Migration of a spiruroid nematode through the brain of a horse

A pregnant 10-year-old Paint mare was examined because of an acute neurologic disturbance. Physical examination revealed signs consistent with extensive, asymmetric brain stem disease. The hemogram, serum chemical panel, and results of lumbosacral spinal fluid analysis were within normal limits. A primary diagnosis of equine protozoal myeloencephalitis was considered, and the mare was placed on treatment with trimethoprim-sulfadiazine. After 5 weeks of steady improvement, an acute exacerbation of neurologic signs necessitated euthanasia of the mare. At necropsy, large, malacic tracts were found extending through the brain stem and cerebral cortex. Cross sections of a nematode were observed microscopically and subsequently were identified as belonging to a single gravid female Draschia megastoma.