Introduced from Russia as a contaminant of alfalfa seed, the plant has become a noxious weed in cultivated fields, pastures, roadsides, often forming dense stands or monocultures.
Recent evidence indicates that a complex compound isolated from yellow star thistle, and also probably present in Russian knapweed (2,3-dihydro-3,5-dihydroxy-6-(H)
-pyran-4- (DDMP) is responsible for causing nigropallidal encephalomalacia. The compound inhibits the dopamine transporter system of the brain. The disease is unique to horses affecting very specific areas of the brain (globus pallidus, substantia migra) that control the muscles of the lips, jaws and tongue. The muscles become hypertonic giving the horse a fixed or wooden appearing facial expression. The affected animal cannot bite-off and chew its food in a coordinated manner. Swallowing however, is unaffected.
The toxic dose of green Russian knapweed is 1.8-2.6 kg/100 kg. body weight PER DAY eaten over a 28-35 day period.
A creeping perennial with a black, horizontally spreading root system. It is an erect, rather stiff, branching plant ranging 2-3 feet in height. The new leaves and stems are covered with gray hairs (knap). The lower leaves are alternate with toothed margins, while the upper leaves are smaller and entire. The flowers are produced terminally and are typically thistle-like, being purple to white in color. The bracts are papery and spineless. The seeds are chalky white. Unlike Canada thistle for which it can be mistaken, Russian knapweed has no spines or prickles.
Affected horses are unable to prehend and chew their food. Attempts at doing so cause excessive salivation. Horses can swallow if food or water can be moved to the pharnyx. Affected horses may learn to immerse their heads into water troughs to get water to the back of their mouth where they can swallow it. Death results ultimately from starvation.
Facial muscles are hypertonic. Severe weight loss due to starvation.
There is no known effective treatment for affected horses. Euthanasia is indicated to avoid the animal starving to death.
Inhalation pneumonia may develop as a result of the animal's attempts at eating and drinking.
Prehension and chewing of food (cranial nerves, V, VII, IX)are affected causing hypertonicity of the facial muscles.
Horses unable to prehend and chew their food are highly suggestive of Russian knapweed and yellow star thistle (Centaurea solstitialis)poisoning. Confirmation of the diagnosis is based upon the presence of typical brain lesions of niropallidal encephalomalacia.
Horses, cattle, sheep and goats find the plant quite palatable. Sheep and goats are useful biological controls of Russian knapweed as they are unaffected by the plant.
There is no evidence at this time to show that spotted or diffuse knapweed is toxic to horses.