Eastern equine encephalitis-104 horses (1979-2017)

Eastern equine encephalitis (EEE) is a highly fatal neurologic disease of horses, but data describing clinical presentation, epidemiology, and prognostic factors are limited.

Describe the clinicopathologic and epidemiologic features of horses with EEE and identify factors associated with survival.

One hundred four horses diagnosed with EEE presented with signs of diffuse brain disease.

Retrospective case series of horses evaluated at a Veterinary Teaching Hospital (1979-2017). Medical records were reviewed for clinical findings, diagnostic results, and outcomes. Logistic regression assessed associations between demographic, epidemiologic, and clinicopathologic variables and survival. Generalized linear models evaluated associations between climatic variables and annual case count.

Median age was 1.4 years (0.1-13.4), predominantly Thoroughbreds and Quarter Horses. Most horses (66/104) presented during summer. In negative binomial regression adjusted for temperature and year, each 100-mm increase in annual precipitation was associated with a 43% increase in cases (IRR 1.43; 95% CI, 1.22-1.82; P < .001). Common findings included abnormal behavior (96%), hyperthermia (99%), recumbency (51%), and seizure-like activity (46%). Cerebrospinal fluid (CSF) abnormalities included discoloration (65%), high protein concentration > 80 mg/dL (80%), and neutrophilic pleocytosis (62%). Case fatality rate was 90% (94/104). Recumbency, seizure activity during hospitalization, band neutrophilia, monocytosis, and high CSF protein concentration and neutrophils were associated with death.

Eastern equine encephalitis is a rapidly progressive, highly fatal neurologic disease of horses, with increased case numbers during periods of higher precipitation. Clinicopathologic findings, particularly recumbency, seizure-like activity, leukogram changes and CSF abnormalities, provide prognostic indicators to guide clinical decision-making and client communication.