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Equines as Unique Among Grazing Livestock
The digestive system of all equines
differs from other large livestock such as cattle, sheep, and goats, because equids are monogastric and not ruminants. Ruminants have the advantage of
possessing multiple stomachs: their large forestomach is adapted to degrade and
detoxify toxins.
Horses have a tendency to forage for longer periods than
cattle: 50% longer in one study, sometimes up to 24 hours per day.
This
may result in higher amounts of possibly toxic material in the digestive system,
which could be dangerous to the equine, as the effects of many toxins are
cumulative.
The particular grazing habits of equines may influence
severity of toxicity or the number of occurrences of toxic events.
Horses have two sets of incisors, which makes them unique among
herbivores, and are therefore highly efficient grazers, capable of eating
forages down to the soil surface, often including root material and discouraging
quick re-growth of desirable plant matter.
Equines are notorious for searching for new, green forage in the
spring (“chasing green”), which may result in the consumption of
toxic weeds and plants that emerge early in the growing season.
Chasing green can result in an early spring diet low in fiber and high in
soluble carbohydrates. that can contain high concentrates of potentially toxic
plant material.
Herd characteristics of equines, such as grouping and the
common tendency to defecate in certain areas, may lead to areas that are avoided
as well as areas that receive much higher levels of use.
Horses tend to graze areas that were previously grazed, and will
choose old patches first, resulting in excessive grazing in those
patches. This can result in areas of concentration of toxic plants, including
weeds.
There is often great difficulty in recognition or diagnosis
of plant poisoning in an equine. Death from plant poisoning may also be attributed to an
animal’s advanced age, in which case it may not be questioned or investigated.
Symptoms resulting from the ingestion of toxic plants very often
resemble other physical illnesses, disorders, or ailments, and occur with
varying speeds and levels of intensity.
The speed with which many toxic reactions occur in equines may make diagnosis problematical
for both owner and veterinarian. An
ornamental, the Japanese yew (Taxus cuspidata), is one plant highly cited
for its ability to fatally poison with only a few ounces, with death occurring
within hours. With other plants, by
the time symptoms present, it may be too late for treatment.
Colic is often attributed to a change in pasture or diet
but not to a toxin, and it is extremely difficult to identify its cause.
It may be assumed that an animal with a history of chronic colic is
simply having another episode, when in actuality poisoning may be taking place.
Plant toxins take three types of action in mimicking colic: a direct irritant to
the gastric system, acting upon the nervous system to stimulate the gastric
system, and causing obstruction or impaction.
Some toxic compounds affect vital organs without any symptomatic
display until the organ is largely compromised, as in the case of severe
liver disease. Toxins may also
affect more than one organ or physiological system, presenting somewhat minor
symptoms in one system, while the fundamental problem is severe organ disease or
failure that isn’t apparent until after treatment of the primary disorder.
Identification of the plant or toxin causing the disorder may therefore be very
difficult for the veterinarian.
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