- NEW YORK (Reuters Health)
- Genetic differences between mosquitoes may help explain why the U.S.
has experienced more severe and widespread outbreaks of the West Nile virus
(news - web sites) than Europe has, researchers report.
A genetic analysis of mosquitoes in several countries suggests that a strain
of mosquitoes believed to play a role in spreading West Nile in the U.S.
may be a hybrid of two strains. One of the strains normally bites birds,
while the other targets people.
Because the hybrid mosquitoes bite both birds and people, they may contribute
to the spread of West Nile in the U.S., according to a report in Friday's
issue of the journal Science.
Although the research deals with the genetics of mosquitoes and does not
prove a connection with West Nile disease in humans, the findings suggest
a way that changes in species can spread diseases, according to the study's
lead author.
"Worldwide traffic of species can generate new genetic combinations
that have the potential to become particularly dangerous disease vectors,"
Dr. Dina M. Fonseca of the Smithsonian Institution (news - web sites)'s
National Museum of Natural History in Washington, D.C., told Reuters Health.
Fonseca's team studied mosquitoes in the Culex pipiens complex. These insects
have been implicated in human outbreaks of West Nile virus in the U.S.,
although the virus is also carried in other species of mosquitoes.
In northern Europe, some populations of these mosquitoes live aboveground,
but others thrive underground, in places like subways and sewers. The underground
mosquitoes are thought to have developed from above-ground mosquitoes living
in the same areas, according to Fonseca.
But a genetic analysis of Culex pipiens mosquitoes suggests that underground
and above-ground mosquitoes in northern Europe are two distinct species,
Fonseca said.
"We have shown that below-ground populations are derived from northern
African specimens that must have migrated north when urban development
generated habitats they could survive in," such as sewers and subways,
Fonseca said.
Even though above- and below-ground mosquitoes in Europe live in the same
area and seem identical, genetic analysis showed that the two types of
mosquitoes were not mixing.
That does not seem to be the case in U.S. mosquitoes. Fonseca and her colleagues
found that more than 40 percent of Culex pipiens mosquitoes in the U.S.
were hybrids. The hybrids mixed genetic material that was separate in above-
and below-ground mosquitoes in northern Europe.
Northern Europe had an insignificant amount of hybrids, the researchers
found. In southern Europe, about 10 percent of mosquitoes were hybrids.
"We have yet to show a direct link between our findings and West Nile
disease," Fonseca said. But if the hybrid mosquitoes in the U.S. do
prove to be an important cause of the differences between West Nile outbreaks
in the U.S. and Europe, "then preventing their introduction into northern
Europe is paramount," Fonseca said.
But not all scientists agree on the importance of hybrid mosquitoes. "The
evidence here is anything but solid," Dr. Andrew Spielman of Harvard
School of Public Health in Boston notes in a related article.
According to Spielman, birds and people in the U.S. who have never been
exposed to West Nile virus may be more susceptible to it, unlike in Europe,
where the virus has circulated for centuries.
The West Nile virus was first identified in Uganda in 1937. The first human
cases of the disease in the U.S. occurred during the summer of 1999 in
the New York City area. Since then, cases of the disease have been reported
in many other states.
The virus spreads to people via mosquitoes that have picked it up from
infected birds. Most cases of West Nile virus infections in humans are
mild, causing flu-like symptoms that eventually go away. In rare cases,
however, the virus can cause a fatal inflammation of the brain.
Source: Science, March 5, 2004.
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