What is Milkweed Poisoning?
All varieties of milkweed are poisonous to horses, though Whorled milkweed seems to contain higher levels of toxins than its broad-leaved cousins. Milkweeds are perennial herbs that have leaves opposite or whorled with flowers ranging in colors and can grow 3 to 4 feet tall. They are characterized by a thick, milky sap that exudes from any broken or cut surface. Growing in a range of environments, from dry to moist soils, on roadsides and ditches to swamps, they are usually most prevalent during the warmer months from March until September.
The family of the milkweed plant is unpalatable and bitter to horses, and they will generally avoid them. There are times when a horse can accidentally eat milkweed. Depending on how much and which type of milkweed your horse eats, you may see gastrointestinal disruptions such as a loss of appetite, diarrhea and bloating, or you can see more serious cardiac and nervous system signs, like incoordination, muscle tremors, seizures, and trouble breathing. Immediate medical care is needed, as the toxins in milkweed can cause death within 1 to 3 days of ingestion.
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Symptoms of Milkweed Poisoning in Horses
Symptoms can vary, depending on the amount and type of milkweed consumed, and can appear rapidly, or within several hours. Death generally occurs within 1 to 3 days after the milkweed has been eaten. Signs include:
- Loss of appetite
- Profuse salivation
- Abdominal pain
- Abnormally fast or slow heart rate
- Irregular heartbeat
- Severe colic
- Lying down frequently
- Reluctance to stand
- Falling down
- Paddling gait
- Low blood pressure
- Hypothermia, or low body temperature
- Muscle weakness
- Muscle tremors
- Rear limb paralysis
- Dilated pupils
- Labored or slow breathing
- Violent seizures and convulsions
- Respiratory failure
Plants in the milkweed family, or Asclepiadaceae, can be divided into two categories based on leaf type. Symptoms of milkweed poisoning can differ based on the kind of milkweed that has been ingested.
Broad-leaved milkweeds produce symptoms involving the cardiovascular and digestive systems. Arrow-leaved milkweeds are also in this category, and can cause additional signs of neurotoxicity. Common milkweeds of this category include:
- Showy milkweed, Asclepias latifolia
- Common milkweed, A. syriaca
- Broadleaf milkweed, A. speciosa
Narrow-leaved milkweeds, including verticillate-leaved milkweeds, primarily cause symptoms involving the nervous system, such as colic, tremors, and incoordination. The ensuing respiratory failure generally causes death within 24 hours of plant ingestion. Milkweeds of this category include:
- Mexican milkweed, Asclepias fascicularis
- Whorled milkweed, A. verticillata
- Plains whorled milkweed, A. pumila
- Horsetail milkweed, A. subverticillata
Causes of Milkweed Poisoning in Horses
The cause of milkweed poisoning is the ingestion of any parts, green or dried, of a milkweed plant. Ingestion can be from:
- Lack of a better food source
- Overgrazed pastures
- Accidental hay or pasture contamination
The primary toxins in milkweed are galitoxin, a lethal neurotoxin found in higher quantities in whorled-leafed varieties, and a group of cardiac glycosides, also called cardenolides. Lethal doses of green plant material range as low as 0.1% to 2% of the animal’s body weight, meaning a 1,000 pound horse need only ingest a half pound to 20 pounds of milkweed. While toxicity is highest in green, fresh plants, dried material is still toxic, and is often accidentally baled with hay.
Diagnosis of Milkweed Poisoning in Horses
It may be difficult to diagnose a milkweed poisoning if you have not seen evidence of your horse eating that specific plant. If you do suspect that your horse has eaten milkweed, bring some of the plant material with you for your veterinarian to positively identify. This identification and the symptoms present can be used to diagnose your horse.
If you are not sure why your horse is experiencing symptoms, often your veterinarian will need to eliminate other possible causes, such as infectious diseases or chemical poisoning. Testing can include blood work, serum testing and a urinalysis. Negative results can cause your veterinarian to look at your horse’s environment, and this can reveal a plant poisoning. Be sure to take a survey of what is in your horse’s reach, or if any accidental contamination is occurring. Stomach contents can diagnose milkweed poisoning in deceased animals.
Treatment of Milkweed Poisoning in Horses
Treatment of a case of milkweed poisoning starts with removing milkweed from your horse’s pasture or feed. Place your horse in a quiet and safe area, and give him fresh water, non-contaminated hay, and shade.
There is no specific treatment or antidote for milkweed poisoning, but your veterinarian may prescribe supportive treatments to minimize absorption of the toxins and aid your horse in recovering. This can include sedatives, laxatives, gastrointestinal detoxification through activated charcoal or mineral oil, intravenous fluids, barbiturates for pain, and medications to help with cardiac and nervous system signs.
Recovery of Milkweed Poisoning in Horses
Recovery will depend on the level of toxins in your horse’s system, as well as how fast treatment was sought. If your horse did not consume a lethal amount of milkweed, remained standing and was able to eat, he may recover over several days. A horse that has ceased to move and has had seizures has been lethally poisoned, and the chance of death is significantly increased. It is vitally important to seek medical care as soon as you suspect your horse has been poisoned.
Preventing your horse from consuming milkweed remains the best way to ensure your horse’s safety. Milkweed is unpalatable, and horses will generally not eat it. Consumption occurs through accidental contamination, or when pastures have become overgrazed and milkweed is the only forage left. Minimize milkweed consumption with these strategies:
- Learn to recognize milkweed varieties common in your area
- Survey plants in your horse’s pasture and grazing areas for poisonous milkweed
- Eliminate any milkweed in your horse’s pasture or areas he might seek forage, either by hand or with an herbicide
- Cultivate healthy grasses and legumes that compete with milkweed
- Routinely mow pastures before milkweed can seed to minimize spreading
- Watch for poisonous plants while on trails to ensure your horse does not consume them
- Monitor your horse’s grazing habits, especially when introducing a new pasture, watching for any physical changes
- Create a management plan for weeds along fence and field edges, nearby roadsides and irrigation ditches to ensure milkweed does not jump onto your property
- Prevent overgrazing by managing fields appropriately with the right amount of horses per acre
- Inspect your horse’s hay to ensure milkweed has not been accidentally baled with it
- Since milkweed toxicity can be cumulative, monitor your horse or any behavioral or physical changes daily