The mosquito was never going to win any popularity contests, but as the “deadliest animal in the world” spreads the Zika virus, it’s become the target of tough talk and powerful pesticides.
“The mosquito kills more people than any other animal on earth,” Dr. Tom Frieden, director of the Centers for Disease Control and Prevention, told CNN’s Dr. Sanjay Gupta, this week. “Is there any redeeming feature? … Well, they do provide food for birds and other insects, but I think the world would be a lot better off without them.”
Indeed, mosquitoes can spread diseases such as malaria, yellow fever, dengue and Zika virus. Even the ones that aren’t carrying diseases are buzzing in ears and threatening everyone’s outdoor fun.
Really, why don’t we just kill all of them?
That solution, it turns out, is the stuff of science fiction.
Aside from the potential impacts on the ecosystem when a species disappears, “it is absolutely impossible to kill all the mosquitoes; it’s just not going to happen,” said Roger S. Nasci, executive director of the North Shore Mosquito Abatement District, a public mosquito control program outside Chicago. “No one in the scientific field today has any illusions of being able to eradicate the mosquito.”
The reality is no matter how big of a mosquito-killing effort cities and countries unleash on the pests, “there will always be a remnant population somewhere that will repopulate,” Nasci said.
It’s been tried before
In the not-so-distant past, there were notions of mosquito eradication, but history quickly taught us otherwise.
Nasci points to the Herculean job that Latin America undertook in the 1950s and 1960s to eradicate Aedes aegypti, the mosquito that transmits yellow fever — and also dengue, chikungunya and Zika. The Pan American Health Organization rolled out all the most effective measures — spraying insecticide such as DDT and discarding standing water containers — on a massive scale. But after efforts let up, the mosquitoes came buzzing right back, possibly hitching a ride on shipping vessels from Asia and Africa.
The United States would probably be hard-pressed to achieve the same success if it borrowed Latin America’s strategy to wipe out mosquito populations. For starters, the Environmental Protection Agency banned the use of DDT, still the best pesticide to kill mosquitoes, in 1972 because of its devastating environmental effects and possible risks to human health. There is also the challenge that mosquitoes develop resistance to DDT and other agents such as pyrethrin that limit their effectiveness.
The best hope is to limit the number of mosquitoes in an area to reduce the risk of disease transmission, Nasci said. There are effective tools for doing so. Nasci and his program turn to them when surveillance data show the number of mosquitoes carrying West Nile virus — the major mosquito-borne disease in his area, carried by the Culex mosquitoes — is on the rise. His team gets called into action, too, when a fed-up resident can’t enjoy his deck because of all the backyard biters.
The future of mosquito control
The good news is that tools used for controlling mosquito populations wreak less environmental havoc than in the days of DDT — and at least so far have not been associated with resistance in mosquitoes. One approach is to spray a formula of bacteria around mosquito habitats. Mosquitoes eat up the bacteria, which then kill them and only them (and related insects) by destroying the lining of their guts.
Another strategy could be to develop tools that only kill the few mosquito species that transmit diseases to people and animals and spare the other 3,000 or so species that don’t do any harm, said Laura Harrington, professor and chair of entomology at Cornell University.
This plan would involve using genetically modified mosquitoes. It could take several shapes, but one would be to breed male mosquitoes to contain a gene toxic to their offspring. These tweaked males would be released into the wild to mate with females, but their larval babies would quickly die off. Studies testing the effect of releases in Brazil and the Cayman Islands have found 80% drops in Aedes aegypti levels. Harrington is also developing genetically modified males that would effectively kill all the females with which they mate.
The downsides of this approach are that genetically modified mosquitoes could be expensive, and they would probably have to be released at least once a year to keep populations in check, Harrington said. It is probably still years before these genetically modified mating menaces are ready to join the armamentarium of mosquito control tools, she added.
Learning to live with mosquitoes
In the meantime, it is important to remember that “mosquitoes can play a very important role in the ecosystem,” Harrington said. They are particularly key in their larval stages when, just because of their sheer mass, they are major food sources for aquatic predators. “If you removed all mosquito larvae in the Everglades, you’d probably have a huge impact,” Harrington said.
For his part, Nasci said his team does not see a “wholesale ecological collapse” when it does mosquito control in the Chicago area, which he would expect to see if the insects played a clutch role in the food chain.
Just because mosquitoes don’t appear to be going anywhere anytime soon does not mean that we must resign ourselves to their biting and buzzing.
There are a number of ways to keep mosquitoes at bay. At the most basic level, make sure you don’t have pools of standing water, such as in plant pots or bird feeders, around your home and that your window screens are in good shape.
If you are outside and insects are really making a nuisance of themselves, you can wear bug spray and a long-sleeve shirt, Nasci said. It can also be a good idea to keep an eye any reports of increases in the level of mosquitoes in your area harboring disease, such as West Nile. Many local public health agencies have monitoring programs, Nasci said.
Even if better mosquito control measures develop — and perhaps one day there are genetically modified mosquitoes — it will still be important to mix up approaches, Harrington said.
Even if levels of Aedes aegypti can temporarily be brought down, people should still take personal precautions, such as wearing repellent. And experts need to work on developing anti-viral drugs and vaccines against the diseases that that handle of mosquito species carry.
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