If you want to protect people from malaria, keep the disease out of mosquitoes. It's an intriguing approach, but implementing it is harder than it sounds: In the wild, malaria-free mosquitoes tend to fare worse than their susceptible counterparts. Enter a mosquito genetically engineered to fight the disease. According to a new study, these transgenic bugs beat out nonengineered mosquitoes when both are feeding on parasite-infected blood. If confirmed, the study means that plans to replace entire mosquito populations with disease-resistant ones may have better chances of success.
After mosquitoes bite a host with malaria, the parasite that causes the disease proliferates in the insect, readying itself to infect the next human victim. It's no fun being infected, and one might think that mosquitoes would have developed a resistance to the malaria parasite over time. But several studies have suggested that mosquitoes engineered to build defenses against malaria are less fit than insects that chose to live with the parasites.
Now there's reason to take heart. Several years ago, a group of medical entomologists at Johns Hopkins University created a strain of Anopheles stephensi (a mosquito that bites rodents) equipped with a gene called SM1 that makes the mosquitoes resistant to infection with Plasmodium berghei, a rodent malaria parasite. In the new study, published online this week in Proceedings of the National Academy of Sciences, the group carried out a series of experiments in which 250 of these insects were put in a cage together with 250 wild-type counterparts and allowed to feed on malaria-infected mice. The resistant insects lived longer and produced more eggs than did those not resistant to the parasite, and after nine generations, some 70% of the population was resistant. The researchers speculate that carrying SM1 is a less costly strategy than whatever defenses malaria-resistant mosquitoes develop in the wild.
Still, the apparent benefits of resistance may not be big enough to help the gene spread through a population in the wild, where not all the hosts are infected, says co-author Jason Rasgon. Nevertheless, he says study is the first demonstration that natural selection may help, rather than hurt, engineered insects. And mosquitoes with the SM1 gene could well help keep malaria out when it has been cleared from an area using other means, such as bed nets, drugs, or a future vaccine, he adds.
Entomologist Bart Knols of Wageningen University in the Netherlands cautions that cage conditions are very different from those of the field, where many other factors influence an insect's survival; what's more, the results may not hold up in human malaria, he says. Still, it's "very encouraging" to see a case where lab-engineered mosquitoes actually have the evolutionary edge, says Kenneth Vernick of the University of Minnesota.