OAHN Equine Project: Spatiotemporal patterns of eastern equine encephalitis virus and its vectors in areas of past outbreaks in Ontario

Project Lead: Dr. Katie Clow

Collaborators:  Dr. Alison Moore

Project Proposal

Eastern equine encephalitis (EEE) is a mosquito-borne viral disease endemic to eastern North America. It causes severe and usually fatal disease in horses and humans, and to a lesser extent, other mammalian and bird species (1). Southern states, especially Florida, have documented the highest incidence, followed by the northeastern states, including Connecticut, Massachusetts, and Vermont (1). Equine cases are sporadically reported in Ontario, with several recent notable outbreaks (2). In 2018, there were 12 reported equine cases, with 11 from the Niagara peninsula. In 2023, there were 18 reported equine cases, with 13 reported from eastern Ontario. This past year, there were 24 reported cases from several regions across Ontario, with the highest number in eastern Ontario (n=17) (2). This summer also marked the first human case of EEE in Ontario (3,4).

Eastern equine encephalitis has a complex disease ecology. The black-tailed mosquito, Culiseta melanura, is the primary vector of EEE virus (EEEv) (1,5). This species resides in hardwood swamps and prefers to feed on birds. Virtually all passerine birds are reservoirs for EEEv and contribute to viral amplification and geographic dissemination (1). It is believed that spillover into horse and human hosts is likely driven by other competent vector species that have wider host and geographic preferences than the primary vector. These include Coquillettidia perturbans, Culex salinanus, Cx. pipiens, Aedes vexans, and Ae. canadensis (1,5).

We currently have limited knowledge on spatial and temporal patterns of EEEv, the primary vector and the bridge vectors. Provincial mosquito surveillance conducted by public health units is targeted towards West Nile virus (WNV) and thus efforts are focused on urban areas where WNV vectors predominate (6). This creates significant data gaps on mosquito and viral dynamics in rural areas where equine populations primarily reside.  Research in the eastern United States has elucidated some patterns, but clear geographic differences in virus and vector patterns have been documented, further illustrating the need for targeted rural sampling in Ontario (1).

Relevance to the equine industry

Eastern equine encephalitis is a fatal disease in horses. A highly effective vaccine is available and recommended as a core vaccine for all adult horses. The American Association of Equine Practitioners recommends annual vaccination in the spring or vaccination every six months in endemic areas with prolonged mosquito activity (7). Our proposed research will identify the seasonal patterns of EEEv and its vectors in areas of recent outbreaks in Ontario to document the current risk period and ensure current vaccination practices align with this risk period.

 References

1. Armstrong PM, Andreadis TG. Ecology and epidemiology of eastern equine encephalitis virus in the northeastern United States: An historical perspective. J Med Entomol. 2022 Jan 12;59(1):1–13.

2. Ontario Ministry of Agriculture, Food and Agribusiness. Equine Immediately Notifiable Disease Surveillance in Ontario. 2024. Available from: https://experience.arcgis.com/experience/4bc735584cbc426fa7df13e9c9db2ef7/page/Home/?views=EEEV

3. Public Health Ontario. Eastern equine encephalitis virus: History and enhanced surveillance in Ontario. Toronto, Ontario, Canada; 2014.

4. City of Ottawa. Local human case of eastern equine encephalitis virus is confirmed. 2024 [cited 2025 Feb 4]. Available from: https://ottawa.ca/en/city-hall/city-news/newsroom/local-human-case-eastern-equine-encephalitis-virus-confirmed

5. Armstrong PM, Andreadis TG. Eastern equine encephalitis virus in mosquitoes and their role as bridge vectors. Emerg Infect Dis. 2010 Dec;16(12):1869–74.

6. Ontario Ministry of Health. West Nile virus preparedness and prevention plan. Toronto, Ontario, Canada; 2023.

7. American Association of Equine Practitioners. Adult horse vaccination chart. 2023. Available from: https://aaep.org/wp-content/uploads/2024/02/Adult_Horse_Vaccine_Chart_2023_SUB.pdf