The Chatham-Kent Public Health Unit has detected West Nile virus in mosquitoes collected from three different trap sites in Chatham.
In July and early August the health unit found the virus in mosquitoes trapped in Wheatley, Dresden and Ridgetown.
It has not said whether any people have been infected.
Public health officials are reminding the public that the West Nile virus is transmitted to people through the bite of an infected mosquito.
Only 20 per cent of bitten people develop symptoms such as fever, rash and muscle aches.
West Nile can cause serious brain and spinal cord inflammation - encephalitis and meningitis – but that is rare.
