The Gateway | Thursday, 29 January, 2004

New model helps swat West Nile virus

Kevin Ma

Jeffrey Greeniaus

U of A researchers are exploring ways to prevent West Nile virus outbreaks.

Using biology, mathematics, and computers, three U of A researchers have found a way to swat outbreaks of the mosquito-borne West Nile virus before they take off.

Scientists first discovered West Nile virus (WNv) in Uganda in 1937, and for many years it remained isolated to the African region. In 1999, an outbreak of the virus occurred in New York. From there, migrating birds carried it north to southern Ontario by summer 2002, and then to Alberta by July 2003. Although it kills under 0.1 per cent of humans that it infects, the virus can cause fever, meningitis, and encephalitis.

Dr Mark Lewis, Dr Marjorie Wonham and Tomas de-Camino-Beck, all from the U of A’s Centre for Mathematical Biology, hope their new mathematical model will help North Americans prevent an outbreak of WNv here.

The researchers wanted to know what conditions produced the New York City outbreak. An outbreak, explained Lewis, occurs whenever a given instance of a disease produces one or more subsequent infections in other creatures.

Scientists knew the WNv spread from birds to horses and humans via mosquitoes, but didn’t know how many mosquitoes or birds had to be eliminated to stop it.

The critical factor turned out to be the ratio of birds to mosquitoes. Mosquitoes transmit the virus, said Wonham, while birds merely contain it.

Keep the mosquito population below a certain outbreak threshold, and the virus will stay under control, she explained.

“But there’s an open question about birds, whether removing birds would do the same thing,” she explained.

To their surprise, the researchers found decreasing the number of birds in their simulated experiment actually increased the chance of a West Nile outbreak.

“By killing birds, the ones that are left are bitten even more,” thus causing the virus to spread faster, explained Lewis.

What do these results mean? According to Lewis, it means cities don’t have to use massive amounts of expensive, environmentally hazardous pesticides.

“You don’t have to kill all the mosquitoes to get rid of the disease. You just have to bring them down to a threshold level, after which the disease will die out on its own. You don’t have to spray everywhere and get rid of all the wetlands.”

The computer model, coded by de-Camino-Beck, can help any region calculate their unique outbreak threshold. However, Lewis cautions that even if mosquitoes are kept below this outbreak threshold, it will still take many years for the virus to die out. In other words, the model can prevent, but not stop, an outbreak.

Nor have they finished working on it, he added. The team wants to add a spatial control to the model.

“You can’t control [mosquitoes] everywhere,” said Lewis.

“We’re trying to calculate the size of a buffer zone around a city that would be necessary to prevent the disease from coming in from the outside.”

Expanding it to include multiple species of bird and mosquito (instead of the one apiece used now) and long-term climatic and geographic data would also make the model more effective.

Biologists have used models like this one for years.

“It turns out that people have been using mathematical models for diseases for almost a century now,” said Lewis, noting how one such model was used to study malaria in the 1930s. “In the field of epidemiology, these models have worked extremely well.”

As models become more and more complex, interdisciplinary projects like the Centre for Mathematical Biology have become more and more common.

“I’m a mathematician,” said Lewis.

“Marjorie’s a biologist, and Tomas is a computer scientist. Together we’ve created something that none of us could have done individually.”

“Biology by itself has its limits,” Wonham agreed.

“And math by itself has its limits. But when you put both of them together, you can constantly go back and forth and get a fruitful cross-pollination of knowledge.”

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