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

Early egg-laying by one of the mosquito species known to carry West Nile virus suggests the possibility of a summer outbreak of the disease in southern Manitoba.

Entomologist observes activity by Culex restuans, a mosquito that transmits the disease to birds

Closeup of a mosquito on skin.
The city's mosquito populations are experiencing a late-summer surge. (U.S. Centers for Disease Control)

Early egg-laying by one of the mosquito species known to carry West Nile virus suggests the possibility of a summer outbreak of the disease in southern Manitoba, entomologists say.

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

Anderson says this is 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 tarsalis — which feeds on both birds and people — begins 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 tarsalis mosquitoes can tolerate relatively dry summers, said former City of Winnipeg entomologist Taz Stuart, the director of technical operations for exterminator Poulin'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 producing them," Stuart said.

The City of Winnipeg`s insect-control branch began larviciding to control both ​Culex restuans and tarsalis earlier this spring, city spokeswoman Lisa Fraser said. The city also has plans to control adult Culex mosquitoes if the province deems it necessary to combat a West Nile virus outbreak

Winnipeg does not, however, monitor the mosquitoes known to transmit West Nile virus. That task falls to the province, which began collecting adult Culex tarsalis mosquitoes in traps set at 29 communities this week, said Dr. Richard Baydack, director of the communicable disease control unit within the province`s public health branch.

So far, Culex numbers 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 weekly surveillance reports.


Bartley Kives

Senior reporter, CBC Manitoba

Bartley Kives joined CBC Manitoba in 2016. Prior to that, he spent three years at the Winnipeg Sun and 18 at the Winnipeg Free Press, writing about politics, music, food and outdoor recreation. He's the author of the Canadian bestseller A Daytripper's Guide to Manitoba: Exploring Canada's Undiscovered Province and co-author of both Stuck in the Middle: Dissenting Views of Winnipeg and Stuck In The Middle 2: Defining Views of Manitoba.