There are several plants or weeds that commonly infest horse pasture that are known to be toxic to the liver. The medical term for this is hepatotoxic. Often the toxin is pyrrolizidine but in some cases the toxin is not known. Some of the more common plants that are known toxic include:

• Pyrrolizidine Containing Plants
  Pyrrolizidine alkaloids (PAs) are potent liver toxins that have been identified in over 6,000 plants throughout the world. About half of the identified PAs are toxic and several cause cancer though this is not reported in horses. PA-containing plants are the most common poisonous plants that affect livestock.
• Crotalaria (rattlebox or rattlepod)
• Heliotropium
• Amsinckia (fiddleneck)
• Senecio (dozens of common names including ragwort and groundsel)
• Unknown Toxin
• Alsike Clover (Trifolium)
• Kleingrass (Panicum)

This article discusses the plants which are hepatotoxic, their characteristics, how to identify them, and how to control them. Symptoms, treatment, and prognosis are briefly covered but covered in great detail in the article on Liver Disease and Failure, ...more..

All of the plants covered in this article are hepatotoxic so the symptoms seen are common to liver dysfunction. However symptoms can be remarkably different depending on the specific type of effected cells, the rate of degeneration, and the severity of the liver damage,. For instance hoses with acute liver failure from a large dose of tansy ragwirt are more likely to have dementia (depression, aggression, seizures, coma) and incoordination as their initial and predominant sign or if the dose large enough, simply found dead.

Horses with exposure to alsike clover are more likely to have chronic liver damage and suffer weight loss and photosensitivity as the first symptoms. Gastric impaction and bilateral laryngeal paralysis are two of many complications that may be seen with equine hepatic failure. Horses with liver disease that is most pronounced in the biliary system responsible for processing old worn out red blood cells are often more jaundiced, have photosensitivity, and less likely to have CNS signs.

Because of the differential toxicity of these plants some of them typically cause chronic symptoms of photosensitivity while others more often cause acute symptoms. But it is important to remember the whole range of possibilities as ingesting small amount of the very toxic plants may cause chronic symptoms and large doses of the less toxic plants may cause more acute symptoms.

Another important difference is the effect of acute and chronic poisoning on the level of liver enzymes. Most of the plants cause death of the hepatocytes or biliary cells. As they die they release specific enzymes into the blood that are easily tested for. So while the presence of elevated levels of these enzymes is indicative of possible liver problems, chronic poisoning or advance cirrohis of the liver may present with normal or very mild elevations, despite serious disease. Below are general guidelines of what happens most commonly with exposure to a particular plant. For more on the secific symptoms and diagnosis of liver disease see Liver Disease and Failure in the Horse

Alsike clover (Trifolium) appears to be toxic under some conditions but the toxic principle remains unknown. Alsike clover is commonly included as a part of a prepackaged pasture seed mix and well adapted to cool climates and poorly drained clay soils. Alsike clover is an inferior forage compared to other commonly used legumes and should be avoided in pastures used by horses. There are two syndromes associated with horses grazing rapidly growing alsike clover: secondary photosensitization and alsike clover poisoning.

Syndromes and Sign Associated with Alsike Clover

Identification

Control of Alsike Clover

Kleingrass (Panicum sp.) is good quality forage for cattle, but can cause liver damage in horses. A warm season grass not native to the US it was imported in the 50's because of its drought tolerance and now can be found through out the Southern US. Green growth after moisture or grazing is reported to be more toxic than old or dormant growth. The toxin survives drying and curing so hay can be toxic.

Symptoms

Control

Crotalaria (rattlebox or rattlepod), Heliotropium, Amsinckia (fiddleneck), Senecio (dozens of common names including ragwort and groundsel) are known causes of pyrrolizidine poisoning in horses. Identification of these plants is covered below. The toxic principle are many different forms of pyrrolizidine alkaloids (PA's). PA's are metabolized to pyrroles in the liver which then bind to the DNA, impairing cell division and causing cell death. Histologically hepatocytomegaly, necrosis, and when advanced cirrohis of the liver can be seen.

These plants are tough and bitter so under normal conditions avoided by horses but may be eaten when other forms of forage are sparse. In particular overgrazing and drought conditions make poisoning far more likely. The toxin survives cutting and curing so may be present in in hay, silage, or pellets. Seeds are often the most toxic part of the plant and toxicity has occured when harvested with grains.

Young, growing animals are more sensitive to poisoning. Pyrrolizidine alkaloids are excreted in milk, urine and will cross the placenta so can adversely effect the fetus and nursing foal. They are considered to be carcinogens, and a potential public health risk.

Species Affected

Clinical Signs

Treatment

There are many different species of Senecio that vary widely in their appearance and toxicity. In general the flowes are yellow with the above appearance.

Tansy ragwort, Groundsel, and possibly other Senecio species are bienniel herb with basal growth the first year. In second year, grows tall, erect (0.5-1 m), and flowers. Leaves dissected, palmate with narrow segmentation, whorled, bracts form a cup under the head. Numerous yellow flowers clustered at the top.

Toxicity

Distribution

Control

Crotolaria has worldwide distribution. There are many species of Crotolaria that are toxic and they very widely in their appearance. Usually the flowers and seed pods are typical however. Notice the pea-like appearance of the flowers and the shape of the seed pods. When dry the seeds rattle in the pod hence the common name of rattelbox.

The plant may appear following land clearing where trees are removed and the soil disturbed. The total amount of ingested plant material that will result in serious harm may be equivalent to 1.5 percent to 3 percent of the animal's body weight. This can be ingested in small amounts over several days or weeks.The seeds and foliage contain pyrrolizidine alkaloids that are readily absorbed from the digestive tract. Liver enzymes convert the alkaloids into toxic pyrroles that poison liver and lung cells in horses. Cattle, swine and poultry are also susceptible, with acute or chronic damage to the liver being the major effect. Sheep and goats may be more resistant to these alkaloids.

Toxicity

Amsickia (fiddleneck, tarweed, burr weed, fireweed and buckthorn) is an erect, sparsely branched annual weed that grows up to 3 ft (1 m) tall and is covered with numerous, fine white hairs. Leaves are hairy, lanceolate, and alternate. The perfect five-parted, small, orange to yellow flowers are born terminally on a characteristic fiddleneck-shaped raceme, with the flowers all inserted on one side of the axis. Mature fruits separate into two to four blackridged nutlets.

The plants appear in late winter and early spring. The seeds are the most toxic parts of these plants and fortunately the mature plants are unpalatable to most horses. Most instances of poisoning occur when mature amsinckia species are bailed in early cuttings of hay or when contaminated grain was threshed for horses. Some poisonings have occurred when horses were pastured on wheat stubble in contaminated fields.

Toxicity

Heliotrope Heliotropium is a genus of flowering plants in the borage family, Boraginaceae. There are 250 to 300 species in this genus, which are commonly known as heliotropes and have worldwide distribution and some species reported as a source of PA poisoning of horses. The name "heliotrope" derives from the fact that these plants turn their leaves to the sun. The old English name is "turnsole". Several heliotropes are popular garden plants, most notably Garden Heliotrope (H. arborescens). The sap of heliotrope flowers, namely of European Heliotrope (H. europaeum), was used as a food coloring in Middle Ages and Early Modern French cuisine. Common heliotrope is grown in Southern Europe as an ingredient for perfume.	European Heliotrope (H. europaeum) Garden Heliotrope (H. arborescens)
Cynoglossum There are many species in the plant genus Cynoglossum They are coarse-appearing, small-flowered plants of the family Boraginaceae. Cynoglossum officinale, the common hound's-tongue, is a native of Asia, Africa, and Europe. It has been introduced into North America, and there have been multiple reports of horses poisned by this plant. More Information on J Am Vet Med Assoc. 1984 Sep 15;185(6):647-50. Cynoglossum officinale (hound's-tongue)--a cause of pyrrolizidine alkaloid poisoning in horses. Knight AP, Kimberling CV, Stermitz FR, Roby MR. The death of 10 horses was attributed to feeding dried grass hay containing hound's-tongue, Cynoglossum officinale. Affected horses developed weight loss, icterus, photosensitization, and hepatic encephalopathy. Histologic examination of the liver of 3 of the horses revealed megalocytosis, biliary hyperplasia, and fibrosis characteristic of pyrrolizidine alkaloid poisoning. Hound's-tongue was found to contain large quantities (0.6% to 2.1%, dry matter basis) of toxic pyrrolizidine alkaloids, which, when fed to a pony for 20 days, caused liver fibrosis and biliary hyperplasia. The toxicity of the many other species is not known but should be suspect until proved differently. Cynoglossum virginianum, known as Wild comfrey, is a common plant from New York to Florida. Cynoglossum boreale occurs in the northern United States as far west as Minnesota and in Canada. Cynoglossum amabile, known as Chinese Forget-me-not and Chinese Hound's Tongue, originally a wildflower from Southwest China, is now found in Europe and North America.	Cynoglossum officinale Cynoglossum amabile
Echium Echium plantagineum (Purple Viper's Bugloss) is a species of Echium, native to western and southern Europe (from southern England south to Iberia and east to the Crimea), northern Africa, and southwestern Asia (east to Georgia). It has also been introduced to Australia and is an invasive plant, killing horses who accidentally consume it. It is an annual or biennial plant growing to 20-60 cm tall, with rough, hairy, lanceolate leaves up to 14 cm long. The flowers are purple, 15-20 mm long, with all the stamens protruding, and borne on a branched spike. Patterson's Curse is now a dominant pasture weed through much of New South Wales, the Australian Capital Territory, Victoria, South Australia and Tasmania and also infests native grasslands, heathlands and woodlands. The plants contain pyrrolizidine alkaloids and, when eaten in large quantities, cause reduced livestock weight or even (in severe cases) death. Paterson's Curse can kill horses and irritate the udders of dairy cows and the skin of humans. After the 2003 Canberra bushfires over 40 horses were officially put down after eating the weed.

Treatment

Specific treatment for sunburn and liver disease are covered in detail elsewhere, links are provided below.

Sunburn

The first step of treatment is to remove the horse from access to the offending plant and protect him from sunlight until the blood has a chance to clear the toxins. Depending on the severity of liver disease this may take several weeks to several months. Following the levels of toxins that have built up in the blood, like bilirubin, can help decide when it is safe to allow the horse back out into the daylight. Treatment for the skin disease would be the same for sunburn...more.

Hepatoencephalopathy

If hepatoencephalopathy is a concern, a low high quality protein (high branch chain / low aromatic amino acid ratio) diet should be fed and neomycin or lactuloseb should be given per os (via syringe) to decrease enteric ammonia production. Feeds with small amounts of carbohydrates and branch chain amino acid should be given every 2-4 hours. For a more complete description of treatment see, Liver Disease and Failure.

Prognosis

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