Yellow Starthistle is a plant that grows 1-3 feet high. It is easily recognized by its yellow flowers that have large thorns under them. The mature plant has a greenish-gray color to it.
This plant is not native to the United States and was accidentally introduced through contaminated seeds. It is competitive plant that easily overpowers native plants and requires strong measures to control it.
It is mostly found in the western United States, usually in areas that have drier or compromised soil, though according to the USDA website, it can be found throughout the United States.
The entire plant is toxic, both in fresh and dried forms. Thankfully it takes a good amount of this plant to cause toxicity -- 60-200% of the horse's body weight. It can cause problems when baled into dried hay, as well as when the horse eats it in a pasture situation. Unfortunately, horses can develop a taste for this plant, consuming it even when other plants are available.
One unique note about this plant is that it appears that only horses are poisoned by it. Other animals, such as cattle, have no problem with it. In fact, even mules and burros appear to be unaffected by large consumption of it!
The toxin that is produced by the plant causes "Chewing Disease" which is officially called nigropallidal encephalomalacia. The disease is called Chewing Disease because it makes it impossible for the horse to swallow. The toxin affects the brain, causing a softening of the tissues.
Symptoms of yellow starthistle poisoning include:
Other signs of poisoning include symptoms similar to early rabies symptoms, since the toxin affects the brain and can cause neurological signs. Therefore, if your horse is displaying symptoms, treat him as though he has rabies until it is verified that the problem is yellow starthistle poisoning.
The toxin does not cause immediate effects; rather, the toxin builds up over time, and by the time symptoms appear, it is extremely likely that the horse will die. The effects are long term, and it has been proven through stomach tube feeding in clinical studies that once horses lose the ability to swallow, they will not regain that ability, even after consumption of the plant has stopped and long periods of time have passed.
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