Eastern equine encephalitis-104 horses (1979-2017).

EEE is highly fatal (90% mortality) with recumbency, seizures, and CSF abnormalities predicting death.

This retrospective case series analyzed 104 horses with Eastern equine encephalitis (EEE) presenting to a veterinary teaching hospital over 38 years (1979-2017). The disease predominantly affected young horses (median age 1.4 years), particularly Thoroughbreds and Quarter Horses, with peak incidence during summer months. Clinical presentation included abnormal behavior (96%), hyperthermia (99%), recumbency (51%), and seizure-like activity (46%). Cerebrospinal fluid analysis revealed discoloration (65%), elevated protein (80%), and neutrophilic pleocytosis (62%). The case fatality rate was extremely high at 90%. Environmental analysis demonstrated that each 100-mm increase in annual precipitation correlated with a 43% increase in case numbers. Prognostic indicators associated with death included recumbency, seizure activity, band neutrophilia, monocytosis, and elevated CSF protein and neutrophils. These findings highlight EEE as a rapidly progressive, highly fatal neurologic disease with identifiable clinicopathologic markers that can guide clinical decision-making and prognosis estimation.

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