Russian Knapweed Poisoning of Horses

Tentative Identification

The plant in their pasture was tentatively identified as Russian knapweed, an uncommon perennial weed in our region. Scientific analysis is currently being conducted to correctly identify the plant. Russian knapweed is quite common in parts of the Dakota’s, Minnesota, and some states out west. We do have a common relative in Eastern US area (spotted knapweed), but it is mostly a roadside weed and rarely consumed by livestock. 

What Is Russian knapweed?

• Russian knapweed (Centaurea repens) is a poisonous plants in that is toxic to horses—causing “chewing disease”—but cattle and sheep consume the plants without any apparent signs of toxicity.
  
• Since this weed is an aggressive invader of pasture and vacant lands, an occasional poisoning of horses has been reported – like the one case in a horse north of Pittsburgh.
• Russian knapweed is a woody stem perennial that grows to approximately 3 feet tall. It is characterized by gray hairs (knap) that cover its leaves and stems. The terminal branches of the stem give rise to purple thistle-like flowers (Figure 1). Not to be confused with Spotted knapweed that has been in the state as an invasive weed for years.
• The exact chemical compound responsible for toxicity in Russian knapweed has not been defined; however, a sesquiterpene lactone, repin, is believed to be the key neurotoxin present.
  
• The toxic effects of Russian knapweed is cumulative, meaning that poisoning normally results when levels of the toxin build up in the body over time due to horses routinely grazing these plants.
  
• Horses must consume relatively large quantities of the green or dried plants before the toxic threshold is reached. It has been suggested that a horse must consume 60% of its body weight in green Russian knapweed plant material before toxicity symptoms appear.
  

Clinical Signs

• The clinical signs of poisoning observed in horses that have consumed large quantities of these plants result from accumulation of the toxin in the brain, resulting in necrosis, or death, of neural tissue. Initial symptoms of the disease include impaired ability to eat or drink, as well as anxious or confused behavior.
  
• In the following couple of days, the horse will begin showing the classic symptoms of hypertonicity (sustained contraction) of the muscles of the muzzle, lips, and tongue. The mouth may be held open or closed with the tongue hanging out in a curled manner. This is accompanied by constant chewing-like motions of the mouth, which can injure the tongue and other mouth parts. During this stage of chewing disease, horses are unable to eat pasture or hay, but their ability to swallow is not compromised.
  
• Muscle paralysis means that they are unable to drink water in a normal fashion, and horses may learn to submerge their muzzles deeply so that water will flow to the esophagus, allowing it to be swallowed.
  
• Other abnormal behaviors observed include yawning, violent head tossing, drowsiness, and other locomotive impairments. 
  
• If left untreated, horses normally die of starvation, dehydration, or inhalation pneumonia.  Symptoms can become lessened if removed from the infested pastures and treated by a vet.

Prognosis

• Since clinical symptoms result from irreversible damage to brain tissue, the outlook for recovery of horses showing signs of poisoning is poor.
  
• If a horse survives, the owner can expect permanent impairment of the horse’s nervous system. Therefore, preventing consumption is the only certain means of preventing clinical symptoms and death.