- It's not an aggressive mosquito. It doesn't like to go
far for a "blood meal." In fact, the mosquito most responsible
for spreading the West Nile virus would prefer to snack on birds rather
than humans.
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- And that's the problem.
"The female mosquito feeds on a bird and gets infected with West Nile,"
says Scott Sawlis, entomologist for Dallas County. Then, "the virus
moves back into her salivary gland, and when she bites a human later, she
injects the virus into the person."
"It's a bird virus that really is not intended to get into people,"
he says. But the main transmitter of West Nile, the ordinary Southern house
mosquito, provides a short path between birds and people.
Only a small percentage of these mosquitoes are thought to be infected
with the virus, but they often live very close to humans.
- "The message we want to give to people is that you
don't want these infected mosquitoes coming into your yard or into your
neighbor's yard to lay their eggs," says Jonathan Day, a professor
of medical entomology at the University of Florida. He and the staff at
the Florida Medical Entomology Laboratory in Vero Beach have studied the
transmission routes of West Nile since it first appeared in the United
States in 1999.
"The best way to avoid infection is to avoid exposure," he says.
The female Southern house mosquito, known also by the Latin name, Culex
pipiens quinquefasciatus, lives only a month or less. But her short life
is a frenzy of reproduction, a continuous cycle of laying a batch of eggs
that can number in the hundreds every three to four days. All a female
mosquito needs, after she has mated once with a male, is a fresh supply
of blood animal or human to provide enough protein to make the eggs
and a small amount of stagnant water in which to lay them.
- "These mosquitoes love standing, fermented water.
It can't get stinky enough for them to breed in," says Jim Olson,
professor and medical entomologist at Texas A&M University. "Usually,
they'll go to a septic system in the country or a rural feedlot that is
contaminated with sewage. But they like urban settings as well."
Thus, the Southern house mosquito lives up to its name by its willingness
to reside in an urban back yard and take every opportunity to slip into
the house in search of a meal.
"We're getting the sense that most of the West Nile infections that
occur in people happen inside the house or near the house, where people
sit around on the patio," says Dr. Day of the more than 150 confirmed
human cases in the three years the virus has been in the United States.
Eighteen people have died.
West Nile appeared in Texas in late June, when dead birds and later mosquitoes
and horses tested positive for the virus. So far, there have been no confirmed
cases of the virus among Texans, although state officials were awaiting
tests on eight residents of southeastern part of the state.
Human cases of the virus typically show up in July through September, says
Dr. Day. "If you start seeing a lot of horse cases in an area, probably
the human cases aren't far behind."
The tenacity of the mosquito spreading the virus should not be underestimated,
Dr. Olson warns. "Don't let these mosquitoes inside the house to sleep
with you. I'm urging people to get their darned houses mosquito proofed."
Mr. Sawlis and his colleagues say they have become experts at routing mosquitoes
out of their backyard habitats. They often turn phone complaints from citizens
angry about mosquito bites into an inspection that points the blame back
to the citizen.
"In about 25 percent of our service requests, we find mosquito sources
on the property," he says. "If you've got flowerpots, tires and
containers all over the place that hold rain and leaves, you've made a
nice organic soup for mosquito breeding.
"And all this stuff is sitting right outside your door, just waiting
for you to step outside so the mosquitoes can have a nice blood meal and
complete their life cycle," Mr. Sawlis says. "They don't have
to fly more than 10 feet" to meet all their needs.
Inspectors for the city and county frequently are able to point out tiny
black specks in standing water, which actually are "rafts" of
hundreds of mosquito eggs. At that stage, each raft is no bigger than a
grain of rice.
"We can spray chemicals in the streets 24-7, and it will not abate
the mosquito problem," says a frustrated Mr. Sawlis. "It'll take
citizen involvement to get rid of them."
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- http://www.dallasnews.com/health/stories/072902dnlivskeeters.d87c5.html
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