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Health Professionals Update - Vol. 18, No. 21

August 20, 2025

From:
Hetal Patel, MD, MPH, CCFP, FRCPC
Associate Medical Officer of Health

In this issue:

  • Update: West Nile virus risk in Peel
  • Update: Eastern Equine Encephalitis virus risk in Peel
  • New: Public Health Ontario Arbovirus Test Requisition Form

Key messages:

  • West Nile virus (WNV) remains endemic in Peel region.
  • Mosquitoes in Peel began testing positive for WNV in late July 2025.
  • No confirmed human cases have been reported in Peel as of August 18, 2025.
  • Clinicians should consider WNV neuroinvasive disease in patients with acute onset of encephalitis, viral meningitis, acute flaccid paralysis, or Parkinsonian-like illness.

Update: West Nile virus risk in Peel

WNV has been present in Peel since 2001. Activity varies each year, largely influenced by weather conditions. Peel Public Health monitors WNV through 33 mosquito traps collected weekly from mid-June through September. In 2025, the first positive mosquito pools were detected in late July. Human cases in Ontario are most common between late July and September. For current provincial surveillance data, visit PHO Vector-borne Disease Tool.

Transmission and clinical presentation

WNV is transmitted to humans primarily through mosquito bites. The incubation period ranges from 2-15 days or up to 21 days in immunocompromised individuals. Most people infected with WNV are asymptomatic. Approximately 20% develop a mild, flu-like illness which can include:

  • Fever, headache, and body aches
  • Fatigue
  • Rash on the chest, stomach, or back
  • Swollen lymph nodes
  • Nausea, vomiting, or photophobia

Fewer than 1% of infections progress to neuroinvasive disease, which is more severe and can present as:

  • Meningitis
  • Encephalitis
  • Acute flaccid paralysis

Additional neurological features such as tremor, myoclonus, or Parkinsonian-like symptoms may also occur. Individuals over 50 years of age and those who are immunocompromised are at greatest risk for severe illness.

Testing

As per Public Health Ontario (PHO) recommendations, cerebrospinal fluid (CSF) serology is the preferred method for suspected neuroinvasive disease; a paired serum specimen must also be submitted. Consider WNV neuroinvasive disease for patients with fever and acute onset of encephalitis, viral meningitis, acute flaccid paralysis, tremor, myoclonus, or Parkinsonian-like condition.

For patients with non-neurological illness, acute and convalescent serum specimens should be collected 2–3 weeks apart. Consider ordering WNV serology for patients who have at least two symptoms with no apparent alternative cause and a recent exposure to mosquitoes.

Testing is ordered using the PHO General Test Requisition and should indicate “West Nile Virus - Serology” along with onset date, symptoms, exposure history, travel history, and whether the specimen is acute or convalescent. Consultation with PHO or Peel Public Health may be required for test interpretation. For more information, see: PHO Test Information Index: West Nile Virus Serology and PCR.

Reporting:

WNV disease is reportable. All suspect or confirmed cases must be reported to Peel Public Health by fax at 289-801-0257.

Update: Eastern Equine Encephalitis virus risk in Peel

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.

Contact us

The Health Professionals Update (HPU) is distributed to physician offices in Peel region via fax or email. Share this update with other health professionals in your clinic.

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