How Mutant Mosquitoes Are Fighting Dengue Fever

If mosquitoes are responsible for spreading an infectious disease, you have to find a way to control them.

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Joao Paulo Burini / Getty Images


But because the insects are such prolific breeders, killing them is a never-ending task. Unless you can use genetics to sabotage the bugs even before they’re born.
That’s what scientists in Brazil are doing to control dengue fever, which is spread by Aedes aegypti mosquitoes infected with any of four dengue viruses. Dengue fever is one of the fastest growing mosquito-borne diseases, with over 40% of the global population living in areas where the disease is endemic. The World Health Organization (WHO) estimates there are 50-100 million infections each year, occurring primarily in tropical and sub-tropical regions like India, Malaysia and Brazil.

The New York Times recently reported that government officials in India have yet to publicly acknowledge the severity of the epidemic in their country, despite the fact that 30,002 people have contracted the disease through October of this year. Within the past month, Reuters reported that 52 people in the Portuguese archipelago of Madeira have confirmed diagnoses of dengue fever and just over 400 are likely to have the disease.

(MORE: Genetically Modified Mosquitos Released In Malaysia)

Brazil, too has long experienced similar outbreaks, but the country is embracing a cutting-edge approach that uses genetically modified mosquitoes to combat the spread of the disease. In 2011, desperate health officials in Malaysia also took advantage of the approach to control burgeoning numbers of dengue cases there after researchers in the Cayman Islands reported some success with the genetically modified bugs. So in certain neighborhoods in the Bahia state , lab workers are developing and  releasing a modified version of Aedes aegypti based on research by scientists at the University of California Irvine and the London-based company Oxitec.

Lab-bred male mosquitoes are endowed with a gene that, when they mate with a female in the wild, either prevents the females from having offspring, or is passed on to the next generation of mosquitoes, which then lives a shorter than average life — hopefully short enough to prevent it from biting and infecting anyone with dengue. The Los Angeles Times reports that after weeks of trucking in millions of the genetically altered males in the Bahia neighborhood of Itaberaba, 84% of the mosquito larvae now carry the gene. The Brazilian government is allowing the project to seed five more neighborhoods before it ultimately gives the OK to release the mosquitos into the entire city of Jacobina.

In one of those neighborhoods, in which the climate is more seasonal, the researchers are looking at how releasing the altered mosquitoes during the dry season can curb the number of infected mosquitoes when the rainy season comes. Mosquitoes breed and lay their eggs in stagnant water, so rain, and the puddles it creates, are a fertile time for the insects. “If you can control the numbers in the dry season, then you don’t have an egg bank of mosquitoes when the rainy season starts,” says Oxitec CEO Hadyn Parry. “We have pretty much eliminated the [infected mosquito population] in that area.”

(MORE: Spermless Mosquitoes: A New Way to Curb Malaria?)

Researchers at Oxitec say current methods to control dengue fever–like spraying with pesticides–are largely ineffective since the mosquitoes simply build up immunity to the chemicals. Spraying with potentially toxic compounds is also challenging since Aedes aegypti lives and breeds where people do.

“In Brazil and the Cayman, dengue is very prevalent and the people have a high awareness of the disease and we have great receptivity for what we are trying to do,” says Parry. “”Whenever you get Aedes aegypti, it is just a matter of time before you get the virus.””


During the project in the Cayman Islands, Oxitec reared the mosquitos in the UK and mailed the eggs once a month to the site in the Cayman Islands. In Brazil, the state-owned company Moscamed is producing the mosquitoes in collaboration with Oxitec and researchers from the University of Sao Paulo.

In the U.S., where mosquitoes are common but dengue is not, it’s a little harder to convince health officials to give the modified bugs a try. According to the Centers for Disease Control, almost all of the U.S. cases were acquired outside of the country. “When there is not a perceived threat, people say well, we didn’t have it last year,” says Parry. Then there is the persistent mistrust of genetic modification, and the fear that changing DNA, even in an insect, can result in a superbug that can spawn unanticipated environmental and health problems. So far, the experiments with the modified mosquitoes in Malaysia and the Cayman Islands haven’t produced any untoward events.

If the process is indeed relatively safe, then more communities facing infectious disease outbreaks like dengue or malaria that are spread by mosquitoes may soon have to decide whether they are comfortable enough with the idea of a man-made mosquito in their midst, or mosquitoes infected with disease-causing virus.

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