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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.
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