Hemlock Poisoning in Horses

Hemlock Poisoning in Horses - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost

Most common symptoms

Circling / Collapse / Disorientation / Mouth Salivation / Seizures / Shaking


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Most common symptoms

Circling / Collapse / Disorientation / Mouth Salivation / Seizures / Shaking

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Hemlock Poisoning in Horses - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost

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What is Hemlock Poisoning ?

Hemlock, hailing from the Umbelliferae family and commonly known as poison parsley, spotted hemlock, winter fern, California fern, Nebraska fern, poison hemlock, water hemlock, deadly hemlock and cowbane, this plant is pretty common across the country.  It can be found not only in pastures, but also along roadsides, creek beds and fence lines virtually anywhere in the United States.  

It is not a particularly tasty plant to the average horse and, as such, isn’t usually the first choice of forage except in those cases when pastures have been overgrazed or more nutritious and tasty forages are unavailable.  Depending on the type of hemlock ingested, the amount and the portion of the plant consumed, the toxic effects can be fatal to both humans as well as equine and other livestock.

Hemlock poisoning in horses is a poisoning occurring from toxins contained in all types of hemlock to some degree.  Because the toxins affect the nervous system and muscular coordination, ingestion of even small amounts can be fatal to animals as well as humans.

Symptoms of Hemlock Poisoning in Horses

The intensity of the symptoms will vary based upon the type, amount and part of the plant ingested; here are some symptoms you will likely see:

  • Nervous trembling
  • Dilated pupils
  • Uncoordination
  • Confusion  
  • Disorientation
  • Loss of appetite
  • Excessive salivation
  • Bloating
  • Rapid but weak pulse
  • Lack of muscular coordination
  • Great abdominal pain
  • Muscle tremors
  • Frequent urination and defecation
  • Nervousness followed by severe depression
  • Seizures
  • Respiratory paralysis occurs before cardiac arrest

If any of these symptoms are noted to any degree of severity, seek medical care for the animal or human immediately.  



There are several types of hemlock of which we all need to be aware, both for our personal protection as well as for protection of our horses.  Here is a list of those types:

  • Water hemlock Cicuta species - Maculata (East) (the most toxic type)

  • Poison hemlock Conium maculatum
  • Spotted hemlock 
  • Various trees are given the “hemlock” classification but are unrelated to the herb  hemlock, the trees being non-toxic while the herb is quite toxic

Causes of Hemlock Poisoning in Horses

The toxin contained in hemlock is a grouping of nicotinic alkaloids which are relatively low in concentration in the first year of growth of the plant but increase with each stage of maturation.  Here are some things to know about this plant:

  • This alkaloid content increases when the climatic conditions are sunny and decreases when sunshine is depleted
  • The plant is toxic in the spring when the blooms come on and again in the fall when they bloom and seed again
  • The plant can be confused with wild carrots, parsley and parsnips due to the look and smell of various parts of it.  This confusion is what leads humans to consume them, thinking they’re eating parsley, parsnips or even wild carrots
  • The plant also resembles Queen Anne’s Lace and can be confused by children who pick them as pretty “flowers”

The nicotinic alkaloids which are contained in all parts of the plant are at the root of the poisoning, causing metabolic and electrolyte imbalances.  Even getting oils from the stems of this plant on your hands from handling it can result in poisoning. These imbalances can have a significant effect on the central nervous system and muscular control in all parts of the equine body.  Death usually ensues due to respiratory paralysis and cardiac arrest.

Diagnosis of Hemlock Poisoning in Horses

Diagnosis of hemlock poisoning in horses isn’t always possible as frequently the horse is found dead and no antemortem diagnosis could be attempted.  This is especially so when the offending poison has come from ingestion of water hemlock, with acute symptoms developing within minutes of ingestion of the roots or seeds.  Water hemlock grows very near the water’s edge in very moist soil which allows the entire plant to be pulled up and eaten, root and all, giving the horse the full benefit of the toxicity of the plant.  And it only takes a few grams of the plant, roots or seeds to be consumed for the poisoning to become severe and ultimately fatal.  

In those cases in which the symptoms are noted quickly and medical care can be provided, after the history you provide and the examination done by your veterinary professional, a treatment plan will be implemented which will likely only be supportive in nature as there is no antidote or cure for hemlock poisoning in horses.


Treatment of Hemlock Poisoning in Horses

If you’re fortunate enough to have seen and recognized the earliest symptoms of hemlock poisoning in your horse and if you are fortunate enough to be able to access medical intervention within an hour of ingestion, treatment options are very slim at best.  There is no antidote for the poisoning so removing the horse from the source of the poisoning should be your first action while waiting for the vet to arrive.  In the case of water hemlock poisoning, the morbidity rate is quite high because the neurological effects of the toxins take over very quickly and the horse can expire within an hour of ingestion of sufficient amounts of the plant.  Treatment options, even supportive care, will not likely have an opportunity to be of any real healing use to the afflicted animal.  

In the case of poison hemlock, whose toxins aren’t as concentrated, treatment options are better utilized.  Removal of the horse from the toxic environment, providing plenty of clean, fresh water and safe, nutritious feed along with close monitoring and rest and quiet in the paddock will likely be all that can be done for your horse.   Fluids may be given IV or orally and some vets may treat with activated charcoal to help absorb the toxins but this particular option hasn’t proven to be very effective. 

Recovery of Hemlock Poisoning in Horses

If your horse survives beyond 8 hours of ingestion of the hemlock, the prognosis is good.  That being said, it is vitally important for you to make yourself aware of the various types of poisonous plants germain to your area and make every attempt to eradicate them from the pastures, fields, hay and grains that you offer to your herd as feed.  Protection of the remainder of your herd depends on restricted or reduced access to these poisonous plants, whether in pasture and fields or in hay and grains.

Some poisonous plants are rendered non-toxic when they are dead and dried while others retain their toxicity in all phases of its life cycle, both fresh and dried - hence the reason you need to check the hay being offered as feed. The continued optimal performance and productivity of your herd should be uppermost in your priorities after a poisoning episode.

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Hemlock Poisoning Questions and Advice from Veterinary Professionals