Ragwort Poisoning in Horses

What is ragwort poisoning?

Ragwort (Senecio spp. ) is a tall, daisy-like plant with yellow flowers, that grows in hayfields, pastures, ditches, and other unimproved areas. It is typically found in the Northeast, Midwest, and along the west coast of the U.S. 

Ragwort is a biennial plant, meaning it has a two-year cycle. In the first year, it forms a flat crown of branched leaves. This flat crown is resistant to mowing and often goes unnoticed. In the second year, it produces yellow flowers on stems that can grow up to 3 feet tall. 

Ragwort contains pyrrolizidine alkaloids, which cause liver damage. Ragwort is mainly a concern for grazing livestock, such as horses and cattle. The plants taste bitter, so grazing animals usually do not eat them unless the pasture or hay is contaminated or there is little other food available. Poisoning usually occurs from chronic ingestion. A single ingestion of ragwort is not likely to cause poisoning unless very large amounts are consumed. 

How much ragwort is needed to cause poisoning?

Generally, horses need to ingest 1% to 5% of their body weight (totaling 50 to 150 pounds) for several weeks for poisoning to occur.

What are the signs of ragwort poisoning?

Although poisoning occurs over time, signs may develop suddenly once the liver fails, and the effects are irreversible. Most horses will be described as “poor doers” in the early stages of ragwort poisoning, with weight loss, diarrhea, poor haircoat, and a non-specific decrease in appetite and activity level. More severe signs include yellow coloration of the eyes or skin (jaundice), distended abdomen, and excessive sensitivity of the skin to sunlight (photosensitization), resulting in burns and skin lesions, with hairless or light-colored haired areas being more sensitive.

When liver failure occurs, certain toxic substances are no longer removed from the blood and may cause effects on the brain (hepatic encephalopathy). Abnormal behaviors may occur, including profound depression, compulsive walking, head pressing, apparent blindness, and convulsions. Horses developing these signs may become dangerous to handle. Most severely affected cases die within days of symptoms appearing.

How is ragwort poisoning diagnosed?

Most cases are diagnosed in horses that have the expected signs and a known or suspected ingestion of ragwort. A history of ragwort ingestion may be difficult to confirm due to the time lag between ingestion and the development of clinical signs. Depending on the season, a person skilled in pasture management may identify the weed in the pasture or hay. Bloodwork confirming liver damage supports the diagnosis. A liver biopsy is required to identify the typical microscopic changes within the liver to confirm the diagnosis.

How is ragwort poisoning treated?

All animals with suspected ingestion of ragwort should have a thorough physical examination and bloodwork performed to evaluate liver function. Prevent further ingestion of ragwort by removing animals from contaminated pastures or replacing hay.

"If poisoning is detected before severe liver damage occurs, supportive care is recommended."

If poisoning is detected before severe liver damage occurs, supportive care is recommended. The liver has a remarkable ability to regenerate following injury, if addressed soon enough. Less severely affected horses may be able to compensate for the liver damage, although they may still have long-term liver effects. Bloodwork may be needed for weeks to months, to monitor liver function.

Once extensive scar tissue develops in the liver, it cannot recover. Treatment is rarely successful once more severe signs have developed. If you suspect that your horse has been exposed to ragwort, contact your veterinarian or Pet Poison Helpline, a 24/7 animal poison control center, at 1-800-213-6680 for treatment recommendations.

How can I prevent ragwort poisoning?

Pastures should be routinely searched for evidence of ragwort. Small areas of growth can be controlled by manually pulling the entire plant, including the root. Mowing alone is not effective for control, as roots left in the ground may re-sprout. If flowering plants are pulled, it is best to seal them in a plastic bag for disposal to avoid releasing seeds. Dried plants are more palatable than live plants and must be disposed of properly to avoid ingestion by horses.

Biological controls may be useful to control growth of ragwort, including the ragwort flea beetle (Longitarsus jacobaeae), ragwort seedhead fly (Botanophila seneciella), and cinnabar moth (Tyria jacobaeae). Distribution of these insects may be discouraged in some areas to prevent damage to native plant species. Consult local agricultural extension agencies before releasing these insects.

Weed killers may be effective if applied to rosettes in the spring or new growth in the fall. Application of these products after flowering will not prevent the spread of seeds. Grazing animals should be restricted from areas treated with these products, as directed by the manufacturer, typically for two weeks.

Pasture contaminated with ragwort should not be used for making hay because the poison remains active even in dried plants. Plants on adjacent land should be removed to avoid spreading the seed back into the pasture or hayfields. Planting non-invasive plants in grazing pastures provides competition to suppress germination of ragwort. Always ensure that there is adequate grazing or alternative food sources, such as hay, so the horses are less likely to eat ragwort.

Pet Poison Helpline, an animal poison control center based out of Minneapolis, MN, is available 24/7 for pet owners and veterinary professionals that require assistance treating a potentially poisoned pet. The staff provides treatment advice for poisoning cases of all species, including dogs, cats, birds, small mammals, large animals, and exotic species. As the most cost-effective option for animal poison control care, Pet Poison Helpline’s per-incident fee includes follow-up consultations for the duration of the poison case. Pet Poison Helpline is available in North America by calling 800-213-6680. Additional information can be found online at: www.petpoisonhelpline.com.