It's back -- and it can be very bad. But just how bad will the
2004 return of
West Nile virus be?
In 2002, West Nile virus caused the largest
encephalitis epidemic in history. Last years' epidemic was arguably twice
as large -- possibly because new tests made it possible to diagnose mild cases
of West Nile
The number of serious cases was about the same: 284 deaths in
2002 and 262 deaths in 2003. Thousands of people suffered dangerous brain
infections with lingering effects. Many of them -- especially those with what's
come to be called West Nile polio -- may never fully recover.
That was one of last year's surprises, says Grant L. Campbell,
MD, PhD. Campbell, based in Ft. Collins, Colo., heads the branch of the CDC
that keeps track of West Nile cases.
"Last year we saw quite a few cases of acute flaccid
paralysis -- the so-called West Nile polio," Campbell tells WebMD. "We
have more than 30 such patients here in Colorado. Usually they don't have
fever, then -- boom -- they get paralyzed in one limb or another. It often
leads to respiratory failure and death. We saw some cases as far back as 1999,
but we became more aware of it last year."
West Nile Lessons from 2003
The good news, Campbell says, is that the West Nile virus
hasn't mutated into a more dangerous form. Although there have been minor
changes in the bug's genetic makeup, the virus seen today is still the same
virus that made its 1999 American debut in New York City.
Spread by migrating birds -- and perhaps by infected mosquitoes
that find their way onto trucks and airplanes -- the virus moved south and then
relentlessly marched westward. Last year, the hardest-hit states were Colorado,
Nebraska, South Dakota, Texas, and North Dakota.
The virus did move west of the Rocky Mountains, but there were
few human infections. Oregon and Washington reported no cases at all.
California reported only three.
For reasons nobody can entirely explain, there tends to be a
lot more West Nile disease when the virus finds its feet in a new area --
usually the second year after it first appears. It never, ever, goes away. But
the number of cases tends to drop off. New York, for example, was where West
Nile virus first landed in the U.S. But last year, that state reported only 71
human infections. Colorado, on the other hand, zoomed from 14 cases in 2002 to
2,326 cases in 2003.
"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.