* So far this year there are 1,590 cases, 66 deaths
* Virus has reached every state except Alaska, Hawaii
* Climate factors boost virus and mosquito numbers
(Adds latest figures from Texas, death in Ohio)
By Sharon Begley
NEW YORK, Aug 29 A total of 1,590 cases of West
Nile virus, including 66 deaths, were reported through late
August this year in the United States, the highest human toll by
that point in the calendar since the mosquito-borne disease was
first detected in the country in 1999, health officials said on
The toll is increasing quickly. "We think the numbers will
continue to rise," said Dr. Lyle Petersen, director of the U.S.
Centers for Disease Control and Prevention's Division of
Vector-Borne Infectious Diseases.
Through last week, 1,118 cases and 41 deaths had been
reported. The updated figures represent a 40 percent increase in
the number of cases and a 61 percent spike in the number of
deaths, but are short of the all-time record for a full year:
9,862 cases and 264 deaths in 2003.
In hard-hit Texas, the number of confirmed cases soared to
894, with 34 people dead, this year as of Wednesday, according
to the Texas Department of State Health Services. Over half of
the deaths occurred in the north of the state.
"It looks like it is going to be our worst year ever," said
Dr. David Lakey, commissioner of the Texas Department of State
Health Services. "As I look at the data, I'm not convinced we
All 48 contiguous states have reported cases of West Nile
virus in birds, which act as hosts; in mosquitoes, which
transmit it by biting birds and then mammals including humans,
or in people. Only Alaska and Hawaii have been spared. And 43
states have at least one human case.
The Ohio Department of Health reported on Wednesday that a
76-year-old man had died from the virus, the state's first
fatality this year.
"The virus is endemic at this point throughout the United
States," with the possible exception of high-altitude regions
such as the Rocky Mountains, said the CDC's Petersen. "There is
a risk almost everywhere."
So far, however, more than 70 percent of the human cases
have been reported in just six states: Texas, South Dakota,
Mississippi, Oklahoma, Louisiana, and Michigan.
Only 2 percent to 3 percent of cases of West Nile fever are
reported to health officials, said Petersen, which suggests that
the actual number of cases is 30 to 50 times higher than
That is partly because an estimated 80 percent of infected
people have no symptoms, said Dr. Robert Haley, of the
University of Texas Southwestern Medical Center in Dallas, in an
essay last week in the Journal of the American Medical
About 20 percent of infections cause only mild symptoms,
including aches and fever, explained Haley. One in 150 people
infected with the virus develop neurological conditions such as
meningitis, encephalitis and other illnesses that can cause
disorientation, cognitive impairment, muscle weakness, and
movement problems that resemble those of Parkinson's disease.
Of those who develop this "neuroinvasive" form of West Nile,
an estimated 4 percent to 18 percent die, said Haley, mostly
those who are older or suffering from other illnesses.
THE HOTTER THE WEATHER, THE FASTER THE VIRUS
There is no treatment for West Nile infection, and no
vaccine. The disease is transmitted by Culex pipiens mosquitoes,
also known as common house mosquitoes, and the only preventive
measure is to avoid being bitten.
People can reduce their risk by eliminating the small pools
of standing water - in bird baths, outdoor flower pots and the
like - where C. pipiens breed.
Public health experts and entomologists are baffled about
why 2012 is such a big year for West Nile. But Petersen said a
U.S. heat wave has been an important contributing factor.
"Higher temperatures foster faster reproduction of both the
mosquito and the virus," said Tony Goldberg, professor of
epidemiology at the University of Wisconsin, Madison, who has
studied urban outbreaks of West Nile since shortly after the
virus arrived in the Midwest.
"The hotter it is, the faster the virus can replicate," said
entomologist Gabe Hamer of Texas A&M University.
As the virus reproduces more quickly inside its bird host --
it likes species from robins and blue jays to sparrows -- there
is a greater chance that a mosquito biting the bird will pick up
a large number of viruses and transmit them to its next victim.
Higher temperatures also increase mosquito activity, making
them more likely to be flying around, especially at night, and
looking for a meal.
Another factor contributing to this year's outbreak is the
continuing cycle of droughts and downpours, a precipitation
pattern predicted by models of climate change.
Intense rain fills drainage ditches, storm sewers and
culverts, and washes grass clippings, leaves and other organic
matter into those pools of standing water, explained Wisconsin's
Goldberg. Mosquitoes prefer to breed in water that has rotting
With normal rainfall, those breeding pools are washed away
in the next storm. But when heavy spring rains are followed by
summer dry spells, as has been the case in much of the United
States this year, the breeding pools remain for weeks or months,
said Goldberg, and the mosquito population explodes.
"As we keep getting more climate extremes," he said, "there
will be more years with many more cases of West Nile."
(Additional reporting by Marice Richter in Texas, editing by
Michele Gershberg and Vicki Allen)