West Nile Virus Update – from MLHU

West Nile Virus Season

On August 8th, the first two mosquito traps positive for West Nile Virus (WNV) this year were identified in London. WNV typically circulates within the mosquito population from July to end of September.

Human Disease:

Reported human cases of WNV in Middlesex-London average 2-3 cases per year, though this is expected to be an underestimate of the true burden of disease.

  • Most people who are infected are asymptomatic, with only 20% developing symptoms, and less than 1% becoming seriously ill.
  • People >50 years of age are at increased risk of WNV neuroinvasive disease, such as encephalitis or viral meningitis, which can cause permanent neurologic damage or death.

1) During the summer, consider ordering WNV serology for patients who have at least two of the following symptoms, with no apparent alternative cause:

  • Fever
  • Myalgia or arthralgia
  • Headache
  • Fatigue
  • Lymphadenopathy
  • Maculopapular rash

2) Note that routine bloodwork may reveal leukocytosis and other non-specific findings associated with viral infections.

3) Suspect and test for WNV neuroinvasive disease for patients with fever and acute onset of encephalitis, viral meningitis, acute flaccid paralysis, tremor, myoclonus, or Parkinsonian-like condition.

4) Remind patients to take effective measures to prevent mosquito bites and exposure to WNV:

  • Wear light-coloured clothing with long-sleeves, pants, and socks in areas where mosquitos are present.
  • Regularly empty standing water where mosquitoes can breed from garbage cans, wheelbarrows, toys, flowerpots and saucers, pool covers, tires, and other items.
  • Apply a Health Canada approved insect repellent, following the manufacturer’s instructions, such as those containing DEET.
  • Take extra care during peak mosquito biting times, at dusk and dawn.

Report all suspected and confirmed cases of WNV illness to the Middlesex-London Health Unit by phone: 519-663-5317, or fax the Reportable Disease Notification Form to 519-663-8241.

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