Halton - West Nile virus, Lyme disease and Anaplasma are active

Article

Jul 24, 2025

Halton Region Public Health: Office of the Medical Officer of Health

Adult mosquitoes in Halton Region have tested positive for West Nile virus (WNV). Human WNV cases often follow up to 6 weeks after the first positive mosquitoes are detected.
As of June 30, 2025, Public Health Ontario requires a Vector-Borne and Zoonotic Virus intake form (external PDF) to be submitted along with the General Test Requisition form (external PDF) when requesting specific vector-borne or zoonotic virus tests, including those for WNV PCR.
There has been a recent increase in Lyme disease tick positivity rates in Halton and locally acquired human cases of Lyme disease are trending upwards.
Cases of anaplasmosis, while not as common as Lyme disease, have been increasing in Ontario. Two ticks collected through active tick surveillance tested positive for Anaplasma for the first time in Halton Region in spring 2025

West Nile virus and other arboviruses present in Ontario, such as California serogroup viruses and Eastern equine encephalitis, should be considered in any patient with febrile or acute neurological illness and recent exposure to mosquitoes, blood transfusion, or organ transplantation.
Serology is the preferred method to detect WNV infection. Visit PHO’s West Nile Virus – Serology and PCR webpage (external link) for testing criteria.
For WNV PCR testing, it is mandatory to provide the clinical information, relevant travel, and relevant exposures on the Vector-Borne and Zoonotic Virus Intake form (external link). Test requests that are submitted without the appropriate mandatory information are subject to cancellation.
Report any suspected or confirmed cases of WNV illness to Halton Region Public Health by calling 311.

Active tick surveillance in Halton provides evidence that the blacklegged tick populations are expanding, and more ticks are testing positive for the bacterium that causes Lyme disease. The positivity rate increased from 17% in 2023 to 27% in 2024, with ticks collected in spring 2025 showing a further increase to 38%.
For Lyme disease clinical guidance, including treatment information, refer to the Management of Tick Bites and Investigation of Early Localized Lyme Disease (external link).
As of June 30, 2025, PHO has changed the testing algorithm for anaplasmosis. To improve test utilization and align with best practice recommendations, PHO now recommends PCR over serology for diagnostic testing. For additional laboratory information, see PHO’s Anaplasma – PCR and Serology (external link).
In 2024, 160 cases of anaplasmosis were reported in Ontario. There were no human cases of anaplasmosis reported among Halton region residents in 2024.
Public Health Ontario has launched an Ontario Vector-Borne Disease Tool (external link) which offers weekly surveillance data on vector-borne diseases in Ontario and risk assessment maps using data from 2014 to the present.

Includes information that can be shared with patients on safe tick removal and risk reduction strategies