Equine parkinsonism

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Equine Parkinsonism

Equine Nigropallidal Encephalomalacia, Russian Knapweed Poisoning, Russian Centaurea, Yellow Star Thistle, Toxic Equine Parkinsonism

Equine parkinsonism is a neurological condition of horses caused by repetitive ingestion of yellow star thistle (Centaurea solstitialis) and/or Russian knapweed (Centaurea repens). The condition is thought to resemble human parkinson's disease and is characterized by the impairment of the horse's ability to eat. One of the earliest signs is paralysis of the lips and tongue. Horses experience a reduction in jaw tone, which causes their mouth to remain partially open with their tongue hanging out. Sometimes the horse's upper teeth are exposed or hypertonicity of the facial and upper lip muscles. The effects of toxins are irreversible and prove to be ultimately fatal to the horse. They will become weak and emaciated, and eventually die from starvation.

Clinical signs in horses usually appear suddenly and unexpected, as they take at least 28 to 35 days for Russian knapweeed and 33 to 81 days for Yellow star thistle for symptoms to appear in horses. There is no specific treatment for the condition, and horses will not recover.


Paralysis of lips and tongue
Anxious or confused behavior
Reduced jaw tone
Constant chewing-like motions of the mouth
Excessive yawning
Violent head tossing
Severe depression
Head hanging low


  • History
  • Clinical signs
  • Physical exam
  • Plant identification
  • MRI
  • Necropsy



Vitamin E: Administered to help reduce clinical signs


  • Make yourself aware of the weeds and plant species that can be invasive in pastures and/or poisonous to horses.
  • Take periodic walks around pastures to check for the presence of potentially poisonous plants
  • Check that hay does not contain dried up poisonous plants
  • If you borrow or hire farm machinery ensure it is clean prior to arriving on your property, the same goes for lending of your own equipment.
  • Quarantine new animals in a separate paddock the first 10 days to 2 weeks after arrival. Weed seeds can be passed through an animal's digestive tract.


Outlook for recovery is poor since the damage to the brain tissue is irreversible.

Scientific Research

General Overviews

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Risk Factors

  • Overgrazed horse pastures
  • Unbalanced nutritional diet