Early mosquito egg-laying raises West Nile virus concern, entomologists say

Early egg-laying by one of themosquito speciesknown to carry West Nile virus suggests thepossibility of a summer outbreak of the diseasein southern Manitoba, entomologists say.

University of Winnipeg entomologist Rob Anderson says he's observed "early andsignificant" egg-laying activity by Culex restuans, a mosquito that feeds entirelyon birds and is known to transmit West Nile virus among feathered creatures.

Anderson says thisis important because West Nile virus must be present in the bird population before another mosquito species, Culex tarsalis, can transmit the infectious disease to people.

The presence of a larger-than-usual number of Culex restuans egg rafts this early in the season could spell trouble if the weather remains warm and relatively dry until Culex tarsaliswhich feeds on both birds and peoplebegins to develop later.

"This is not a predictor of a West Nile outbreak. It's only one precondition for an outbreak that would also depend upon what happens with the weather over the rest of the summer," said Anderson, an associate professor of biology at the University of Winnipeg.

"The early start in the egg laying, coupled with a really healthy overwintering (adults) population is a precondition. The other factors are sustained heat and enough water to keep a decent amount of habitat available."

West Nile outbreaks can occur without torrential rains, as birds and mosquitoes seek out whatever wet habitat remains, Anderson said.

Culex tarsalismosquitoescan tolerate relatively dry summers, said former City of Winnipeg entomologistTaz Stuart, the director of technical operations for exterminatorPoulin's.

"That's the fun part about this mosquito. It likes hot and dry conditions. The smaller water bodies that have a high organic matter are more conducive to producingthem," Stuart said.

The City of Winnipeg`s insect-control branch began larviciding to control bothCulex restuans andtarsalisearlier this spring, city spokeswoman Lisa Fraser said. The city also has plans to control adult Culexmosquitoes if the province deems it necessary to combat a West Nile virus outbreak

Winnipegdoes not,however, monitor the mosquitoes known to transmit West Nile virus. That task falls to the province, whichbegan collecting adult Culex tarsalismosquitoes in traps set at 29 communitiesthis week, saidDr. Richard Baydack, director of thecommunicable disease control unit within the province`spublic health branch.

So far, Culexnumbers are low."What we've seen to date this year isn't different,`` Baydack said.

Four people and one horse tested positive for West Nile virus in Manitoba in 2015, according to the province`s weeklysurveillance reports.