Ahearn and his colleagues run a computer project for the City of New York that uses a sophisticated formula to chart clusters of dead birds across the city. Still in its testing phase, it is helping the city to link non-random dead bird clusters to pools of mosquitoes that show signs of the West Nile virus and then to possibly predict where human infection has a high probability of occurring.
"The next phase of the project will be to factor in the ecological context, things like air quality and weather," Ahearn said. "I think this is the only logical way to go."
Virus Takes the Blame
Stephen Guptil of the geographic sciences branch of the U.S. Geological Survey agrees. He too says that his unit may consider mapping an air quality layer in their tracking of West Nile events.
"We may find via this tracking that higher ozone levels lead to greater susceptibility to replicate more virus," Guptil said.
Maybe so, but is there more to this than meets the eye? Guptil is assuming that only the virus (whatever the amount of viral replication) leads to harm, either in crows or humans.
That assumption may turn out to be premature.
In a paper published in the journal Science on Dec. 17, 1999, the authors claimed they had isolated the West Nile virus. It's since been widely accepted (as Guptil does) that whenever the "virus" is injected into, say, a crow, and the crow dies, it means that here is proof positive that the cause of West Nile disease is solely the virus.
That conclusion suggests to many scientists that you might as well forget about any environmental factors being necessary in causing West Nile-associated illness. Some scientists, including Lipkin and McLean, allow for the possibility that environmental factors might enhance the opportunities for infection.
Either way, the virus remains the culprit.
Filtering Out the Virus
John F. Anderson, the director of the Connecticut Agricultural Experiment station in New Haven and the lead author of the study, explained that his group ground up mosquitoes and the brains of dead crows and after several steps eventually filtered out the virus. He used a filter of 0.22 micrometers because he wanted to "exclude bacteria and fungi" from the culture.
The purpose of isolating a virus is to ensure that viral particles are pure and therefore separated from everything else, including possibly harmful materials from the very cells they infect. That way you are reassured that further tests may tap the virus as the likely cause of an illness.
But was a filter of 0.22 micrometers small enough, given that a West Nile viral particle is said to be 0.04 micrometers, about six times smaller than the filter? In other words, there is enough room for cellular matter to move through the pores along with the West Nile viral particle. That cellular matter could incorporate the biochemical products of other toxic insults, including those from the environment, to the organism.
"Yes, small molecules [from tissue] can pass through," Anderson admitted, but didn't think this might be a problem of any kind.
"We don't have a purified form of the virus," said Robert McLean matter-of-factly. "Cellular material could interact with the virus to enhance the replication of viral particles."
What McLean left unsaid was the possibility that the cellular material itself could be sufficiently toxic to cause damage to tissue, with or without the virus.
Clearing the Air