Neuropathology of Naturally Occurring Trypanosoma evansi Infection of Horses

The clinical signs and pathology of the central nervous system in 9 horses with naturally occurring neurologic disease due to Trypanosoma evansi are described. The clinical course was 2 to 20 days; clinical signs included marked ataxia, blindness, head tilt and circling, hyperexcitability, obtundity, proprioceptive deficits, head pressing, and paddling movements. Grossly, asymmetric leukoencephalomalacia with yellowish discoloration of white matter and flattening of the gyri were observed in the brain of 7 of 9 horses. Histologically, all 9 horses had necrotizing encephalitis that was most severe in the white matter, with edema, demyelination, and lymphoplasmacytic perivascular cuffs. Mild to moderate meningitis or meningomyelitis was observed in the spinal cord of 5 of 7 horses. T. evansi was detected immunohistochemically in the perivascular spaces and neuropil of formalin-fixed, paraffin-embedded brain tissue in 8 of 9 horses.