The number of West Nile virus cases reported in the United States
through early September is the highest year-to-date total since the
mosquito-borne disease was first detected in this country in 1999,
federal officials said Wednesday. The number of fatalities had jumped by
nearly a third from the previous week, they said.
The Mid-Atlantic region has not been heavily affected,
although local officials are monitoring the situation closely. So far
this year, the number of reported cases in the Washington area is about
the same as in other years or fewer.
In Maryland, 20 cases and one adult death have
been reported this year. D.C. health officials confirmed the city’s
first fatality Wednesday, a Northeast Washington resident. A Southeast
resident is hospitalized. No additional details were available. Virginia
has five confirmed cases and no fatalities.
Texas continues to
be the state hit hardest, accounting for about half of all reported U.S.
cases this year. Aerial spraying of insecticide in some areas has
reduced the population of mosquitoes that carry the virus, officials
there said. But the number of human cases is expected to rise through
October because of the lag time between infection and reporting of the
illness.
As of Tuesday, a total of 1,993 cases nationwide,
including 87 deaths, had been reported to the U.S. Centers for Disease
Control and Prevention, a 25 percent increase in the number of cases and
a 32 percent increase in deaths from the previous week.
Asked
about recent disease outbreaks, including the hantavirus outbreak traced
to Yosemite National Park, Lyle Petersen, director of the CDC division
of vector-borne infectious diseases, said pathogens are spreading faster
because people and goods are moving around the planet at record rates.
“The world is a smaller place right now,” he said.
U.S. health
officials have notified 39 countries that their citizens might be at
risk from the rodent-borne hantavirus after traveling recently to
Yosemite. Six hantavirus cases, two of which were fatal, have been
linked to the park. The CDC said that as many as 10,000 people were at
risk after staying in Yosemite’s “signature tent cabins” between June 10
and Aug. 24.
For Texas, 2012 is “the worst year ever for West
Nile virus,” the state health commissioner, David Lakey, told reporters
during a conference call. The state had 1,013 confirmed cases and 40
deaths.
More than 70 percent of the cases this year have been
reported in six states: Texas, South Dakota, Mississippi, Oklahoma,
Louisiana and Michigan.
Most people who become infected have no symptoms.
In Texas, 89 people found out they had been infected during routine
screening for blood donation, Lakey said. A spokeswoman for the American
Red Cross said volunteer blood donors are routinely screened for West
Nile virus. As of Wednesday, more than 200 would-be donors in 28 states
had tested positive this year for West Nile, and the numbers are
expected to rise, Red Cross spokeswoman Karen Stecher said. Blood
infected with the virus is destroyed or used for research, she said.
Public
health experts said it is hard to know for sure what is behind this
year’s large outbreak, or why Texas — particularly the Dallas area — has
been hit so hard. But this summer’s heat waves and record temperatures
are likely factors, the CDC’s Petersen said.
“We know that West Nile virus tends to occur when temperatures are above normal,” he said.
The
record number of U.S. cases for a full year was reported in 2003, with
9,862 cases and 264 deaths, but the most West Nile deaths were reported
in 2002, with 284.
Mosquito-borne outbreaks have always been
difficult to predict, experts said. A complex set of environmental
factors is involved in West Nile transmission, said Katherine Feldman,
Maryland’s public health veterinarian. The virus lives in the blood of
birds, and mosquitoes spread it to people and horses.
There is no vaccine for humans.
The
extent of an outbreak is influenced by the number of mosquitoes and how
infectious they are, the population of susceptible bird species,
rainfall and temperature. Bringing “all these things together in the
right combination at the right time facilitates that the virus go faster
and to a greater degree in certain areas of the country,” said Roger
Nasci, chief of arboviral diseases at the CDC.
“The hotter it is, the mosquitoes tend to be more infectious, and it also affects how long a mosquito may live,” Petersen said.
Floods wash out mosquito breeding sites. But the right amount of rain can produce ideal breeding conditions.
Robert
Haley, director of the epidemiology division at the University of Texas
Southwestern Medical Center at Dallas, said he suspects that particular
local weather conditions made Dallas ground zero for West Nile virus
this year.
A mild winter allowed more female mosquitoes to
survive. West Nile infection among birds was relatively mild last year,
meaning more birds would be susceptible this year. Haley, who lives in
North Dallas, said he suspected that something was amiss when he saw two
dead bluejays in his yard in July. Bluejays and crows are among those
species that tend to die from the virus, he said.
Dallas also had a
hot, dry summer with rain every three to four weeks that replenished
the stagnant pools in which mosquitoes breed, he said.
West Nile
disease can vary in severity. The onset of symptoms can take from a few
days to two weeks. People 50 or older have the highest risk of severe
illness.
About 80 percent of people who are infected will not
develop any illness. About 20 percent will develop West Nile fever.
Symptoms include fever, headache, tiredness and body aches.
Occasionally, there will be a skin rash and swollen lymph glands.
The
most severe type of infection causes inflammation of the brain or of
the tissues surrounding the brain and spinal cord. In those cases,
symptoms include headache, fever, stiff neck, muscle weakness and
paralysis.
Of the West Nile disease cases reported to the CDC this year, 1,069, or 54 percent, were considered severe.
Health
officials say residents should use insect repellent when outdoors,
especially at dusk and dawn, when mosquitoes are most active. Residents
should also eliminate mosquito-breeding areas by emptying birdbaths,
flowerpots, buckets and barrels where rainwater collects.
-By Lena H. Sun,
