Yellow Star Thistle Poisoning (Nigropallidal Encephalomalacia or Chewing Disease) in Horses

Key takeaways

Yellow star thistle poisoning, also called nigropallidal encephalomalacia or “chewing disease”, occurs when a horse eats a toxic amount of the yellow star thistle plant. Yellow star thistle is an annual weed commonly found in western North America, the Mediterranean, Argentina, and Australia. 

  • The specific toxin and mechanism of toxicosis in horses is unknown, however the toxin appears to target and damage the brain
  • "Chewing disease" symptoms include difficulty and inability to swallow and chew, progressive weight loss, and abnormal gait
  • Diagnosis is based on symptoms
  • There are no treatment options for nigropallidal encephalomalacia; severely affected animals are usually euthanized
  • Prevention centers on removing the thistle from pastures and ensuring adequate alternative forages are available
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A closer look: Yellow Star Thistle Poisoning (Nigropallidal Encephalomalacia or Chewing Disease) in Horses

Yellow star thistle poisoning is also known as chewing disease because it leads to difficulty chewing and eating in horses. If enough of the thistle is consumed, affected horses eventually die of starvation as they are unable to consume enough nutrients to survive.

Yellow star thistle toxicosis is cumulative. Once a horse has ingested more than a critical amount of thistle, symptoms occur rapidly, and chances of survival are extremely poor.

Toxicosis occurs once a horse consumes between 20 and 200% of its body weight over the course of one to three months. The exact neurotoxin that causes "chewing disease" is unknown.

Prevention is the only way to ensure safety for at-risk horses. The best prevention strategies are removing yellow star thistles from pastures and providing adequate amounts of alternative feed sources, such as hay or grass pasture.

Risk factors

Yellow star thistle toxicosis is rare in horses. No treatment or antidote exists, and affected animals die from starvation due to inability to eat.

Any horse with access to yellow star thistle is at risk of poisoning. The plant is native to Africa and the Mediterranean (particularly Turkey) and is present all over the USA. In North America, yellow star thistle is most concentrated in California.

Possible causes

Ingestion of a toxic amount of the yellow star thistle causes yellow star thistle toxicosis or "chewing disease." The exact mechanism of the disease process is currently unknown.

Main symptoms

Testing and diagnosis

Diagnosis of nigropallidal encephalomalacia is based on symptoms, physical examination, and a history of ingesting yellow star thistle. At some specialty hospitals, MRI imaging may be available to confirm the diagnosis. Once the symptoms appear, the animal is in the last stage of the toxicosis, and generally does not survive.

Steps to Recovery

There is no treatment for yellow star thistle toxicosis, and by the time symptoms appear, it is highly likely to be fatal.

Affected horses generally die as a consequence of starvation and dehydration.

Euthanasia is recommended for horses showing symptoms of chewing disease to minimize suffering. Other horses sharing a pasture or feed source with the affected horse must have alternative feed provided as soon as possible.

In rare cases of survival, the neurological damage caused by the toxicosis is permanent, and may lead to long-term deficits in ability to chew and consume food.


Yellow star thistle toxicosis is not contagious.

As there are no treatment options, prevention is paramount. Prevention strategies include:

  • Removal of yellow star thistles from pastures
  • Ensuring hay is not contaminated
  • Providing access to adequate amounts of hay or grass pasture to discourage eating of thistles

Removal of yellow star thistles from pastures is the best way of preventing "chewing disease," however care must be taken as a number of herbicides and pesticides can also be toxic to horses.

Is Yellow Star Thistle Poisoning (Nigropallidal Encephalomalacia or Chewing Disease) in Horses common?

Yellow star thistle toxicosis is rare in horses. Horses with access to adequate feed generally do not eat yellow star thistles.

Typical Treatment

  • Euthanasia

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