Ragwort Poisoning in Horses

By Deidre M. Carson, BVSc, MRCVS & Sidney W. Ricketts, LVO, BSc, BVSc, DESM, DipECEIM, FRCPath, FRCVS.

Tansy Ragwort ( Senecio jacobea ) is a tall daisy like plant with yellow flowers that grows in hayfields, pastures, ragwort_poisoning-1ditches, and unimproved areas.   Tansy ragwort contains pyrrolizidine alkaloids, toxins that are found in many other plants that affect horses and livestock. Ingestion of Tansy ragwort can ultimately lead to scarring in the liver and eventually complete liver failure. Both fresh and dried plants are poisonous. The plants are not palatable and horses will not usually eat them unless the pasture is heavily contaminated or there is little other food available. However, the poison is very stable and remains toxic even when the dried plant is incorporated into hay. The most common cause of ragwort poisoning is therefore from chronic (long-term) eating of hay that includes dried ragwort.

What are the signs of ragwort poisoning? 

Unless very large quantities of fresh plants are eaten the signs of poisoning are usually not seen until 4 weeks to 6 months after horses begin eating the plants. Damage to the liver accumulates over this time period and is irreversible.  It generally takes about  50-150 pounds or about 1-5% of a horse's body weight eaten over several weeks to cause significant liver damage. Signs generally develop after ingestion of 50-150 pounds or about 1-5% of a horse's body weight for several weeks.

Signs generally aren't present until the liver has failed and then they occur quite rapidly.  Early signs include loss of appetite, depression, diarrhea, weight loss and mild jaundice. More severe signs include marked jaundice, photosensitization (excessive skin sensitivity to sunlight, especially in pink skin areas) and collapse or abnormal behavior, that can range from profound depression to compulsive walking and pressing the head against objects, e.g., walls, apparent blindness, and convulsions. These behavioral abnormalities are caused by toxic effects on the horse's brain (hepatic encephalopathy).  These cases become dangerous to handle. Most severely affected cases with behavioral abnormalities die within approximately 10 days.

How is ragwort poisoning diagnosed and treated?

ragwort_poisoning-2The diagnosis of ragwort poisoning is based primarily on clinical signs and laboratory tests. A history of ingestion of ragwort may be unclear due to time lag between ingestion and the development of clinical signs. Depending on the season of the year, a person skilled in pasture management may identify the weed on the pasture or in the hay. Laboratory tests, including the measurement of liver enzymes, bile acids and bilirubin levels in the horse's blood, support a  diagnosis of liver disease. A liver biopsy is  required to demonstrate the typical microscopic abnormalities and help confirm the diagnosis .  Follow-up blood samples help to monitor the progression of the condition in horses receiving treatment for ragwort poisoning.

If you suspect that your horse has been exposed to ragwort, contact your veterinarian or Pet Poison Helpline* (800-213-6680) immediately for treatment recommendations.

Treatment is rarely successful once signs have developed, especially those with behavioral (neurological) abnormalities. Feeding a special diet to try to reduce the severity of nervous symptoms can help in the short term in some cases. If ingestion is suspected and complete liver failure has not developed, supportive care is ragwort_poisoning-3recommended, but the horse may never return to its previous healthy state. The scar tissue that develops in the liver cannot be replaced by normal liver tissue, but less severely poisoned horses can sometimes be helped to compensate for their loss of liver tissue. Intravenous fluids, electrolytes, glucose, and B vitamins are useful as is protecting the horse from the sun. Other in-contact horses should be examined for signs of poisoning so that they can receive treatment and extra care. A good physical examination and screening the blood for liver enzyme abnormalities are important first steps.

How can I prevent ragwort poisoning?

Ragwort is a biennial plant. In the first year,  a flattish crown of branched leaves is formed. This flat crown is fairly resistant to mowing and often goes unnoticed. In the second year, yellow flowers are produced on stems that are up to approximately 31.5" (80 cm)  high (see photo).  The pastures should be searched for evidence of tansy ragwort. Any plants that are found should be pulled up by their roots and disposed of far away from horses and other livestock. Do not leave cut or pulled plants in their environment or they may be eaten when they have dried. Do not use pasture that is contaminated with ragwort for hay making because the poison remains active even in dried plants.   Plants on adjacent land should be removed to avoid the spreading of seed back into the pasture or hayfields. Always ensure that there is adequate grazing or alternative food sources such as hay, so that your horse or pony is not tempted to eat any ragwort that may have been missed.  New weed-killing sprays are available and manufacturers claim they are effective in killing ragwort while being safe to use near horses.

*Pet Poison Helpline, is an animal poison control service available 24 hours, seven days a week for pet owners and veterinary professionals who require assistance treating a potentially poisoned pet – including birds! Pet Poison Helpline is available in North America by calling 800-213-6680. Additional information can be found online at www.petpoisonhelpline.com. Pet Poison Helpline is not directly affiliated with LifeLearn.

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