. Recovery of your horse will depend on the quantity of leaves ingested and how quickly your horse receives veterinary care. It is important to note that silver and sugar maple species may also contain the same toxic elements but in less toxic amounts. Be cautious if these species exist on your property.

Yellow Star Thistle – This weed with spiny yellow flowers contains toxins that affect the brain. The areas of the brain affected include nerves that control the horse’s ability to bite off and chew. In order to ingest a toxic dose, a horse must consume 50‐200 % of their body weight over a 1‐3 month time period. If poisoned with this weed, your horse may have a tensed or clenched facial expression and be unable to bite or chew their food. Weight loss can be seen over time and ataxia or incoordination may also be seen. Unfortunately, there is no treatment for this damage to the nervous system. Euthanasia is recommended if the horse is too debilitated to eat.

Yew – Many of us have these ornamental shrubs nearby. Unfortunately, all parts of the yew are toxic except for the fleshy portion of the berries. The toxin is called taxine which can cause respiratory and cardiac collapse. Even after the leaves are dried, they are still toxic. Even just a single mouthful can be deadly to your horse within minutes, therefore, making sudden death the most typical sign of ingestion. Animals that are found alive may be trembling, colicky, have a slowed pulse, or have difficulty breathing. There is no treatment for yew poisoning. Prevention is key. Avoid disposing trimmings of the yew anywhere near your horse and avoid planting yew in or around horse pastures.

In conclusion, if you suspect your horse has come in contact or ingested a poisonous plant, prompt removal of the plant is key. Seek veterinary help immediately and if you need to save a portion of the suspected toxic plant for your veterinarian to identify then do so. If you can determine the amount of plant ingested by the horse it is helpful to know since small amounts of some toxins can be fatal in a short period of time.

Be prepared for your veterinarian to initiate supportive care and stabilize your horse, if possible. This may be followed by referral of your horse to a medical facility for ongoing, 24 hour management if poisoning is advanced. At these facilities, nursing care and veterinary care is around the clock and your horse can be well monitored and cared for. Remember that even after treatment is initiated, poisoning in some cases can cause permanent or irreversible damage.

Prevention, common sense and good horse management are key. Offering your horse good quality hay and grain is important. In addition, inspection of your hay at regular intervals is also important. If you have any questions about your feed sources, ask your veterinarians about sampling and inspection of them. Furthermore, enlist the help of your veterinarian or local county extension office if you have questions about poisonous plants on your farm or in your pastures.


Information in this document has been provided with the help of the American Association of Equine Practitioners. For more horse health information also visit the AAEP website for horse owners: