PULLMAN, Wash.–A Washington State University veterinary student’s research with yellow star thistle suggests the exotic weed that is advancing into Washington is the cause of a fatal disease in horses with symptoms similar to Parkinson’s disease in humans.
Commonly known as “chewing disease,” equine nigropallidal encephalomalacia causes symptoms that include fatigue, a lowered head and a characteristic rapid uncontrolled movement of the upper lip. This lip twitching prevents horses from efficiently picking up and swallowing food.
For decades, the cause of chewing disease has remained somewhat elusive. While yellow star thistle has been implicated all along, several factors have perplexed investigators. For instance, why does the disease only affect horses and not cattle, sheep or wild animals that consume it? And why did previous feeding trials of yellow star thistle conducted in California fail to induce the disease?
Sean Sanders, a senior veterinary student in the research track at WSU’s College of Veterinary Medicine, has isolated a compound which may prove to be the cause of chewing disease. Feeding studies indicate a suspected toxin in yellow star thistle, or a nutrient deficiency as a result of eating the plant are linked with chewing disease. Daily consumption of the plant produces clinical symptoms within 30 to 80 days, depending upon how much the horse eats.
“This disease damages areas in the brain that control fine motor movements, including the horse’s ability to chew, swallow or drink, and eventually leads to starvation and dehydration,” said Sanders. “The symptoms occur rapidly, and currently there is no treatment.”
Sanders became interested in this study after one of his professors mentioned the disease in class.
“Research on horses with this disease hadn’t been conducted since the 1960s, and most of the research tested chicken brain tissue that is very difficult to use, plus this disease only affects horses,” said Sanders. “I wanted to study the effects on horse brain tissue and the areas of the brain that chewing disease damages.”
Sanders was able to isolate a dopamine-like compound from yellow star thistle that he believes may be responsible for the disease. Dopamine is a molecule normally produced by brain cells in the area of the horse’s brain that controls its ability to chew and swallow.
“We know dopamine is present in many areas of the brain, but it is particularly prominent in areas controlling movement and acts as a messenger in these areas,” said Joe Harding, a WSU neuroscience professor in the veterinary college. “It is released by one brain cell to carry a chemical message to another brain cell.”
Harding explained that in order to convey a message from one brain cell to another, the dopamine must cross a synapse, the space between brain cells. Once the message is delivered, the chemical signal needs to be turned off. To do this, dopamine is reabsorbed from the synapse by the brain cell that released it. The brain cell recognizes the dopamine by looking for a specific shape on the dopamine molecule.
“Because there are different types of dopamine receptors, brain cells will not absorb dopamine if they are not designed to. When the brain cell recognizes its specific dopamine molecule, the brain cell absorbs it,” said Harding.
The compound Sanders found in yellow star thistle resembles the shape and pattern of the dopamine molecule in the horse’s brain that helps to control its chewing and swallowing. When these brain cells absorb the dopamine molecule after sending a message, they may mistakenly absorb the compound produced by yellow star thistle. Sanders proposes that once the compound is absorbed, its toxicity kills the cell.
Once all the cells that control specific functions like chewing and swallowing are dead, the horse is unable to perform that vital behavior and dies.
By isolating the toxin, Sanders hopes to create a toxoid that would act like a vaccine. The toxoid would alert the horse’s immune system to create antibodies that would destroy the toxin and prevent the brain cells from mistakenly absorbing it.
Yellow star thistle is a drought resistant weed, originally from the Mediterranean. It out-competes most native plants and can become the major green foliage in a horse’s diet.
“Yellow star thistle has migrated north from California to eastern Washington, and chewing disease may become more common to our area,” Sanders said. “This weed primarily hurts small land owners and pleasure horse owners who cannot move their horses to other pastures not infested with the weed. Control of the weed is difficult with herbicides and not economically feasible in most cases.”
WSU’s College of Veterinary Medicine has not received a large number of chewing disease cases yet, but last summer a horse was diagnosed with chewing disease near Lewiston, Idaho.
Sanders’ research, while in a preliminary phase, has caught the attention of the National Parkinson Foundation. Many of the symptoms and the location of the brain affected by chewing disease are similar to Parkinson’s disease in humans.
“Veterinary medicine has adapted many treatments from human medicine to treat animals and vice versa,” said Sanders. “I hope that we can develop a cure for chewing disease and in the process contribute to the research on Parkinson’s disease.”

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