Rapid and Unexpected Death
Part A - Toxins
Table of Contents
- Ionophore Toxicity
- Yew Poisoning
- Poison Hemlock
- Red Maple Leaf Poisoning
- Oleander Toxicosis
- Cantharidiasis (Blister Beetle Poisoning)
- Toxic Plant Resources
- 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
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.
- 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
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
- Feed only mineral and commercial rations intended for horses.
They are manufactured to prevent accidental contamination with
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.
- Yews are deadly and should not be located on horse or other
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).
|| Time from Ingestion
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.
- Areas where poison hemlock is present should be fenced off from
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.
- 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 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
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
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.
- Hanson LJ, Eisenbeis HG, Givens SV. Toxic effects of lasalocid
in horses. Am J Vet Res 1981;42 (3):456-61.
- Keeler RF, Balls LD, Shupe JL, Crowe MW. Teratogenicity and
toxicity of coniine in cows, ewes and mares. Cornell Vet 1980;70:19-26.
- 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.
- 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.
- Brown CM, Kaneene JB, Taylor RF. Sudden and unexpected death
in horses and ponies: an analysis of 200 cases. Equine Vet J 1988;20
Toxic Plant Resources
- Kingsbury, JM. Poisonous Plants of the United States and Canada.
- Ontario Weeds. OMAFRA Publication 505, 2001.
- Kershaw L. Ontario Wildflowers. Lone Pine Publishing, 2002.
- 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
- The Canadian Poisonous Plants Information System.