- TORONTO (CP) -- In a finding
that challenges dogma about West Nile transmission, researchers have shown
that infected mosquitoes can pass the virus to their non-infected, blood-sucking
siblings as they feed on the same animal.
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- Scientists know that female mosquitoes become infected
with West Nile while feeding on birds with high levels of the virus in
their blood. The birds get West Nile after being bitten by infected mosquitoes,
but it takes several days for the virus to build up in the blood.
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- Until now, most animals were considered dead-end hosts
that did not pass along West Nile to new swarms of mosquitoes.
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- But researchers at the University of Texas Medical Branch
found that when infected and non-infected mosquitoes fed simultaneously
on a healthy mouse, the virus-free insects picked up West Nile within an
hour " even though the blood of the lab animal showed no evidence
of the virus.
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- "We were surprised. We didn't expect to see any
of these recipient mosquitoes infected," principal investigator Stephen
Higgs said Monday from Galveston.
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- "We don't know how it's getting from one mosquito
to another."
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- In five separate experiments, the scientists exposed
anesthetized lab mice to scores of infected (donor) mosquitoes, then to
about an equal number of non-infected (recipient) mosquitoes. Between 2
and 6 per cent of the recipient insects which fed on the animals' blood
were found to be infected after only an hour.
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- Dr. Higgs, whose study is published in this week's Proceedings
of the National Academy of Sciences, called the findings "scary."
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- "Animals that we have never considered as relevant
in West Nile transmission may now be implicated," he said. "We
know that thousands of horses are being bitten by infected mosquitoes because
thousands have died.
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- "But nobody has considered that they have actually
been a source of infecting more mosquitoes."
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- Infected mosquitoes can also pass West Nile to humans.
While most people have virtually no symptoms, about one in 150 will develop
severe illness and may even die.
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- While people can take precautions to avoid being bitten,
animals have no such defences and can be attacked by hundreds of the pesky
bugs per hour, said Dr. Higgs. That means not just birds are possible hosts
for the disease, but all kinds of animals - everything from elk and moose
to raccoons and field mice to cats and dogs.
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- "What this new method of transmission does is it
accelerates the time, because you don't need that period of the animal
becoming sick or viremic (having high blood virus levels)," he said.
"Mosquitoes can be basically infected instantaneously."
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- The team plans to study other mosquito-borne diseases,
such as dengue fever, which strikes up to 100 million people around the
world each year and kills thousands.
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- "Maybe if this is happening with West Nile, maybe
it's happening with other viruses," Dr. Higgs said. "We don't
know."
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