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

Overview

• This study analyzed 104 cases of Eastern equine encephalitis (EEE) in horses over a 38-year period to describe clinical characteristics, identify epidemiologic patterns, and determine factors that influence survival.
• The disease is highly fatal and presents with neurological symptoms, with case numbers linked to environmental factors such as increased rainfall.

Study Purpose and Objectives

• To describe the clinicopathologic features of horses with Eastern equine encephalitis (EEE).
• To characterize the epidemiologic features of EEE in horses from 1979 to 2017.
• To identify prognostic factors associated with survival in affected horses.

Methods

• Study Design: Retrospective case series conducted at a Veterinary Teaching Hospital.
• Sample: 104 horses diagnosed with EEE presenting with clinical signs of diffuse brain disease.
• Data Collection: Medical records were reviewed for clinical signs, diagnostic results including cerebrospinal fluid (CSF) analysis, and outcomes.
• Statistical Analysis:
  • Logistic regression to identify associations between demographic, epidemiologic, and clinicopathologic variables and survival.
  • Generalized linear models to investigate climatic factors (temperature, precipitation) and their associations with annual case count.

Demographic and Epidemiologic Findings

• Median age of affected horses was 1.4 years; range from 0.1 to 13.4 years, indicating mostly young horses were affected.
• Predominantly affected breeds: Thoroughbreds and Quarter Horses.
• Seasonality: Majority of cases (66 out of 104) occurred during the summer months.
• Environmental Factors:
  • Increased annual precipitation correlated with higher incidence; specifically, each 100 mm increase of precipitation associated with a 43% increase in cases.
  • No explicit detailed association mentioned with temperature or other climatic variables after adjustment.

Clinical Presentation and Laboratory Findings

• Clinical signs indicated diffuse brain disease, consistent with neurologic dysfunction caused by EEE.
• Cerebrospinal fluid (CSF) findings:
  • Elevated protein concentration (>80 mg/dL) found in 80% of cases, indicating inflammation or blood-brain barrier disruption.
  • Neutrophilic pleocytosis (an increase of neutrophils in CSF) in 62% of horses, suggesting bacterial or viral inflammation of the central nervous system.
• Peripheral blood abnormalities associated with worse outcomes included:
  • Band neutrophilia (presence of immature neutrophils in blood).
  • Monocytosis (elevated monocytes).

Prognosis and Survival Factors

• High fatality rate: 90% (94 out of 104 horses died).
• Factors associated with death included:
  • Recumbency (inability to rise or stand).
  • Seizure activity during hospitalization.
  • Blood leukogram abnormalities such as band neutrophilia and monocytosis.
  • High CSF protein concentration and neutrophils.
• These clinical and laboratory indicators could help veterinarians predict prognosis and inform owners during clinical decision-making.

Conclusions

• Eastern equine encephalitis is a swiftly progressing neurologic disease in horses with a very high mortality rate.
• Environmental factors, particularly increased precipitation, lead to more cases, likely due to mosquito breeding and virus transmission dynamics.
• Specific clinicopathologic findings such as neurological signs, CSF abnormalities, and blood leukogram changes serve as important prognostic indicators.
• The study provides valuable information to improve clinical management, client communication, and potentially guide preventive strategies based on epidemiologic patterns.