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Knowledge is key to safety; Plants that poison horses


The second, called "big liver disease" is severe liver disease or recurrent bouts of liver disease that is seen clinically as icterus, weight loss, CNS depression, anorexia, incoordination, dark and discolored urine and an enlarged fibrotic liver.

The third syndrome is associated with excessive salivation or slobbering, when horses eat clover that is infected with a fungus that causes brown leaf spot. Horses stop slobbering when exposure is discontinued.

Signs of poisoning depend on the syndrome and include anorexia, loss of body condition, jaundice, hepatoencephalopathy (neurologic disease) and death. Signs of other syndromes include sunburn with dermal edema, necrosis and sloughing of skin and possibly excessive salivation.

Treatment includes removing horses from exposure to the plant, treating photosensitivity and supportive care. Recovered animals often are hepatic cripples and more susceptible to liver failure or other liver diseases. It is recommended that alsike clover not be included in pasture seed mixes for horses.

Xanthium spp (Cocklebur).

Cocklebur seeds and seedlings contain a potent toxin called carboxyatractyloside, a plant-growth inhibitor that allows cockleburs to dominate competing plants. In animals, this toxin disrupts cellular metabolism, causing severe liver disease (centrilobular liver necrosis).

Poisoning most often occurs when horses consume feed contaminated with seed or when they eat small seedlings. The minimum lethal dose of seeds is 0.3 percent of body weight. All animals are susceptible to seedling poisoning.

Common signs include neurologic disease related to liver failure, depression, weakness, prostration, abnormal eye position and movements, paddling, convulsions and coma. Other changes include stocking up (swelling and edema of the feet and legs) and vasculitis. Severely poisoned animals generally die or are hepatic cripples that do poorly.

Diagnosis is made by documenting exposure and identifying blood-related changes of liver failure. Microscopic changes in liver cells and blood vessels can be detected in biopsy or post-mortem samples. Stomach or intestinal contents also can be analyzed for carboxyatractyloside.

Treating poisoned horses is symptomatic, with little response, as liver damage is extensive when animals become sick. It is important to mow or remove cockleburs before they form seeds and cause heavy infestations.

Crystalline hepatopathy – Panicum coloratum (Kleingrass), P. virgatum (switchgrass), Tribulus terrestris (puncture vine), Nolina texana (sacahuiste), Agave lechuguilla (lechugilla).

These plants are native (Nolina, Agave, and Tribulus spp.) and introduced cover crops (Panicum spp.) that can be found across North America. Poisoning is variable and incompletely understood. Likely toxins include saponins (diosgenin and yamogenin) that damage the liver biliary system. Because toxin concentrations vary and poisoning is sporadic, risk is difficult to predict. Some animals never develop disease. Panicum grasses are not very palatable and are poor forages. In most cases, animals must be forced to eat them.

Poisoning signs usually are related to sunburn or photosensitization with elevated blood biomarkers and serum enzymes suggestive of liver disease.

Lesions include severe sunburn with necrosis and sloughing of skin. The photosensitivity cause is liver failure; characteristic microscopic changes of necrosis and bile-duct lesions can be seen in the liver.

It is difficult to predict dose or risk of poisoning, so horses should not be fed monocultures of these forages.

Treatment should include supportive care for both the liver disease and sunburn.

Precautions necessary

Because most poisoning occurs when hay and prepared feeds are contaminated, caution should be used when buying hay to ensure that it is safe and of good quality.

Because most weed infestations are patchy, the best way to do this is to inspect the hay before it is harvested.

Hay should be closely inspected when it is opened prior to feeding.

Other horses are poisoned when they are pastured with toxic plants with few other forage choices.

Most of these poisonings can be prevented by monitoring pasture and grazing animals.

For additional photographs and more detailed descriptions of the plants, toxic syndromes and toxin-related pathology, see Burrows and Tyrl 2001; Knight and Walter 2001; Stegelmeier 2005.

Dr. Stegelmeier is a researcher at the United States Department of Agriculture's Poisonous Plant Research Laboratory in Ogden, UT.


  • Burrows, G. E., and Tyrl, R. J. (2001). Toxic Plants of North America, pp. 1-1277. Iowa State Press.
  • Knight, A. P., and Walter, R. G. (2001). A Guide to Plant Poisoning of Animals in North America, pp. 1-367. Teton NewMedia, Jackson WY.
  • Stegelmeier, B. L. USDA ARS Poisonous Plant Research Laboratory. USDA/ARS Location Website. 2005. Last updated 8-8-2006. http://


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