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What is Cowbane Poisoning?

Cowbane is from the Apiaceae family, and makes up the Cicuta species. A perennial native to North America, cowbane can grow from 2 to 10 feet tall, with purple streaked stout stems. The tuberous roots and hollow stems produce a yellowish oil that has been likened to smelling like parsnips. The large leaves are alternate and toothed, and the white flowers form in small umbrella-like clusters. Found in many parts of the United States, cowbane grows in swamps, wetlands, and on the edges of water, such as along streams.

Cowbane, more commonly known by the name of water hemlock, is a highly poisonous plant to horses. Only a little amount of cowbane needs to be ingested to cause violent seizures that can lead to coma and death. Owing its name to the cattle it generally affects, cowbane is considered one of the most dangerous plants in the United States.

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Symptoms of Cowbane Poisoning in Horses

Cowbane poisoning progresses very rapidly, with symptoms appearing anywhere from 10 minutes to an hour after ingestion. Animals are commonly found dead. Cowbane affects the nervous system, and toxicity is characterized by extremely violent seizures, often within 30 minutes. These seizures can cause further damage, such as broken bones, and can have intermittent periods of relaxation. Death ultimately occurs from respiratory failure, usually within an hour. Signs include:

  • Nervousness
  • Dilated pupils
  • Excessive salivation
  • Diarrhea
  • Stomach pain
  • Fever
  • Bloat
  • Irregular pulse
  • Muscle tremors, usually in the lips, face, ears, and nose
  • Respiratory depression
  • Incoordination 
  • Rolling eyes
  • Walking in circles
  • Twisting the neck
  • Falling down
  • Violent seizures that involve teeth grinding, frothy saliva, tongue lacerations, and broken bones
  • Paralysis 
  • Coma 
  • Respiratory failure 
  • Death
  • Skeletal deformities in chronic low dose cases

Causes of Cowbane Poisoning in Horses

The cause of poisoning from cowbane consumption is due to the alcohols cicutoxin and cicutol. These are present in all the parts of the plant, but are in the highest concentration in the roots. Seen as an oily and fragrant yellow resin that exudes from cut stems or roots, these alcohols affect the central nervous system within minutes to an hour of ingestion. While death usually occurs from an acute poisoning, a case of chronic low dose ingestion can result in skeletal deformities.

A toxic dose has been estimated at 1 gram of cowbane per kilogram of body weight. For horses, this amounts to around 8 ounces of plant material, but death has been reported for an ingestion of only 2 ounces. Some species of cowbane can kill with a piece of root the size of a walnut. While all parts of the plant are toxic, the upper parts decrease in toxicity during the growing season. The roots, however, remain fatally toxic year round, even when dry. 

Horses generally find cowbane unpalatable, but there are times when they can ingest this deadly plant. 

  • Drinking water can become contaminated from cowbane roots that have been trampled
  • Roots of cowbane can easily be pulled up since they grow in wet areas, and can be ingested while grazing
  • Leaves, seeds, and roots can be rolled into bales of hay and accidentally ingested

Diagnosis of Cowbane Poisoning in Horses

Diagnosis can be difficult if you do not know that your horse has eaten cowbane. However, due to the rapid progression and type of symptoms, your veterinarian will likely assume your horse has been poisoned and may administer treatment before a diagnosis is definite. Tell your veterinarian any and all symptoms you have seen, and the timeframe the symptoms have occurred. Make available a sample of the plant you believe your horse may have ingested for proper identification. 

Tests may include blood work, serum testing, and a urinalysis. In the case of an unknown cause, your veterinarian may need to rule out other possible causes before looking at your horse’s environment for a toxic plant. The stomach contents of deceased animals can also be checked for plant material, and can provide a positive diagnosis, allowing you to eliminate cowbane from your fields and pastures.

Treatment of Cowbane Poisoning in Horses

There is no antidote for cowbane poisoning. Treatment is often of little value in cases of cowbane toxicity, and may only be supportive. Animals that survive for many hours without the presence of seizures have the highest rate of recovery, though most animals succumb to death within a much shorter time. 

Activated charcoal can be administered to reduce absorption of the toxin, and sedating medication is often given. Sodium pentobarbital may be intravenously administered at the onset of the first seizure to help prevent a fatality.

Recovery of Cowbane Poisoning in Horses

Recovery from a cowbane poisoning is often poor, due to the fact that the toxins involved work so quickly in the body, and cause breathing paralysis within minutes to hours of ingestion. In many cases, an owner will find a poisoned animal dead before they see any symptoms. If a smaller, less lethal dose is ingested, your horse may survive, but may incur heart or skeletal muscle damage that can be temporary or permanent. If your horse survives for over 8 hours without seizures, he is likely to recover. 

Prevent this fatal toxicity by effective management of the plant growth on your property. Herbicides can be used, as well as hand pulling, but be sure to wear gloves and wash any equipment used as cowbane is poisonous to humans as well. Cowbane is seen most in the summer months.