Ominous Start for West Nile Season
West Nile Virus 2007: Fastest Start Yet for Mosquito-Borne Illness
July 27, 2007 -- The U.S. West Nile virus season is off to its fastest start
ever. By July 24, the mosquito-borne bug sickened 122 people and killed three,
the CDC says.
It's too soon to know if this means 2007 will be a record year for West Nile
virus. But in 2003 -- the worst year in the brief eight-year U.S. history of
the virus -- there were only nine official cases this early in the year. The
2003 caseload eventually reached nearly 10,000, with 264 deaths.
"It is looking sort of similar to 2003 at this point," CDC
researcher Emily Zielinski-Gutierrez, DrPH, tells WebMD. "The question is
not just how many cases, but where the focal points of activity will
West Nile virus doesn't spread evenly across the U.S. Last year's epicenter
was Boise, Idaho. This year, the virus seems to be disproportionately severe in
"The Dakotas and the central plain states have just been hit hard year
after year," Zielinski-Gutierrez says. "If you look at the impact on
these rural counties, with people getting sick and hospitalized and needing
mosquito control without a huge population base to fund it, that represents a
huge challenge to those more rural states."
West Nile virus first appeared in New York in 1999. It spread inexorably
until it finally crossed the country in 2002. From 2004 through 2006, the virus
relaxed its hold in most Southeastern states. That's probably because the virus
replicates a bit better in the mosquito species found in the Central and
Another factor is agriculture. The mosquitoes that spread West Nile in the
Central states have an affinity for irrigation water. And farm workers, who are
outside from morning until dark, are highly exposed to mosquito bites.
Four out of five people who get West Nile virus have no symptoms at all. The
other 20% get a fever that can last for only a few days, or for several weeks.
One in 150 infections, however, invades the brain or other parts of the nervous
system (neuroinvasive). The result can be death or permanent polio-like
Because so many infections have no symptoms, the CDC's official case count
is a huge underestimate. But nearly every case of West Nileencephalitis or meningitis (affecting the brain or lining around the
brain) is reported -- so by multiplying the number of these cases by
150, it's possible to get a rough estimate of the true number of
So far this year, there have been 42 cases of neuroinvasive West Nile
infection. That suggests there have already been about 6,300 total infections
with West Nile. By the end of worst year, 2003, there were 2,866 reports of
neuroinvasive disease, suggesting there were nearly 430,000 infections.