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 The battle against the insects on the streets of Brazil is the
latest in an ancient war between humankind and the Culicidae, or
mosquito, family which the pests frequently win.
Today, mosquito invaders are turning up with increasing regularity
from Washington DC to Strasbourg, challenging the notion that the
diseases they carry will remain confined to the tropics, scientists
documenting the cases told Reuters.
Ironically, humans have rolled out the red carpet for the invaders
by transporting them around the world and providing a trash-strewn
urban landscape that suits them to perfection.
The Aedes aegypti species blamed for transmitting Zika breeds in car
tires, tin cans, dog bowls and cemetery flower vases. And its
females are great at spreading disease as they take multiple bites
to satisfy their hunger for the protein in human blood they need to
develop their eggs.
Around the world, disease-carrying mosquitoes are advancing at
speed, taking viruses such as dengue and Zika, plus a host of
lesser-known ills such as chikungunya and St. Louis encephalitis,
into new territories from Europe to the Pacific.
"The concern is that we have these species spreading everywhere.
Today the focus is on Zika but they can carry many different viruses
and pathogens," said Anna-Bella Failloux, head of the department
that tracks mosquito viruses at France's Institut Pasteur.
In 2014, there was a large outbreak of chikungunya, which causes
fever and joint pains, in the Caribbean, where it had not been seen
before, while the same virus sickened Italians in 2007 in a wake-up
call for public health officials.
Europe has seen the re-emergence of malaria in Greece for the first
time in decades and the appearance of West Nile fever in eastern
parts of the continent.
Out in the Atlantic, the Madeira archipelago reported more than
2,000 cases of dengue in 2012, in a sign of the northerly advance of
what - at least until Zika - has been the world's fastest-spreading
tropical disease.
In the past 40 years, six new invasive mosquito species have become
established in Europe, with five arriving since 1990, driven in
large part by the international trade in used vehicle tires.
Mosquitoes lay their eggs in the tires and they hatch when rain
moistens them at their destination.
North American health experts are also racing to keep up, with the
first appearance of Aedes japonicus, an invasive mosquito, in
western Canada last November and Aedes aegypti found in Washington
DC, apparently after spending the winter in sewers or Metro subway
stations.
SPREAD UNPRECEDENTED
The speed of change in mosquito-borne diseases since the late 1990s
has been unprecedented, according to Jolyon Medlock, a medical
entomologist at Public Health England, a government agency.
For many experts, the biggest potential threat is Aedes albopictus,
otherwise known as the Asian tiger mosquito, which is expanding its
range widely and is capable of spreading more than 25 viruses,
including Zika.
"There is strong evidence that Aedes albopictus is now out-competing
aegypti in some areas and becoming more dominant," said Ralph
Harbach, an entomologist at London's Natural History Museum, who has
been studying mosquitoes since 1976.
In the United States, Aedes albopictus has been found as far north
as Massachusetts and as far west as California. In Europe it has
reached Paris and Strasbourg.
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Adding to the challenge for public health authorities are the
blurred lines between diseases carried by different mosquitoes, as
shown by research in Brazil this month that another common mosquito,
Culex quinquefasciatus, may also be able to carry Zika.
Both Aedes aegypti and Culex quinquefasciatus probably first arrived
in the Americas from Africa on slave ships, scientists believe. In
the centuries since, commerce has shuttled other species around the
world, while air travel has exposed millions of people to new
diseases.
"You've got a global movement of mosquitoes and a huge increase in
human travel. Humans are moving the pathogens around and the
mosquitoes are waiting there to transmit them," said Medlock.
Human incursions into tropical forests have aggravated the problem.
Deforestation in Malaysia, for example, is blamed for a steep rise
in human cases of a type of malaria usually found in monkeys.
DON'T KILL THE GOOD GUYS
There have been some victories against mosquitoes, thanks to
insecticide-treated bed nets and vaccines against viruses like
yellow fever and Japanese encephalitis, as well as a new one for
dengue approved in December.
But mosquitoes still kill around 725,000 people a year, mostly due
to malaria, or 50 percent more than are killed by other humans,
according to the Bill & Melinda Gates Foundation.
Climate change adds a further twist. A 2 to 3 degree Celsius rise in
temperature can increase the number of people at risk of malaria by
3 to 5 percent, or more than 100 million, according to the World
Health Organization.
Hotter weather also speeds up the mosquito breeding cycle from
around two weeks at 25 degrees to 7 to 8 days at 28 degrees,
according to the Institut Pasteur's Failloux.
So is it time to wipe out mosquitoes altogether?
Aggressive action in the 1950s and 1960s, including the use of the
pesticide DDT, certainly pushed them back for a while.
Today, genetic modification, radiation and targeted bacteria are
being considered.
Trying to eliminate all mosquitoes, however, would make no sense,
since there are 3,549 species and fewer than 200 bite humans.
"It might be possible to wipe out a few species but we don't want to
wipe out the good guys because a lot of them serve as food for
frogs, fish and bats," said Harbach. "Many also visit flowers to
feed on nectar and may play a role in pollination."
Some are even our friends. Harbach has a soft spot for the
Toxorhynchites genus, which have a convenient penchant for eating
Aedes aegypti larvae.
(Editing by Giles Elgood)
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