Rapid and Unexpected Death in Horses
Part A - Toxins

Table of Contents

  1. Introduction
  2. Botulism
  3. Ionophore Toxicity
  4. Yew Poisoning
  5. Poison Hemlock
  6. Red Maple Leaf Poisoning
  7. Oleander Toxicosis
  8. Cantharidiasis (Blister Beetle Poisoning)
  9. Summary
  10. References
  11. Toxic Plant Resources
  12. Related Links


There is an old farmer adage - "Where there is livestock, there is deadstock." Unfortunately, it comes true too often. In this series, we will be describing conditions that affect all ages of horses with the exception of conditions specific to the neonate (less than 6 months of age). Horse owners and veterinarians should be aware of the common toxins that can affect horses. Eliminating these toxins from horse farms can prevent some sad and avoidable deaths.

The most common acute toxins that kill horses in a few hours to 36 hours include:

  • Botulism - often associated with haylage feeding
  • Ionophore toxicity - associated with feed contamination
  • Yew toxicity - associated with horses consuming clippings from this common ornamental shrub
  • Poison-hemlock - found in swampy areas
  • Red maple leaf poisoning

Less common are oleander and cantharidin (blister beetle) poisoning.

In addition to intoxications, there are other causes of sudden or unexpected death in horses, including electrocution and lightening strike, which also need to be ruled out when dealing with sudden death. These will be discussed in Part B of this series.


Botulism is caused by the bacteria, Clostridium botulinum. In Ontario, botulism is most commonly associated with the feeding of haylage containing the preformed toxin. However, botulism has also been associated with the ingestion of lawn clippings and, occasionally, other forms of forage, including: acid treated hay, dry hay and corn silage. The greater the moisture content of the material, the greater the danger of providing the ideal conditions for the bacteria to grow. The risk of botulism increases as you move from feeding dry hay to wetter forms (dry hay < acid treated < bagged small bales < wrapped or bagged large bales). Horses are susceptible to botulinum toxin at two (2) parts per trillion. Typical clinical signs include: muscle weakness, tremors, difficulty swallowing, drooping eyelids and mydriasis (dilation of the pupils), occasionally leading to recumbency and death due to cardiac and respiratory failure.

Take-Home Message

  • If you are feeding your horses haylage/silage, vaccinate for botulism. The vaccine only protects against Type B but, when a problem occurs, Type B is the most common type.
  • Anti-toxin may be available at your nearest veterinary college.
  • Round-bale silage baled at moisture levels of 16 - 30% may be an increased risk factor.
  • Hay fields fertilized with poultry manure may pose a higher risk.

Ionophore Toxicity

The ionophores are coccidiostats used in the poultry industry but may be included in some cattle feed and mineral as growth promotants. The ionophores include monensin (Rumensin) and lasalocid (Bovatec). The most common clinical signs of toxicosis include: lethargy, cyanosis, depression, pulmonary edema, myocardial degeneration and death.

The lethal dose 50 (LD50) of monensin for horses is 1-2 mg/kg of body weight. The LD50 of lasalocid in horses is estimated to be 21.5 mg/kg of body weight (1).

Take-Home Message

  • Feed only mineral and commercial rations intended for horses. They are manufactured to prevent accidental contamination with ionophores.

Yew Poisoning

The genus Taxus consists of three commonly grown ornamental shrubs: English yew (Taxus baccata); Canada yew (Taxus canadensis), a native shrub; and Japanese yew (Taxus cuspidata). They are commonly used as landscape shrubs. The needles and seeds of all yews are highly poisonous to horses and cattle, both in the fresh and dry form. However, the red fleshy seed covering is not poisonous. The toxic principle is taxine. As little as 0.1% of a horse's body weight or one pound of English yew is toxic to a horse. Horses eating yew will die within 1 - 3 hours. Death is attributed to cardiac arrest and asphyxia.

Take-Home Message

  • Yews are deadly and should not be located on horse or other livestock farms.

Poison Hemlock

Poison hemlock (Conium maculatum) is a naturalized herb with an umbrella-form shape similar to wild carrot (Queen Anne's Lace). The plant grows in swampy areas, in wet meadows, and along the edges of streams and drainage ditches. Cattle, goats, horses, swine, and sheep, as well as rabbits, poultry, deer, and humans have been poisoned after ingesting poison hemlock. Animal species vary in their susceptibility to acute toxicity (2).

Animals Susceptibility Time from Ingestion to Death
3.3 mg/kg body weight
1.5-2 hr
44 mg/kg body weight
1.5-2 hr
15.5 mg/kg body weight
30-40 min

Poison hemlock contains a neurotoxin, which causes muscle tremors and violent convulsions and death in 2 - 3 hours in some species. Horses tend to exhibit less violent signs. Poison hemlock contains the chemical coniine, a potential teratogen, which can cause "crooked calf disease" in young pigs and cattle. Poisonings occur from the ingestion of the foliage or tubers. It has 2 - 8 thick tubers. Dried foliage is less toxic but can be a problem in hay.

Take-Home Message

  • Areas where poison hemlock is present should be fenced off from all livestock.

Red Maple Leaf Poisoning

Red maple (Acer rubrum) leaf poisoning is associated with horses eating wilted red-maple leaves from broken branches or when horses reach over fences, consuming leaves that were disposed of into the manure pile. The amount of toxin increases in leaves during the summer. Fallen leaves remain toxic for a few weeks or more. Ingestion of fresh leaves does not appear to cause disease. The ingestion of 1.5 - 3 grams of wilted leaves per kilogram of body weight (0.7 - 1.5 kg for the average 450-kg horse) will cause haemolytic disease characterized by severe depression, anemia and hemoglobinuria (presence of free hemoglobin in the urine). The toxic principle is thought to be gallic acid. Gallic acid has also been found in silver maple and sugar maple. Both of these tree species are capable of hybridizing with red maple.

Take-Home Message

  • Not all trees with red leaves are red maple. Red maples should not be grown in or adjacent to horse pastures.
  • Fallen red-maple leaves should not be placed in locations where horses can ingest them.

Oleander Toxicosis

Oleander poisoning in livestock has been associated with plant clippings in those areas where oleander is common (i.e. California). Sudden death was the most common presenting complaint. Other signs reported included: diarrhea, pulmonary edema, tachycardia, cardiac arrhythmias, colic and lethargy. Presumptive diagnosis of oleander poisoning is based on matching clinical signs with evidence of consumption of oleander. A new two-dimensional, thin-layer chromatography analysis of ingesta for oleandrin and an awareness of lesions in heart muscle have greatly improved the ability to diagnose oleander toxicosis (3).

Cantharidiasis (Blister Beetle Poisoning)

Blister beetle poisoning is associated with insect contamination of forage, usually diets containing alfalfa hay. The onset of signs of disease is rapid with signs of gastrointestinal tract distress, non-specific neurologic signs and dying in shock terminally, with duration of clinical signs ranging from 3 to 18 hours. In fatal poisonings, gross lesions may be minimal or unapparent, and diagnosis must be confirmed by chemical detection of cantharidin in urine, blood, or stomach or cecal contents (4). Blister beetle poisoning is seen in the midwestern United States where blister beetles are abundant.


For owners and veterinarians, it is extremely frustrating when a horse dies suddenly and no explanation can be found. While intoxications as a cause of unexpected death are relatively uncommon, it is important to be aware of them, as they are potentially preventable. In a study of 200 cases of sudden and unexplained death in horses and ponies older than one year, no cause of death was found in roughly one-third of the cases. The remaining died from cardiovascular, gastrointestinal or respiratory lesions (5). These conditions will be discussed in Part B of this series.

Full descriptions of botulism, red maple leaf and yew poisoning can be found on the OMAFRA website. Excellent sources of resource material are listed below.


  1. Hanson LJ, Eisenbeis HG, Givens SV. Toxic effects of lasalocid in horses. Am J Vet Res 1981;42 (3):456-61.
  2. Keeler RF, Balls LD, Shupe JL, Crowe MW. Teratogenicity and toxicity of coniine in cows, ewes and mares. Cornell Vet 1980;70:19-26.
  3. Galey FD, Holstege DM, Plumlee KH, Tor E, Johnson B, Anderson ML, Blanchard PC, Brown F. Diagnosis of oleander poisoning in livestock. J Vet Diagn Invest 1996;8 (3):358-64.
  4. Helman RG, Edwards WC. Clinical features of blister beetle poisoning in equids: 70 cases (1983-1996). J Am Vet Med Assoc 1997;211 (8):1018-21.
  5. Brown CM, Kaneene JB, Taylor RF. Sudden and unexpected death in horses and ponies: an analysis of 200 cases. Equine Vet J 1988;20 (2):99-103.

Toxic Plant Resources

  • Kingsbury, JM. Poisonous Plants of the United States and Canada. Prentice-Hall, 1964.
  • Ontario Weeds. OMAFRA Publication 505, 2001.
  • Kershaw L. Ontario Wildflowers. Lone Pine Publishing, 2002. ISBN 1-55105-2085-7.
  • McKnight AP, Walter RG. A Guide to Plant Poisoning of Animals in North America. Teton NewMedia, 2001. ISBN 1-893441-11-3.
  • Burrows GE. Tyril RJ. Toxic Plants of North America. Iowa State Press, 2001. ISBN 0-8138-2266-1.
  • The Canadian Poisonous Plants Information System.

Related Links

For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca
Author: Bob Wright - Lead Veterinarian, Equine and Alternate Species/OMAFRA; Margaret Stalker - Anatomic Pathologist, Animal Health Laboratory/University of Guelph)
Creation Date: January 2007
Last Reviewed:
January 2007