Contaminants that are Potentially Toxic to Horses
There are many contaminants that are potentially toxic to horses. This technical bulletin lists and describes some of the more common ones found in forages, toxic plants, bedding and water.
Contaminants
in Forages (hay or pasture)
Mold
and Fungus
Tall fescue (grown on approximately
40 million acres) is the most widely grown forage in the United States. It can be a very safe inexpensive source of
forage for horses. However, it can
become infested with a fungus that can cause many problems primarily related to
foaling. The fungus is an endophyte
(grows inside the plant) that does not adversely affect plant growth. The primary problems associated with
infected fescue are abortions, prolonged gestations, dystocia, agalactia, and
thick or retained placenta. The end
result is increased foal and mare deaths.
The endophyte can be found both in pasture and hay. There is a fungus-free seed available, but
the only way to determine if the fescue presently being fed is fungus-free is
by laboratory analysis.
Sweet clover is an important forage
crop in some areas of the country.
Sweet clover poisoning, caused by dicumerol, is produced from coumarin
in plants that are infected with various species of Penicillim. Mold-free sweet
clover hay is difficult to produce because the thick stems are hard to
dry. Dicumerol interferes with Vitamin
K needed for normal blood clotting thus the horses can hemorrhage internally.
Red clover can also be a high
quality forage for horses. However, if
harvested too wet, red clover hay can be infected with Rhizoctonia leguminicola, which may produce a mycotoxin called
slaframine. This causes animals to
slobber profusely; the condition is commonly called “slobbering disease” or
“blackpatch”. This condition is not
life threatening but does cause alarm.
The slobbering usually subsides within 48 – 96 hours after removal of
the affected hay.
Alfalfa hay grown in the southwestern United States may be infested with blister beetles. Blister beetles are attracted to the bloom of alfalfa plants and are crushed during the processing. The beetles contain cantharidin, which irritates the mucous membranes of the digestive tract. The horses develop a fever, colic, the sweats and a very severe, watery diarrhea. Ingesting five to six beetles can cause death in less than 48 hours. There is no antidote.
Blister beetles are about ¼“ wide
and ¾” long. They can be of several
colors, but have very distinctive heads and long narrow bodies. It is very important to remember that even
though blister beetles are not likely to be a problem in the eastern United
States the hay could have come from another area of the country.
Prussic acid (cyanide) poisoning can
cause death in horses. Toxic levels
often build up in the leaves of a number of plants, including sorghum, Sudan
grass, and hybrids of the two, Johnson grass and wild cherry. Prussic acid is most likely to accumulate
after a killing frost or during rapid growth after a drought. Wild cherry can be a problem after a
windstorm when branches are broken off the tree and the leaves wilt. Death is caused by suffocation due to
interference with the oxygen-transferring ability of red blood cells; the blood
is cherry red in animals affected by Prussic acid.
Some plants accumulate nitrates
during stress periods and/or heavy fertilization with nitrogen. The plants most likely to accumulate
nitrates are Sudan grass, sorghum-Sudan hybrids, corn, wheat and oats. Some weeds that are found in hay such as
nightshade, goldenrod, smartweed, ragweed and lambs quarters can also
accumulate nitrates. Clinical signs of
nitrate poisoning include labored breathing, staggering, muscle spasms and
death. The blood is coffee-colored due
to decreased oxygen-carrying ability of the hemoglobin.
Selenium levels in soils and plants
vary greatly from one area of the United States to another. Selenium poisoning occurs primarily in the
midwestern plains states. Most selenium
toxicities in horses are due to consumption of secondary accumulator plants
(those plants which accumulate selenium).
Other plants that require high selenium soil to grow are called
“indicator plants.” They can build up
100 times or more the selenium levels of other plants grown in the same
soil. Locoweeds, woody aster, golden
weed and prince’s plume are some plants that can produce selenium
poisoning. They are toxic when eaten
fresh or dried.
Foxtail can cause ulcerations of the
lips and mouth due to its porcupine quill-like awns. The plant itself is not toxic but the horse will be unable to eat
normally until the ulcers heal.
The yew and many other ornamental plants are toxic to horses. All parts of the yew are poisonous, whether fresh or dried. Death can occur within minutes of eating as little as one pound of the plant. The horse will quiver and then drop suddenly as if shot. Once signs appear, it is generally too late for treatment. Most horses will not preferentially eat yew, but problems occur when grass clippings containing yew trimmings are fed to horses. In the winter the green yews also appear to be more attractive to animals.
Bracken fern is also toxic because
it causes a thiamine deficiency, but it is not nearly as poisonous as yew. Twenty to twenty-five percent of the total
diet would have to be bracken fern to be fatal. However, bracken fern is very palatable; if available to the
horse they will eat it.
Black locust, red maple, black
walnut and oak trees can be toxic to horses.
Some of these have toxic leaves while others like the black walnut are
totally toxic.
Bedding
If used for bedding, black walnut shavings can be a problem. Juglone is the toxic substance in these shavings. They will cause problems if ingested or if they contact with the skin. Within 12 to 24 hours after contact, the horse will develop signs of acute laminitis. They can recover and live with laminitis, but if severe death can occur.
Water contamination can be a problem
in some areas. Runoff from strip mines
and the dumping of brine water from gas and oil wells can cause mineral
imbalances. Water that is extremely
high in some elements can be toxic or interact with other elements affecting
the animal’s health.
Summary
Contaminants that are potentially toxic to horses are numerous. Fortunately, horses are very selective eaters and will not usually consume toxic substances under normal circumstances. In areas where pastures are short or when severe weather has destroyed forages animals are sometimes forced to eat toxic plants. Horses that are kept in barns or dry lots may also be less selective when turned out to pasture.
If toxic plants or other toxic contaminates are harvested in hay it is more difficult for the horse to distinguish and separate the toxic portion. Under this circumstance, they are more likely to eat something that is toxic even though adequate feed is provided.
If a toxicity problem occurs with a horse it is important to obtain a complete, detailed history. This should include samples of feedstuffs, bedding and water. All pastures and turnout lots should be inspected for any sources of potential toxins. The combination of a complete history and samples should help provide an answer to the cause of the problem.
With good management taking into account the different seasons it is possible to greatly reduce the incidence of contaminants affecting animal health.
