Equine Nigropallidal Encephalomalacia (Star Thistle Poisoning, Knapweed Poisoning)

Robert J. MacKay

Nigropallidal encephalomalacia (NPE) is a disease of adult horses characterized by facial and lingual dystonia and variable obtundation, behavioral signs, and ataxia.

■ Clinical Signs The signs appear suddenly but always after long-term ingestion of large quantities of the plants. Characteristic signs of NPE include weight loss, mild to moderate obtundation, ataxia, yawning, lowered head, protrud­ing tongue, tremor of the tongue and lips, and facial grimacing when feed is offered. The facial hypertonicity causes a retraction of the lips, which results in a fixed grimace with the mouth and lips held half open. Although affected horses can grasp food between the incisors, prehension, chewing, and swallowing of food are uncoordinated and inefficient. There may be constant chewing movements and food retained in the mouth, and cheek pouches may protrude from the commissures of the lips. Affected animals may attempt to drink by immersing their muzzles deeply into the bucket to force the water into the back of the pharynx. There also may be a transient tendency to walk compulsively, often in circles. Occasionally, horses with NPE are hyperexcitable.

After several days of progression, signs stabilize and do not then progress. Affected animals usually do not recover and are euthanized or die of starvation or dehydration.

■ Diagnosis The CSF of affected horses may have high nucleated cell counts.³ There are no characteristic changes in complete blood cell count or serum chemistry profiles. MRI was used to make an antemortem diagnosis in one horse with NPE.⁵

■ Pathophysiology The toxic principle in yellow star thistle and Russian knapweed is not known with certainty, but there is some evidence for repin, a sesquiterpene lactone with high affinity for neural tissue.^6-8 Studies in rats suggest that repin exerts its neurotoxic effects by inhibiting dopamine release.⁸ A role for neurotoxic amino acids, including aspartic and glutamic acids, has been proposed, but researchers could not confirm their presence in yellow star thistle.⁹ Immuno­histochemical staining of affected brains show that NPE is similar to environmentally acquired toxic parkinsonism of humans, whereby clinical findings are attributable to lesions in the globus pallidus and substantia nigra pars reticulata rather than to destruction of dopaminergic neurons.¹⁰

■ Necropsy Findings At necropsy, sharply demarcated areas of yellowish malacia are visible grossly in the substantia nigra and globus pallidus (extrapyramidal system). Lesions are symmetric in most but not all cases.

■ Epidemiology NPE has been reported in horses of the United States, Australia, and South America. Yellow star thistle is a common plant in unirrigated pastures in the arid regions of the western United States. The plant is resistant to the effects of saline or alkaline soil conditions and requires minimal moisture. Russian knapweed belongs to the sunflower family and grows predominantly on flood plains, where it can extract deep subterranean moisture. In the United States, the plants tend to remain green during the dry months, so most poisonings occur during the summer or late autumn. Most horses are reluctant to eat Centaurea and Rhaponticum plants unless other vegetation is unavailable, but some develop a craving and selectively seek it out. Horses that develop NPE are usually being fed a poor-quality, high-roughage diet. Affected horses range from 4 months to 10 years of age (median, 2 years).¹¹ The case-attack rates of yellow star thistle poisoning range from 3% to 31% of horses on infested pastures.¹¹ Feeding studies have demonstrated that as much as 59% to 200% of the body weight of yellow star thistle and 59% to 63% of Russian knapweed must be eaten over 3 to 11 weeks to cause clinical disease.^2,11 Continuous protracted exposure to the weeds seems to be important for expression of clinical disease. The plants retain their toxicity when dried.

■ Treatment and Prevention There is no known treat­ment. Prevention is best aimed at correcting the nutritional problem by daily supplementation with good-quality forage and by not pasturing horses in areas where either of the responsible plants grows.