Dr. Marjorie Wonham and her research team from the Centre for Mathematical Biology at the University of Alberta, created a simple mathematical model using the dead bird counts collected in New York in 2000. Her research is published in the current issue of the Royal Society of London's journal Proceedings B. Tomas de-Camino Beck and Mark Lewis are co-authors on the paper.
West Nile virus is an emerging infectious disease in North America that spreads primarily through contact between birds and mosquitoes. It can be lethal to birds, horses and humans. One of the key findings from Wonham's work is that chance of a virus outbreak is decreased by removing mosquitoes but is actually increased by removing birds. The model provides a new analytical method for determining necessary mosquito control levels.
"This virus is endemic and we're probably never going to get rid of it completely unless we say that we're going to kill all the mosquitoes in the world--that's not going to happen," said Wonham. "What this work does is tell you just what percentage of mosquitoes is necessary to kill to keep the virus below an outbreak. This is a first step towards effective management."
The research group set up parameter values from published scientific papers on mosquito biology, crow biology and West Nile biology. In order for a specific region to use this model, officials could tailor such parameters as mosquito life span, biting rate and crow life span to the data in their area. For example, since summer in Edmonton is short and dry, the mosquito lifecycle might be quite different than in the longer, more humid summer of New York.
Currently mosquitoes are killed through the application of chemical larvicides to the water, filling in the wetlands to remove the habitat and as a last resort, spraying chemicals to kill the adult species.
"Since applying chemicals and filling in wetlands costs money and causes environmental damage, one would ideally use the minimum amount of control that would still be effective in preventing outbreak," said Wonham. "Our model lets you calculate the threshold mosquito population for West Nile outbreak--you just have to keep them below the threshold level. This means, we would hope, minimal economic cost and environmental damage while still preventing outbreak."
Mathematical models have been used to manage diseases such as malaria but this is the first one to deal with West Nile.