Jan. 17, 2004. 01:00 AM
Fewer bugs, less pesticide
Computer model used to track West Nile spread

Model would cut use of mosquito insecticides as well


OTTAWA—Canadian researchers have opened the door to avoiding future outbreaks of West Nile virus while also dumping less chemical poison into the environment.

A University of Alberta team used a unique combination of skills to devise a computer model that calculates how much to reduce the mosquito population to stop the virus spreading.

Once the model is fully developed, local health officials can use dead bird counts to pick the best time to start killing mosquitoes in the late spring. They'll also have a more accurate idea of how much poison is needed.

"This should allow for much more judicious use of insecticides and larvicides," says biologist and lead researcher Marjorie Wonham, a post-doctoral researcher at the university in Edmonton.

The mathematical model, based on New York City's experience with West Nile virus in 2000, is now being extended by Wonham and colleagues to cover different kinds of mosquitoes, infection rates, bird populations and climate conditions across North America.

Some of that work should be ready by this year's mosquito season but other aspects could take several years to complete, Wonham says.

The research, published this week by the U.K. Royal Society, was carried out in the Centre for Mathematical Biology at the University of Alberta. The only one of its kind in Canada, the new centre focuses on investigations that combine mathematics and biology.

The University of Alberta model allows health officials to calculate how dense the mosquito population has to be in a locale before there is a real threat of an outbreak where people would be infected.

The key factor is the number of female mosquitoes — who take a blood meal by biting — in relation to the number of crows and other birds that are a crucial intermediate stage in the spread of the disease.

Cut that ratio by getting rid of mosquitoes and West Nile isn't going to spread, Wonham says.

But killing crows only makes matters worse, the researcher says, because it ups the ratio and increases the chances of the remaining birds becoming infected and passing the virus along to other species of mosquitoes that feed on both birds and humans.

`This should allow for much more judicious use of insecticides and larvicides'

Marjorie Wonham, biologist

"It's mosquito control, not bird control that's effective," she says.

The research also found that controlling West Nile should be easier in regions where the mosquito season is short, even if there are thick swarms in mid-summer.

"It's the season-long average number of mosquitoes that matters, not the spikes in population over a few weeks," Wonham says.

The model showed that reducing New York's population of Culex pipens mosquitoes to fewer than five per crow would have avoided the West Nile outbreak in 2000. That amounted to a drop of between 40 and 70 per cent, depending on estimates of the initial mosquito population.

The research team adapted an epidemiological model initially developed for malaria. But the key numbers used in the West Nile model had to be "cobbled together" from various published sources because research into the spread of the virus in North America is so new.

Wonham says a crucial question is whether birds like crows and house sparrows here that are now susceptible to the virus develop immunity as European birds apparently have. Also unresolved is whether other mammals that become infected with West Nile — like horses — are really dead-end hosts as suspected or could also "amplify" the virus as birds do.

Harvey Artsob, a top federal West Nile expert, says the prospect of wider application makes the research very exciting.

"Lots of public health officials would love to have trigger points that would tell them when to start control mechanisms," says Artsob, who works at the National Microbiology Laboratory in Winnipeg.

More than 1,300 people in Canada came down with the debilitating West Nile virus last year after being bitten by mosquitoes carrying the infection, three times the number in 2002. But deaths last year dropped to 10 from 20 the year previous.

Ontario saw 89 cases of West Nile last year with two deaths compared with more than 400 cases and 18 deaths in 2002.

Large-scale killing of mosquito larvae, plus better public awareness and cool wet spring weather, are credited for the drop in Ontario's cases.

Other members of the University of Alberta research team were Mark Lewis, holder of the Canada Research Chair in mathematical biology, and Tomas de-Camino-Beck, who developed the computer program. The work was funded by the Natural Sciences and Engineering Research Council and the Killam Trust.

Additional articles by Peter Calamai

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