Toxic equine parkinsonism: an immunohistochemical study of 10 horses with nigropallidal encephalomalacia.
Authors: Chang H T, Rumbeiha W K, Patterson J S, Puschner B, Knight A P
Journal: Veterinary pathology
Summary
# Editorial Summary: Toxic Equine Parkinsonism and Nigropallidal Encephalomalacia Chronic consumption of yellow star thistle or Russian knapweed triggers nigropallidal encephalomalacia (NPE) in horses, producing parkinsonian signs including dystonia of the lips and tongue, inability to grasp food, depression and locomotor dysfunction. Chang and colleagues examined brain tissue from 10 affected horses using immunohistochemistry to map dopaminergic pathways and determine whether NPE resembled human Parkinson disease at the histological level. The pathological damage was precisely localised to the substantia nigra pars reticulata and rostral globus pallidus, with partial disruption of dopamine-carrying fibres passing through the pallidus; notably, the cell bodies of dopaminergic neurons in the substantia nigra pars compacta remained intact, and no Lewy body inclusions characteristic of human Parkinson disease were observed. These findings suggest that the clinical signs result directly from disrupted neural circuitry in the globus pallidus and substantia nigra pars reticulata rather than from death of dopamine-producing neurons themselves. For practitioners, this distinction is significant: NPE serves as a rare large-animal toxicological model of acquired parkinsonian syndrome, and the specific neuroanatomical pattern may eventually inform treatment strategies targeting restoration of function in affected circuit regions rather than neuronal regeneration.
Read the full abstract on PubMed
Practical Takeaways
- •Horses with dystonia of lips/tongue, inability to eat, depression and locomotor deficits following pasture access to yellow star thistle or Russian knapweed should be suspected of NPE—remove from contaminated forage immediately
- •NPE causes selective lesioning of specific brain regions rather than neuronal death, which may inform prognosis and recovery potential in affected horses
- •Recognize that toxic plant ingestion (thistle species) is the underlying cause; prevention through pasture management and plant identification is the primary control strategy
Key Findings
- •NPE lesions are localized to substantia nigra pars reticulata and rostral globus pallidus, sparing dopaminergic neuron cell bodies in substantia nigra pars compacta
- •Partial disruption of dopaminergic (tyrosine hydroxylase-positive) fibers occurs in the globus pallidus
- •No Lewy bodies or abnormal cytoplasmic inclusions were observed, distinguishing equine NPE from human Parkinson disease pathology
- •Clinical parkinsonism phenotype is directly attributable to lesions in globus pallidus and substantia nigra pars reticulata rather than dopaminergic neuron destruction