Last week, courtesy of the Hagyard Equine Medical Institute, we began our tour of the top 10 poisonous plants to watch for in your horse’s pasture, based on a list compiled by the American Association of Equine Practitioners. Part 1 of this two-part series spotlighted five toxic plants to keep an eye out for in your horse’s pasture (click the plant name to see an image and additional information): bracken fern, hemlock, water hemlock, tansy ragwort, and Johnson grass/Sudan grass. This week, we’re taking a look at five more: locoweed, oleander, red maple tree, yellow star thistle, and yew.

Locoweed (click link for image and additional information)

This plant often is found in the West and Southwest in dry, sandy soil. The toxin found in the plant, swainsonine, is an alkaloid that 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 horses that consume locoweed may develop a taste for it. Clinical signs can include strange behavior: a horse may acquire exaggerated, high-stepping gaits or stagger, wander aimlessly, bob their heads, or even fall down. Other signs may include abortion in pregnant mares, birth defects, stallion infertility, or vision impairment. Unfortunately, there’s no treatment in advanced poisoning, and the effects are irreversible. If the horse has less severe poisoning, they might recover by prompt removal of the weed and supportive veterinary care.

Oleander (click link for image and additional information)

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 (being down and unable to rise), irregular heart rate, difficulty breathing, tremors, or even death. Symptoms can occur several hours after ingestion and can last more than 24 hours. Supportive care from your veterinary team is imperative if the poisoning is spotted early. Often, poisoning occurs when trimmings of the plant are thrown into the pastures. Avoid planting oleander in or around your horse pastures and initiate prompt removal and disposal after pruning.

Red Maple Tree (click link for image; additional information also available here)

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, wilted leaves are 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), which inhibits the transportation of oxygen to the tissues. The lack of oxygenation leads to organ damage. 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 four or five days after 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 (click link for image and additional information)

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 period of one to three months. If poisoned with this weed, your horse may have a tensed or clenched facial expression and be unable to bite or chew his food. Weight loss can take place 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 (click link for image and additional information)

Many of us have these ornamental shrubs nearby. But all parts of the yew are toxic except for the fleshy portion of the berries, and yew leaves remain toxic even after they have dried. The toxin, taxine, can cause respiratory and cardiac collapse. A single mouthful can be deadly to your horse within minutes, making sudden death the most typical sign of ingestion. Animals 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.

What to do

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 save a portion of the suspected toxic plant for your veterinarian to identify, if possible. If you can determine the amount of plant ingested by the horse, it is helpful to know, because 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 has begun, 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. And 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 by Hagyard Equine Medical Institute with the help of the American Association of Equine Practitioners.

 

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