EEEv is a rare but severe mosquito-borne illness with a high case fatality rate. Risk in Peel is very low because the primary vector, Culiseta melanura, is rarely found locally due to limited suitable woodland swamp habitat. No horses, mosquitoes, or humans in Peel have ever tested positive for EEEv.
Areas of Ontario with suitable habitat, particularly cottage country and eastern regions, are more likely to have EEEv detected in mosquitos and horses in the late summer and early fall. However, reports of human infections with EEEV have been rare; historically there have been four human cases of EEEV in Ontario, the most recent reported in 2024 by Ottawa Public Health.
Clinicians may consider EEEv in patients presenting with acute onset of encephalitis, if they report recent travel or outdoor exposure in these higher-risk regions during mosquito season. Refer to the PHO Vector-borne Disease Tool for maps of where EEEv activity is occurring in the province.
On July 1, 2025, PHO introduced a supplementary Arbovirus Test Requisition Form for diagnostic testing of selected arboviruses. This form must be completed in addition to the General Test Requisition. Use of the correct form ensures timely processing and avoids delays in testing. Forms and guidance are available on the PHO Test Information Index.
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