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Toxic Plants

By: Dr Andrew Hoffman

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Hoof wall cracks and bobtail ("alkali disease")

Too much selenium can be a bad thing – just ask someone in part of the country where certain selenium accumulating plants reside (Southeast, Central, Southwest). Selenium replaces the usual sulfur that is incorporated into hoof and hair, which disastrous consequences for the formation of keratin. Circular hoof cracks develop, and hair falls out, classically fist in the tail. The short tail looks "bobbed."

Plants that accumulate selenium are numerous, but notables include Golden weeds, Milkvetch, Woody astrers, and Prince's plume . Less likely to do so, but reported nevertheless, include Asters, Saltbrush, Beard tongue, Ironweed, Broomweed, and Gumweed. These plants are not likely to be found in lush green areas, and long-term consumption is necessary.


The most serious lameness is caused by Black walnut (Juglans nigra), which induces serious laminitis. Hence, it is imperative that a horse is not bedded on shavings made from these walnuts. Another plant, Hoary alyssum (Berteroa incana) causes limb edema, fever, and laminitis, confusing the diagnosis of Ehrlichia equi and purpura hemorrhagica (secondary to strangles) in some horses. Hoary alyssum toxicity has been reported from consumption of contaminated hay.

Stiffness, soreness

Other causes of lameness are plants that degenerate muscle ( coffee weed -- e.g. Cassia occidentalis), cause calcium deposits in muscle producing stiffness such as jessamine (Cestrum diurnum) . These plants contain a vitamin-D-like substance that stimulates calcium migration into tissues in toxic proportions.


The most important anemia-producing plant is Red Maple (Acer rubrum) The toxin, which is currently unknown, can be found in dried (not green) or wilted leaves, and in the bark. Leaves remain toxic for about 30 days. Only small amounts need to be ingested, and within a couple days, the horse exhibits red-brown urine, severe depression, and signs of shock. A horse with Red Maple toxicity has a poor prognosis, but in some cases can be saved with whole blood transfusions.

Other plants that induce anemia infrequently include onions (Allium spp due to N-propyl sulfide in bulbs and in the plant itself) and moldy Sweet Clover (Melilotus spp – containing molds that accumulate in hay, called dicoumarols). Onions like Red Maple cause red blood cells to break down, and a severe anemia, kidney damage, and shock result. Moldy Sweet Clover causes hemorrhages because the clotting system is impaired. Like sage, onions can be smelled on the breath of the intoxicated horse.

Cyanide poisoning from plants

A special kind of anemia s caused by ingestion of certain berry-laden plants ( Western chokecherry, Serviceberry, Elderberry ) that contain cyanide. Cyanide is found in many plants but is sequestered in a non-digestible form. Free cyanides are liberated in damaged plants. The free cyanide binds to the trivalent iron within cytochrome oxidase, and important enzyme necessary for the function of hemoglobin – the result is that hemoglobin cannot release oxygen to tissues. The blood from an intoxicated horse is bright red because hemoglobin is fully saturated with oxygen. The tissues starve from oxygen, a sort of internal asphyxiation. Rapid death often results, but if treatment is pursued quickly with detoxification chemicals, including sodium thiosulfate and sodium nitrate (intravenously), the life of the horse can be saved.

Hypocalcemia due to oxalates

Some plants contain chemicals called oxalates that bind calcium, reducing the availability to the horse. This is mostly a problem in growing, pregnant, and lactating horses, rarely with healthy mature horses otherwise. Horses have to eat a lot of these oxalate-containing plants to have problems. Plants with oxalates include Pigweed, Sorrel (dock), Sufar beet, Lambsquarter, Rhubarb, Greasewood, Halogeten, Shamrock, Soursob, and Sorrel, with the last 5 being the most common offenders. Horses can get gastrointestinal upsets. In other species, the oxalates settle in the kidneys, blocking their outflow.

Teratogens and abortions

Nothing could be more important than to keep your pregnant mare away from toxic plants. The fetus is most vulnerable to exposure to toxins during the first trimester of pregnancy, when major organ development takes place. The classic example is consumption of Skunk Cabbage / Hellebore (Veratrum eschscholtzii) during the first equine trimester, producing a cyclops foal. A whole host of teratogenic plants exist, but rarely have affects. These include Milkvetch, locoweeds (Astragalus spp) which also cause abortion and congenital skeletal deformities (carpal flexion, hock laxity), and various hemlock species. Suspected teratogenic plants also includes lupine (cause abortions), tobacco, sudan grass, jimson weed, creeping indigo, poppies, groundsel, wild black cherry, periwinkle, mimosa, and wild pea and others. In summary, keep your mares away from ANY unusual pasture or plants.

Heart Damage

Heart damage and even death can result from ingestion of certain plants that make "cardiac glycosides." Digitalis is an example of a cardiac glycoside derived from Foxglove (Digitalis spp) that can stimulate the heart in a beneficial way, but in excess quantities, is quite injurious. The cardiac glycosides produce affects similar to the classic cardiac stimulant, ouabain, by blocking the sodium-potassium pump in the cardiac cell membrance. The most important plants that contain cardiac glycosides include Foxglove, certain Milkweeds, Oleanders, Lily of the Valley, Indian hemp, and Dogbane . Sudden death is a frequent presentation of these toxicities.

Sudden death

Only infrequently will a horse die suddenly from plant toxicity. Death can result from ingestion of cardiac glycosides or cyanide containing plants, as previously discussed. Plants that cause death within a couple hours include Yew (Taxus spp), Poison, spotted, or European hemolock, Death Camas (Zigadenus spp). Avacado peels should never be fed to horses nor should horses be pastured in next to Avacado trees. An unknown toxin derived from this plant causes death following episodes of colic (see above) after a couple days. Avacados are deadly to all large animal species, and goats die within 48 hrs.

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