Eosinophilic encephalomyelitis in horses caused by protostrongylid parasites

Four thoroughbred horses showing lameness, ataxia, circling, depression, recumbency, and seizures, were examined. The horses had gross, pale- to dark-red manifestations and foci in the central nervous system (CNS). Multifocal to coalescing eosinophilic necrotizing encephalomyelitis was observed histologically in the CNS along with intact or degenerated nematodes. Nematodes had polymyarian-coelomyarian musculature, a smooth thin cuticle, and intestines lined by multinucleated cells with microvilli. These traits suggested the nematodes belonged to the family Protostrongylidae, which includes Parelaphostrongylus tenuis. It was concluded that the horses were infected by nematodes, presumably Parelaphostrongylus tenuis, resulting in eosinophilic necrotizing encephalomyelitis.