Merck Manual

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Professional Version

Western Equine Encephalitis in Poultry


James S. Guy

, DVM, PhD, DACVM, DACPV (Deceased), Department of Population Health and Pathobiology, North Carolina State University

Reviewed/Revised Oct 2019 | Modified Jun 2023

Western equine encephalitis (WEE) virus has many characteristics in common with eastern equine encephalitis virus; however, it is rarely associated with disease in avian species. WEE virus is seen mainly in western parts of the USA and Canada, Central America, and South America. In the USA and Canada, it is transmitted principally by Culiseta tarsalis, a mosquito vector that is common west of the Mississippi River. Control is based on reducing exposure to mosquito vectors.

Clinical Findings

Western equine encephalitis virus is a cause of encephalitis and high mortality in turkeys; affected turkeys exhibit somnolence, tremors, and leg paralysis. Turkey breeder hens may have decreased egg production.


Prevention and Control

Prevention and control of western equine encephalitis is based on management procedures aimed at reducing vector populations and/or locating production facilities away from vector habitats.

Zoonotic Risk

Western equine encephalitis virus is a zoonotic agent and a potential cause of neurologic disease in people; however, most of these infections are subclinical. The case fatality rate for WEE virus is approximately 3%–7%.

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