Brazil is latching onto a novel, if controversial, approach to fight the spread Zika virus: genetically modified mosquitoes.
Zika virus is transmitted to humans primarily through the bite of an infected Aedes species, such as Aedes aegypti, the yellow fever mosquito.
No vaccine or treatment exists to combat the infection, which public health officials are worried may be linked to a brain defect in infants and a rare neurological syndrome that could cause paralysis in adults. The World Health Organization has expressed alarm at the explosive spread of the virus in the Americas in recent months and says as many as 3 to 4 million people could become infected.
Releasing even more of these insects into the wild seems like the last thing a Zika-stricken country needs, but Brazil’s National Biosafety Committee recently approved multiple releases of genetically modified Aedes aegypti throughout the country. Essentially, the plan is to turn their own species against them.
Created by Britain-based Oxitec, a spin-off company from Oxford University that is a subsidiary of Intrexon Corp., these mosquitoes contain a self-destruct mechanism within their DNA in the form of an inserted gene. This gene produces a protein called tetracycline repressible activator variant (tTAV), which ties up the cell's machinery and prevents the expression of other genes key to survival. As a result, the insect dies before it reaches adulthood.
But if reared on a diet with a special antidote — the antibiotic tetracycline, which binds and inactivates tTAV — the self-destruct mechanism will never switch on.
Oxitec feeds its genetically modified mosquitoes tetracycline so they can survive and reproduce in the lab. For pest control purposes, mature males carrying the self-destruct gene are gathered up and released into the wild to mate with females. Offspring who inherit the gene will die without the tetracycline antidote, causing the mosquito population to drop drastically — and hopefully with it, the threat of disease.
“We've trialed this technology in the Cayman Islands, in Brazil and in Panama through four different trials, and we have shown up to 99 percent control of the mosquito population,” said Derric Nimmo, Product Development Manager at Oxitec, in a company video. In April, Oxitec started releasing its so-called “self-limiting” mosquitoes in Piracicaba, a city in the Campinas region of São Paulo state, and reported an 82 percent reduction in wild larvae by the end of the calendar year.
Because Aedes aegypti is considered the primary vector for dengue, Chikungunya and Zika viruses, the company has focused extensively on this species for public health applications. For surveillance purposes, its mosquito products also contain a heritable, fluorescent marker to differentiate between altered insects and wild ones. Monitoring the ratio of genetically modified vs. wild mosquitoes in traps after a release can help gauge whether more product is needed to further suppress the pest population.
This genetic approach and others — for instance, those that render insects infertile or disease-resistant — represent a new spin on the classical Sterile Insect Technique (SIT). During the 1930s and 1940s, the idea of SIT was conceived as a method of “birth control” for unwanted insect species. Males would be rendered sterile by exposing them to massive amounts of radiation, then released to mate with females. SIT was used to eradicate the screwworm, a deadly parasite of livestock, from the United States in the 1950s, and to tame pink bollworm moth populations in California's cotton fields since 1967.
Oxitec's scientists have also created self-limiting versions of common agricultural pests in the hopes of minimizing crop losses. One example is the diamondback moth, an insecticide-resistant nuisance that feeds exclusively on brassicas, such as broccoli and cabbage. Oxitec and Cornell University are planning field tests to be conducted in the summer in upstate New York, which have been granted approval by the U.S. Department of Agriculture.
One big question is whether manipulating the ecosystem in this way will have unintended, negative consequences.
Oxitec has come under criticism from various environmental groups that remain concerned about the possible effects of releasing a new strain of organism into the wild. For instance, a drastic drop in mosquito population could lead to harmful insects or other animals multiplying uncontrollably.
Supporters of genetically modified insects say self-limiting species target only one species, can still be eaten safely by predators, and are more effective/safe than insecticides. Also, the altered males and their offspring die off quickly due to the tTAV gene.
“In that sense, we're only removing Aedes aegypti and nothing else from the environment,” Nimmo said. “It's pinpoint accuracy. It's going in with a scalpel and taking away Aedes aegypti, leaving everything untouched.”