Can GM mosquitoes wipe out dengue?

NEW DELHI - Unable to prevent malaria or dengue with current methods, scientists are now turning to genetically modified (GM) mosquitoes to tackle diseases - amid notes of caution that it could lead to unforeseen problems.

The idea behind genetically modified mosquitoes is to release them into the wild to mate with females and produce offspring with shorter lifespans to curb the mosquito population.

Malaysia could be the first country in Asia to use these GM mosquitoes to battle dengue.

It plans to release between 2,000 and 3,000 GM mosquitoes in some areas, said Lim Chua Leng, a Malaysian health ministry official.

The idea of using GM mosquitoes to help tackle both dengue and malaria has been around for a while.

However, Gurmit Singh, head of Malaysia’s Centre for Environment, Technology and Development, said: “There are a lot of risks involved.” Once the mosquitoes are released “you have no control and it can create more problems than solving them”, he added.

Research by Johns Hopkins scientists, reported by the journal Proceedings of the National Academy of Sciences, has shown that GM mosquitoes are fit enough to survive in the wild.

They placed 1,200 GM mosquitoes and 1,200 “wild” mosquitoes in a cage with malaria-infected mice. After nine egg-laying cycles, out of every 100, 70 GM mosquitoes survived as against 30 of the wild mosquitoes.

Mauro Marrelli and colleagues from Johns Hopkins University in the US have found that GM mosquitoes can out-breed their wilder cousins.

Meanwhile, researchers at Oxitec, a spin-off company from Oxford University, are using GM mosquitoes to reduce Aedes aegypti mosquito populations, which spreads all four dengue viruses.

The method targets dengue fever that infects up to 100 million people worldwide every year, including India, Southeast Asia, China and Africa, with no known treatment or vaccine.

The Oxitec team uses a system known as RIDL (release of insects carrying a dominant lethal) to genetically sterilise mosquitoes. Such GM mosquitoes released into the wild can’t produce viable offspring.

“You release sterile males, they mate with wild females and the progeny die before they can bite and transmit disease,” says Luke Alphey, Oxitec’s research director.

“If you release them for long enough, the wild population will decline and collapse,” adds Alphey.

The first strategy was tested successfully in Malaysia during 2007-2008. The second strategy, developed by a group funded by the Gates Foundation, is being tested in outdoor cages in Mexico.

Oxitec is concentrating on this kind of “population suppression” strategy rather than disease resistance because it should have an impact sooner, says Alphey.

Dengue is an infectious disease that arises during and after monsoons in tropical and subtropical areas.

Symptoms are high fever, severe headache, ache behind the eye, severe joint ache, severe muscle ache, nausea and rashes. They usually start within five to six days after a person is bitten by an infected mosquito.

A person can be infected by at least two, if not all four types of dengue virus, at different times during a life span, but only once by the same type. Infection with one dengue virus does not protect against infection with another.

When mosquitoes bite infected humans, they become infected and later transmit it to other people they bite.

In case of malaria, mosquitoes are merely carriers for a parasite called Plasmodium, which actually causes the infection.

GM mosquitoes carry an altered gene that produces chemicals to disrupt Plasmodium’s development so that they are not able to infect humans.

Parthasarthi Bose, a Kolkata-based consultant and chest specialist, who was formerly with Escorts Heart Institute, New Delhi, and Apollo Hospitals, Kolkata, however, cautioned: “This could lead to development of strains of malarial parasite or dengue virus which might adapt themselves to the shorter life span of the mosquitoes and may result in the development of drug resistant mutants.”

“There’s no way to know what the long-term ecological effects might be. Other animal populations could be affected,” he added.

K.N. Prasad, microbiologist at the Sanjay Gandhi Post-Graduate Medical Institute in Lucknow, says unlike the malaria parasite, the dengue virus just requires a drop of water to grow and multiply.

“The genetically modified mosquitoes or their offspring won’t be carrying those kind of enzymes that allow the dengue virus to flourish. But on the flip side, they could become resistant to insecticides.”

(Shudip Talukdar can be contacted at shudip.t@ians.in)