Another West Nile Virus Summer?
Eyes are turning to California as encephalitis season starts.
West Nile Lessons from 2003 continued...
"Nobody really understands the flattening out. We've seen
it with St. Louis encephalitis virus in years before," Campbell says.
"Part of it is bird immunity."
Most researchers think that people once infected with West Nile
virus get very long-lasting immunity. That happens with similar viruses, such
yellow fever virus. For every person who comes down with West Nile
symptoms, four more never have any noticeable symptoms at all. But Campbell
says that even in intensely populated areas with lots of infected birds and
mosquitoes, no more than 4% of people show signs of having been infected with
the virus. An unreleased study of people in Slidell, La., Campbell says, found
that West Nile-infected mosquitoes bit only about 2% of the population.
Hot Spots: California, Colorado
This pattern bodes ill for California. Already this year, a
dead crow and two live house finches in Southern California tested positive for
West Nile virus.
"We worry about Southern California. By that theory,
California should light up this year -- but it all depends on climate and
mosquito control and luck," Campbell says. "If California is going to
be a problem, you might see human cases very early. Cases in May or June or
early July are very unusual and could be a sign of something big. Just a lot of
dying birds and horses could suggest something big coming."
The pattern also suggests that Colorado -- last year's hardest
hit state -- may see fewer cases this year.
"The big question is, what will happen in Colorado this
summer," Campbell says. "If we continue to have a hot, dry summer, we
could have activity again this year. If you start seeing cases in early July
rather than late July in Colorado, it might portend a big epidemic."
It's too soon to tell whether this year's first probable human
case of West Nile virus -- seen in a 79-year-old man from southern Ohio with
severe viral encephalitis -- is an unusual aberration or a sign of things to
come. Ohio reported 108 cases of West Nile virus infection in 2003 -- down from
441 cases in 2002.