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Eastern Equine Encephalitis and West Nile Fever in Equids 2023 Update

Eastern equine encephalitis (EEE) virus and West Nile virus (WNV) are two zoonotic, mosquito-borne viruses that cause fever and neurologic disease in equids. Birds are the reservoir host for both viruses. EEE and West Nile fever (WNF) are typically diagnosed in the late summer and fall in the US. EEE, Western equine encephalitis (WEE) and Venezuelan equine encephalitis (VEE) viruses are in the Togaviridae family, but WEE and VEE have not been present in the US for decades.1 Ratites are also susceptible to EEE and present with hemorrhagic gastric enteritis without neurologic signs.2 WNV is in the Flaviviridae family, and many species of mammals, birds and reptiles are susceptible to infection.3 Annual vaccination of equids against EEE and WNF is mostly protective, but unvaccinated equids are at risk. EEE cases present with severe neurologic signs and have up to 95% case fatality risk, whereas WNV cases are milder with a 22%- 44% case fatality risk.1 

Both EEE and WNF can be diagnosed through serologic testing (presence of IgM antibodies). The EEE IgM ELISA is performed at the National Veterinary Services laboratory (NVSL) in Ames, Iowa, and the WNVIgM/IgG ELISA is performed at the AHDC. The AHDC offers EEE PCR testing on brain as well as EDTA whole blood. The test on the latter sample type is currently in the validation process, and all EDTA whole blood samples should be accompanied with serum for the EEE IgM confirmatory test at NVSL. The sensitivity of these diagnostic tests varies throughout the course of disease; therefore, it is always best to run the EEE IgM ELISA and the EEE PCR together. The NYS Department of Health Wadsworth Center in Albany, NY also performs EEE and WNV PCR testing on equid brain samples during arbovirus season.

In New York, 9 EEE cases and 5 WNF cases were confirmed in equids with diagnostic testing performed at the AHDC, NVSL, or the Wadsworth Center in 2023. EEE cases occurred in the northern part of the state; most horses presented with fever and ataxia and all 9 were euthanized. The WNF cases were spread across the state, 3 had fasciculations, and only 1 horse was euthanized. For both viruses, affected horses ranged from 2 years to 24 years, and the majority were middle aged.


  1. Mackay RJ, Middleton JR, Aleman M. Chapter 35: Diseases of the Nervous System. Editor(s): Smith BP, Van Metre DC, Pusterla N. Large Animal Internal Medicine (Sixth Edition), Mosby, 2020, Pages 1006-1117.
  2. Tully TN Jr, Shane SM, Poston RP, England JJ, Vice CC, Cho DY, Panigrahy B. Eastern equine encephalitis in a flock of emus (Dromaius novaehollandiae). Avian Dis. 1992 Jul-Sep;36(3):808-12. PMID: 1417619.
  3. Spickler, Anna Rovid. 2013. West Nile Virus Infection. Retrieved from