Under normal conditions, grazing animals avoid plants such as ragwort, a common weed which thrives on wasteland, road verges and railway land. The weed spreads easily to pasture land and is poisonous to horses, ponies, donkeys and other livestock, causing death through liver disease. However, during drought conditions, when other food is not available, some animals may eat poisonous plants as roughage when they are available on a lush pasture. Animals are also poisoned when eating these plants in hay, silage, or pellets. The principal toxins of ragwort are the pyrrolizidine alkaloids that are rapidly metabolised to highly reactive and cytotoxic pyrroles, which can escape into the circulation and bind to proteins. Ragwort causes more of a problem when dried in hay, as horses can't easily avoid it and may inadvertently eat large amounts.
Poisoning with ragwort is a common cause of chronic liver disease in horses. Horses with chronic form of pyrrolizidine alkaloidosis (PA) show aimless walking, sleepiness, yawning, incessant licking of objects, head pressing, staggering gait, and irritability. Weight loss, diarrhea, central blindness, colicky pain, oral ulceration, light sensitivity, and poor appetite may also occur. The first signs of PA in a small percentage of horses is difficulty breathing and bloody urine. The major clinical signs are the result of hepatic failure but inflammation of the fourth stomach and gastric impaction followed by rupture has recently been associated with the disease and may cause death or necessitated euthanasia.
Photosensitization may occur in some horses that have areas of non-pigmented skin. In these horses, severe dermatitis develops when the horses are exposed to sunlight results from the liver's failure to excrete phyloerythrin, a photo-reactive by-product of chlorophyll breakdown.
Avoidance of ragwort is very important. The most effective way of ragwort control on pastures is to dig up and remove whole plants before they flower, and to remove and burn the debris to prevent horses to come into contact with wilted plants as they are toxic. Hay should always be bought from a reputable supplier, and any signs of ragwort in hay necessitates disposing of the entire batch.
Recently, scientists have proposed developing an environmentally-friendly fungal spray that would specifically target ragwort, infecting and killing the weed at a critical growth stage.
Death usually occurs many weeks after the last mouthful. Up to 70 pecent of the liver can be non-functional before signes of disease are seen. Acute ragwort poisoning may also occur in 5 to 10 days with symptoms of dullness, abdominal pain, and jaundice.
Diagnosis is best on history, signs and microscopic examination of liver and kidney tissue collected at necropsy. Veterinary treatment may include methionine 10% dextrose solution, administered intravenously.
In full flower, ragwort is easy to recognize. It is a biannual plant, 1-3 ft. high, preferring waste land, roadsides and pastures. It has bright yellow flowers on erect stems and jagged, lobed leaves, hence its name. The plant contains toxic alkaloid and is listed as poisonous. Although some species, notably deer and rabbits, seem resistant to ragwort poisoning, cattle and horses are especially susceptible. Horses rarely take the plant while it is still growing, but will readily consume it in a wilted form, when broken off in the field. Horse owners must be vigilant in removing them from the environment of horses.