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New West Nile virus strain may worsen epidemic 7-4-08
WASHINGTON (Reuters) - A new strain of West Nile virus is spreading better
and earlier across the United States, and may thrive in hot American summers,
researchers said on Thursday.
The virus infected an estimated 175,000 people last year, the U.S. Centers for
Disease Control and Prevention reported in its weekly report on death and
disease on Thursday.
The mosquito-borne virus caused an estimated 35,000 cases of fever, was reported
to have killed 117 people and caused serious disease such as encephalitis and
meningitis in 1,227 people in 2007, the CDC reported.
A second team of researchers said a new strain of the virus that has completely
overtaken the original strain is particularly well suited to hotter weather --
which in turn means West Nile outbreaks may worsen in the north.
It also means that North America may suffer more from West Nile virus than other
parts of the world, said Lyle Petersen, who helps lead West Nile surveillance at
the CDC.
West Nile was introduced to the United States in 1999 -- during a particularly
hot summer in New York City.
"In Europe, Africa and West Asia, where the virus was previously endemic, you'd
see these big outbreaks and then they'd kind of disappear and then not come back
for years on end," Petersen said in a telephone interview.
"What we have seen in the United States, we've had repeated outbreaks every
single year since 2002 -- in fact, big outbreaks. This is an unusual pattern
that not been seen before." Hot American summers may be to blame, said Petersen
and Marm Kilpatrick of the Consortium for Conservation Medicine and the
University of California Santa Cruz.
FASTER AND WARMER
Writing in the Public Library of Science journal PLoS Pathogens earlier this
week, Kilpatrick and colleagues said they showed the new strain, first seen in
2002, replicates faster in the bodies of mosquitoes when it is warm.
"The warmer the temperature, the faster it replicates in mosquitoes and the
faster the mosquito can transmit the virus," Kilpatrick said in a telephone
interview.
"It also indicates that increases in temperatures due to global climate change
would have major effects on transmission of the virus."
West Nile virus infects birds, and it can spread to people via mosquitoes that
bite both.
Petersen and Kilpatrick said it is known that mosquitoes transmit all sorts of
diseases more efficiently when it is warm.
Kilpatrick and colleagues, working in a lab, showed the new, 2002 strain of West
Nile, does particularly well in warmer temperatures.
The new strain appears to have evolved naturally, said Petersen. "We can no
longer find the 1999 strain. It's pretty dramatic," he said.
The Kilpatrick findings fit in with what the CDC has seen, Petersen added.
"What we observed is, at least in temperate climates, these big West Nile Virus
outbreaks tend to occur in heat waves," Petersen added.
Petersen said it is too soon to show any links between climate change and West
Nile Virus, however.
If it gets too warm, mosquitoes die sooner, before they can spread the
infection, Kilpatrick said. So in southern states the new strain may not have an
advantage. But in the northern states and Canada, hot summers could make a big
difference, he said.
"It is probably going to push the northern boundary farther north," Kilpatrick
said.
Comment:
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