Controlling the spread of mosquitoes
using bacteria that halve the insects' lifespan
could virtually eliminate the transmission of
dengue fever, which kills around 12,500 people
a year. Traditional methods for controlling
the spread of mosqui-to-borne disease, such
as using bed nets and draining wetlands, are
ineffective for the Aedes aegytpi mosquitoes
that spread dengue fever virus because they
bite during the day and thrive in urban areas.
Scott O'Neill, a geneticist
at the University of Queensland in Brisbane,
Australia, and his team has now developed a
way to kill the mosquitoes before the dengue
virus is mature enough to infect people if they
are bitten. Dengue fever takes approximately
eight to 10 days to incubate in mosquitoes,
and therefore tends to be spread by older insects.
Adaptation
The team used a strain of the
insect-infecting Wolbachia pipientis bacterium,
which usually infects fruit flies and causes
them to die early. By adapting the bacterium
to infect A.aegtypi, the team hoped to cut the
mosquito's lifespan.
After unsuccessful attempts
to infect the dengue fever mosquitoes with the
naturally occurring form of W. pipientis, the
team grew the bacteria in a culture with the
mosquito cells.
Over a period of three years,
some of the bacteria adapted so that they could
successfully infect female mosquitoes. The scientists
found that the lifespan of the infected mosquitoes
was around 30 days, roughly half the expected
60-day survival rate of laboratory-reared mosquitoes.
The team bred the infected
females to produce whole populations of infected
mosquitoes, which also lived for only 30 days.
"We were able to show that when mosquitoes
carry these bacteria, their adult lifespan is
roughly halved," says O'Neill. The team's
findings are published in Science.
Field
studies
The researchers have now begun
field studies with caged mosquitoes to see if
they get similar results outside the lab. Dengue
viruses can mature at different rates, making
it difficult to predict the absolute age at
which the mosquitoes need to be killed.
But O'Neill says that if the
adapted Wolbachia also halve the average age
of mosquitoes in the wild, transmission of the
virus could be reduced to nearly zero. If the
results of the cage studies are positive, O'Neill
says the next step would be to try the same
technique using wild, free-roaming mosquitoes.
O'Neill says an advantage of his 'biopesticide'
approach is that it is more likely to get regulatory
approval than other ongoing attempts to control
the spread of dengue fever by genetically modifying
the disease-carrying mosquitoes.
That transgenic method is being
researched by geneticist Luke Alphey at the
University of Oxford, U.K., who founded a company
called Oxitec to genetically modify the dengue
fever mosquito. They modified mosquito larvae
so that they need to be fed the drug tetracycline
to survive. When theses lab-reared mosquitoes
are released into the wild, they breed as normal
but their offspring die because they don't have
access to the drug, thereby reducing the total
wild population size.
