Mosquito surveillance remains the mainstay in the Ministry of Health and Long-Term Care's efforts to prevent and control West Nile virus. The purpose of mosquito surveillance is to help determine the risk of contracting WNV in a particular region. This information helps the medical officer of health for each local health unit make decisions in the prevention and control of WNV illness.

Adult mosquitoes are trapped from spring to fall, identified up to species level and counted. A Real Time Reverse Transcriptase Polymerase Chain Reaction test is done to determine the WNV status of the various mosquito pools.

The mosquito species/groups of species that tested positive for WNv from 2001 to 2004 (approximately 20 species/groups of species) are the focus of the MOHLTC's viral testing.

The mosquito traps used in Ontario are mainly Centers for Disease Control (CDC) light traps which use both CO2 and light to attract mosquitoes. Some health units also use 'Gravid' traps, which are units, designed to attract gravid (i.e. pregnant) mosquitoes that have bitten at least once in their lifetime and could have picked up WNV.

WNV positive mosquito pools tend to be area specific, and can be an excellent indicator of the threat from WNV in a specific locality. Traps may be set out by the health unit on a permanent basis throughout the community, and/or be moved about to determine mosquito status in new locations of interest. A mix of fixed and flexible trap locations will be recommended to achieve representative coverage of the jurisdiction.

Positive WNV mosquito results may also be reflective of local bird WNV infection even though the indicator birds may not have been identified in an area, or the birds submitted may have proved negative for WNV. There are three main reasons for this :

1. Birds may fly in a large range, allowing ill birds to fall in locations they are not readily found for pick-up and submission (e.g. wood lots or forest);
2. Dead birds may have died of non-WNV causes; and
3. An increase in bird immunity may develop as the disease becomes enzootic (or "endemic" in human terms) in the avian population. With fewer birds dying there is less opportunity to retrieve and test them for WNV.

Ontario has also initiated a secondary adult mosquito-based surveillance program focused upon Culiseta melanura, Coquillettidia perturbans and other species that carry Eastern Equine Encephalitis (EEE). While EEE was found in only one horse in 2004 and in horses in nine health units across Ontario in 2003 (no human cases have ever been identified in Ontario), it is an important mosquito-borne disease for humans.

This program will help the medical officers of health in decision-making on control strategies as well as building a historical entomological database around EEE.

WNV mosquito surveillance statistics are provided individually to the MOHLTC by Ontario's 36 regional health units. Surveillance statistics are current as of 4 pm (EST) Monday through Friday.

• Humans
• Birds
• Horses

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