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What is Yellow Star Thistle Poisoning?

Yellow star thistle (Centaurea solstitialis) are toxic to horses, resulting in a condition called “chewing disease” or nigropallidal encephalomalacia. The plant is a noxious weed that easily overpowers native plants and is prevalent in range and vacant lands, often in areas with dry or compromised soil, in the western United States. The weed is an annual and has multiple stems that display yellow, star-like flowers with long, spiny bracts for protection. Yellow star thistle can reach a height of around 3 feet.

The whole plant is toxic, whether in its fresh form or when dried. It can be problematic in your horse when it is baled into hay or if your horse eats it when in the pasture. It is thought that only horses experience poisoning by the plant; cattle, mules and burros do not appear to be affected by it.

Toxic to horses, ingesting a large quantity of yellow star thistle can lead to a condition called centaurea solstitialis or “chewing disease” in your horse.

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Symptoms of Yellow Star Thistle Poisoning in Horses

Should your horse experience yellow star thistle poisoning, you may observe the following symptoms:

  • An impairment in your horse’s ability to eat or drink
  • Appearing nervous or confused
  • His mouth may be held open or be unable to open and his tongue may hang out and be curled. 
  • He will make motions with his mouth like he is chewing. 
  • Chewing food and then spitting it out
  • Your horse will be unable to eat pasture or hay
  • Upon developing muscle paralysis, your horse will be unable to drink water normally and you may notice him submerging his muzzle deeply into water in order to allow the water to flow to his esophagus.
  • Yawning
  • His tossing his head violently
  • Acting like something is caught in his throat
  • Appearing very tired
  • Impaired movement
  • Weight loss

Types 

There are two types of yellow star thistle seed; plumed and plumeless. Plumeless seeds make up 10-25% of the total amount of seeds and the rest are plumed. Since two types of seeds are created that disperse at different times and in different ways, the chances of new populations being established are greater than with one type of seed.

Causes of Yellow Star Thistle Poisoning in Horses

The symptoms that are noticed upon your horse experiencing yellow star thistle toxicity are due to the toxin accumulating in their brain, which will lead to the death of neural tissue. It is thought that the key neurotoxin is repin, which is a sesquiterpene lactone. The effects of ingesting yellow star thistle are cumulative; poisoning normally happens when toxic levels build up in your horse over time as a result of your horse grazing the plants on a regular basis.

Before your horse will experience toxicity, he must ingest a large amount of the plant. It is thought that he will have to ingest 50-200% of his body weight of the plant over 30 to 90 days before he will experience problems. Once he has reached this threshold, symptoms will develop quickly.

Diagnosis of Yellow Star Thistle Poisoning in Horses

Should you notice any of these symptoms in your horse, it is important to contact your veterinarian, who will conduct a physical examination. The signs of poisoning may be similar to early symptoms of rabies and your veterinarian will want to rule that out as a cause. Diagnosis will be made based on clinical signs; unfortunately, once symptoms are evident equines are often in the late stages of the toxicity. If there is a chance your horse can be saved it will depend on symptomatic therapy and his response to it. Documentation shows that on the rare instance that an equine does recover, neurological function is most often impaired.

Your veterinarian may choose to walk the pasture and paddocks with you to verify the plants found there. His knowledgeable eye will be able to identify the yellow star thistle and other noxious weeds that may be present on your property.

Treatment of Yellow Star Thistle Poisoning in Horses

Should your horse experience yellow star thistle poisoning, you will want to keep him from the pasture or area where he ingested the plant. Unfortunately, by the time symptoms of toxicity develop, it is very likely that the horse will die. The effects of the toxin are long term. Should your horse lose the ability to swallow as a result of the toxin, he will not regain the ability, even after he has stopped eating the plant and a significant amount of time has passed.

Horses experiencing yellow star thistle poisoning will often die of starvation, dehydration, or inhalation pneumonia. As a result of the neurological damage that occurs and cannot be reversed, euthanasia is often recommended. If the veterinarian is able to provide nutrition therapy, fluids, and medication to perhaps lessen the effects of the toxicity, and your horse responds to the therapy, there is a chance of recovery. However, studies show that sadly, this is not often the case.

Recovery of Yellow Star Thistle Poisoning in Horses

It is important to keep your horse away from this plant in order to ensure that he does not ingest a toxic amount of it. Owners of horses should continually monitor the areas where their horses graze to make sure that these toxic plants are not present. Speak with your veterinarian for advice on the use of safe herbicides for weed control.