Barnaby's Thistle Poisoning in Horses

Barnaby’s thistle, also known as yellow star thistle (Centaurea solstitialis), is an annual with large yellow thistle-like flowers ringed with sharp thorns. This plant is not actually a thistle, instead it belongs in the knapweed genus and it contains compounds that can cause equine nigropallidal encephalomalacia, a fatal disorder more commonly referred to as chewing disease. This poison causes lesions to develop on the brain of the horse, and inhibits the ability of the horse to swallow, causing the animal to expire due to either malnutrition or dehydration.

Barnaby’s thistle contains a compound that can cause a fatal disorder commonly known as chewing disease, that inhibits the horse’s ability to swallow either food or water.

The symptoms of poisoning by Barnaby’s thistle, known as chewing disease, are typically mild to non-existent until very large quantities of this plant are eaten, generally between fifty and two hundred percent of the horse’s weight. Symptoms of equine nigropallidal encephalomalacia, or chewing disease, can include: 

• Clenched facial muscles
• Drowsiness
• “Grinning” appearance
• Depression
• Head-tossing
• Inability to swallow
• Lethargy
• Lolling or portruding tongue
• Loss of appetite
• Open mouth
• Submerging head in water
• Unexplained yawning

Types

Russian knapweed (Centaurea repens) is similar to Barnaby’s thistle in toxicity, as it has the same chemical compounds as Barnaby’s thistle (also known as yellow star thistle). These toxic compounds can cause irreversible lesions on the brain that cause a disorder commonly known as chewing disease. Russian knapweed is physically distinct from yellow star thistle; it is a large woody plant characterized by the tiny grey hairs that are present along the plant’s leaves and stems. It has large, purple thistles at the end of it’s three foot long stems rather than the large yellow thistles found on the Barnaby’s thistle plant, and the large thorns that are present under the yellow thistles are absent under the purple thistles of Russian knapweed.

This plant has a bitter flavor and the flowers are ringed by long, sharp thorns. Because of this, Barnaby’s thistle is generally considered to be unpalatable to horses, however, horses put out to pasture in overgrazed fields may end up grazing on this weed; some horses have even been known to develop a taste for the plant once exposed to it. Occasionally, horses are exposed to this plant when it is accidentally mixed in with dried hay. Both Barnaby’s thistle and Russian knapweed are poisonous only to horses, proving non-toxic to other species, including other equines, such as mules and donkeys.

If you see your horse grazing on pastures rife with this weed, identification is often sufficient enough for a preliminary diagnosis of the origin of your horse’s distress. A sample of the plant that you believe that your horse consumed will help to properly identify the plant and the examining veterinarian will perform a full physical examination at this time, as well as taking samples to employ in standard tests such as a biochemistry profile, urinalysis, and complete blood count.

Even when the consumption of the plants are unwitnessed, the distinctive symptoms of equine nigropallidal encephalomalacia may give your veterinarian a good idea of which plants are responsible, and note will be taken of the plants and weeds that are growing in and around the grazing pastures as well as within reach of the stable area, and if neither Barnaby’s thistle or Russian knapweed are found in the fields, the rations of hay that the horse has been eating may also be checked for contaminants. A sample of the patient’s feces is also typically evaluated, and plant material found in the feces may assist the examiner in determining an accurate diagnosis.

If the ingestion of large quantities of this plant have recently been consumed and no symptoms have emerged the veterinarian may opt to use decontamination methods such as gastric lavage and activated charcoal to eliminate as much of the toxin from the animal as possible to prevent the absorption into the bloodstream. In most cases, however, the ingestion of the plant occurs over a period of thirty to ninety days, and the emergence of symptoms occur unexpectedly.

The signs of poisoning by this weed indicate that irreversible lesions have formed within the substantia nigra and the globus pallidus portions of the horse’s brain and the ability to swallow is permanently impaired. Once the symptoms of toxicity have developed any available supportive treatments are only palliative and the horse will eventually succumb to dehydration or malnutrition even with aggressive therapy attempts. In clinical studies, poisoned horses that were fed through a tube directly into the stomach never regained the ability to swallow even after extended lengths of time.

The prognosis for horses who have developed symptoms related to the ingestion of Barnaby’s thistle is poor, and euthanization is often determined to be the kindest course of action. The best treatment for Barnaby’s thistle poisoning is prevention. Fortunately, this plant is usually distasteful to horses due to both the bitter taste and the sharp thorns and they only sample it when there is no other forage available; however, some horses will develop a taste for this weed. If you do see your horse grazing on these plants it is best to remove the animal from that field until the dangerous weed is eradicated.