West Nile stoppable without mass
extermination of mosquitoes: U of A study
EDMONTON - Preventing a West Nile outbreak doesn't require
killing every mosquito, says a new study by three researchers at the
University of Alberta's centre for mathematical biology.
Using dead-bird counts and other disease data from New York
state, where the North American outbreak of the virus began in 1999,
the trio calculated that a 40- to 70-per-cent reduction in mosquito
numbers in the spring would have been enough to forestall the
continuation of the state's outbreak in 2000.
"What our model tells us is you don't need a mosquito-free city,"
said Marjorie Wonham, who with Mark Lewis and Tomas de-Camino-Beck,
wrote the paper published today in the prestigious British journal,
Proceedings of the Royal Society.
"You can figure out using the model how low a mosquito population
you need to prevent an outbreak," said Wonham, who studied biology
before learning the math used to model disease transmission.
Rather than using dead birds as a West Nile indicator, the model
uses the number of mosquitoes per crow, one of the birds
particularly susceptible to the virus.
In New York, the researchers estimated there were 7.5 to 15
mosquitoes per bird in the spring. If the ratio had been below 4.6
per bird, they suggest, the outbreak could have been prevented.
The model suggests that if you know the mosquito threshold you
need to reach, you could avoid spraying more insecticide and filling
more wetlands than necessary, Wonham said.
They also found that getting rid of birds such as crows, jays and
magpies that are among the most susceptible to the West Nile virus
increases the chance of an outbreak.
What matters is the number of mosquitoes per bird, Wonham and
"If there were fewer birds, each bird would get bitten by more
mosquitoes. Then more birds would be infected. Then in return, when
a mosquito bit a bird, it would have a higher chance of getting
infected," said Lewis, who heads the centre for mathematical
"This was an interesting finding for us because we certainly read
about ideas of controlling birds," he said.
"Our conclusion is that is counter-productive."
West Nile virus was first discovered in the West Nile area of
Uganda in 1937. In the summer of 1999, seven people in New York
state died and the virus subsequently spread rapidly across the U.S.
and most of Canada, reaching Alberta last summer.
Last year, nearly 9,000 cases of West Nile infection and 212
deaths were reported in the U.S.
Alberta had more than 272 cases while across the country Health
Canada reported 851 probable and 466 confirmed cases. Ten Canadians
died of the disease.
One thing the U of A group's model doesn't do is predict how bad
the next West Nile season will be. Nor will it be of much use to
areas outside of New York until more disease data is available.
It's really just a starting point, Wonham and Lewis said, and
will need refinement and further testing. One big assumption, they
added, is that the numbers they used in modelling the New York
outbreak, culled from several sources, were valid.
"The public health surveillance data they have there is
unparalleled," Wonham said. "They've got the number of infected
mosquitoes by species, the number of infected birds, the number of
In Edmonton, it will likely take another couple of years to get
similar data, Wonham said.
One advantage Edmonton does have is that its northern climate
makes mosquito populations a seasonal phenomenon and easier to
control than such areas as Louisiana, where mosquitoes are a