Evaluation of outbreaks of disease attributable to eastern equine encephalitis virus in horses.

OBJECTIVES

To evaluate outbreaks of disease attributable to eastern equine encephalitis virus (EEEV) in horses in Michigan, and the associated environmental patterns and weather conditions, so that factors could be identified that may have predisposed horses in specific areas of the state to infections with EEEV.

DESIGN

Epidemiologic retrospective records analysis.

ANIMALS: Data on EEEV vectors, wild-bird reservoir hosts, and incidental hosts, including horses and human beings, obtained from census reports and medical records compiled between 1942 and 1991.

PROCEDURE: Patterns detected during outbreaks of disease attributable to EEEV infections in horses were compared to associated water drainage patterns, distributions of EEEV vectors, wild-bird reservoir and incidental hosts, and weather conditions.

RESULTS

Michigan has all of the elements required to sustain EEEV on a state-wide basis. Outbreaks of disease attributable to EEEV in horses have recurred in a similar regional distribution in Michigan. Regions of Michigan that have specific patterns for water drainage, specific mosquito species, and areas with higher than expected amounts of precipitation have been associated with outbreaks of disease attributable to EEEV in horses.

CLINICAL IMPLICATIONS: Evaluation of environmental patterns, weather conditions, and vector and reservoir host distributions may be useful to identify areas in Michigan and elsewhere in which horses and human beings are at increased risk for an outbreak of disease attributable to EEEV.