Yellow Star Thistle and Russian Knapweed
Horses that eat yellow star thistle (Centaurea solstitialis) or Russian knapweed (C. or Acroptilon repens) for prolonged periods develop an irreversible brain disease called nigropallidal encephalomalacia, because it causes a softening or dissolution (encephalomalacia) of the nigropallidal areas of the brain. This damage destroys the horse's ability to take in and masticate food. The disease occurs only in horses and only in those areas of the western United States, Australia, and Argentina where these plants are abundant. Cattle and sheep appear to be able to eat both yellow star thistle and Russian knapweed without problem.
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Yellow star thistle originated in the Mediterranean area. It has become extensively established in California and has spread through the southern states to the Atlantic coast. It is an annual weed, with multiple branching stems growing to 3 ft (1 m) in height. The stems have longitudinal wings or ridges formed by the downward extension of the leaf bases. The basal leaves are markedly lobed, with the stem leaves being linear and covered with fine white hairs. The characteristic, star-like yellow flowers are produced at the ends of the branches and are protected by bracts with long spines. The seeds have a terminal tuft of whitish hairs.
Russian knapweed was introduced from Russia and has become a noxious weed in many Rocky Mountain states. It is a perennial with woody stems up to 3 ft (1 m) tall and invasive, branching underground stems. The name knapweed is derived from the grey hairs, or knap, that cover the leaves and stems, giving the plant a grey green appearance. The stems branch terminally and end in a purple thistle-like flower. The bracts are papery white and lack the stiff long spines seen in yellow star thistle. The seed heads tend to remain closed and do not shed the seeds readily.
The specific toxin that causes localized destruction of the globus pallidus and substantia nigra areas of the brain has not been confirmed, but may be repin, a sesquiterpene lactone toxin.
Horses must consume large quantities of the green or dried plants for extended periods before a toxic threshold is reached and clinical signs abruptly appear. Horses have to consume an amount of green Russian knapweed or yellow star thistle equal to 60 to 70% of and 1 to 2 times their body weight, respectively, before clinical signs develop. Thus, Russian knapweed appears to be more toxic than yellow star thistle. Horses apparently develop a preference for the plants and will eat them even in the mature spiny state. In California there are two times of the year, June- July and October-November, when yellow star thistle poisoning is most prevalent, suggesting some seasonal variation in palatability or toxin content of the plants.
The clinical signs occur suddenly in association with the development of lesions in the brain. Initially, affected horses exhibit increased tone of the muscles of the muzzle and lips so that the mouth is held open with the incisors exposed and the tongue hanging out. Continuous movements of the tongue and the effects on the lips may cause frothing of saliva as the horse tries to eat. Some horses may wander about with their lips brushing through the grass; to the unobservant, this could be mistaken for normal grazing. If offered hay, the horse will attempt to scoop up the hay and hold it in its mouth, but normal feed intake and mastication are severely impaired. Swallowing is, however, unaffected, and some less severely affected horses may learn to submerge their heads far enough into a deep trough of water to allow water to reach the pharyngeal area to be swallowed. Some affected horses appear depressed and, if the lesions in the brain affect one side more than the other, the lips may be pulled to one side and they may circle in one direction. Other abnormal behavior may include violent head tossing and excessive yawning. Weight loss becomes severe, as the affected horse is unable to eat. If not euthanatized, they eventually die from starvation and/or feed inhalation-induced pneumonia.
There is no effective treatment, as the affected areas in the brain die and do not regenerate. Affected horses may be kept alive by administering water, electrolytes, and a high-energy liquid diet through a stomach tube. Euthanasia of affected horses is eventually necessary because of the debilitating and irreversible effects of the condition.
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