- An international team of scientists analyzing mosquitoes
from five continents has published a study that appears to solve the mystery
of why West Nile virus spread so rapidly in North America but not in Europe.
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- The mosquitoes that transmit the potentially fatal virus
in North America are hybrids of two strains of Culex pipiens that like
to feast on the blood of both birds and humans, thus facilitating the spread
of the disease from one species to another, according to the study, Why
West Nile is West of the Atlantic, published today in the journal Science.
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- In Europe, the Culex pipiens species normally bites either
birds or mammals, but not both, which explains why the West Nile outbreaks
there have been relatively small.
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- "It was an exciting discovery," said Dina Fonseca,
of the Smithsonian Institution's Museum of Natural History in Washington,
who led the study.
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- "There are an enormous number of hybrid mosquitoes
in the U.S. and we think they are the main vectors of the disease here."
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- West Nile virus, which attacks birds and is transmitted
to humans by mosquitoes that eat the blood of humans and birds, surfaced
in New York City in 1999.
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- Ms. Fonseca and several scientists in Europe and Japan
examined mosquito samples from 33 sites in Germany, Britain, Australia,
Italy, Japan, Jordan and the United States.
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- In the United States, more than 40 per cent of the Culex
pipiens mosquitoes were hybrids. In Southern Europe, no more than 10 per
cent of the mosquitoes were hybrids, while almost none from Northern Europe
were.
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- Andrew Spielman, entomologist at the Harvard School of
Public Health in Boston, does not agree with the conclusions of the study.
He believes West Nile virus spread so rapidly in North America because
it is relatively new on the continent and birds have not built up an immunity
to it.
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- "They have never seen the virus before and that's
why the intensity of transmission is so great now in U.S. and in Canada,"
he said.
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- Mark Loeb, a Canadian West Nile researcher, said the
study does not explain where the next outbreak of the disease will occur,
or why it shifted from the east to the western U.S. states and prairie
provinces.
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- "As well, the disease on the prairies this summer
was transmitted by another kind of mosquito, the Culex tarsalis,"
said the associate professor at McMaster University
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- West Nile in the United States has been isolated in 38
different mosquito species.
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