In This Section |
Rapid and Unexpected Death in
Horses
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| 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 |
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:
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.
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).
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.
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 |
|---|---|---|
| Cows |
3.3 mg/kg body weight
|
1.5-2 hr |
| Ewes |
44 mg/kg body weight
|
1.5-2 hr |
| Mares |
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.
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.
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).
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.
For more information:
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