When I was growing up in Queens, NY, mosquitoes tortured us all through the muggy summers. I ran around with pink splotches of calamine lotion covering my arms and legs. The cold of the lotion soothed the itch for about seven seconds. Never stopped me from scratching. We hated mosquitoes. We wanted them to disappear—not just from Queens, but from the face of the earth.
Of course, my childhood take on mosquitoes was shaped by the hell of itchiness, not fear of disease. After the Centers for Disease Control and Prevention declared in 1949 that malaria was no longer a significant pubic health problem in the United States, Americans enjoyed the luxury of viewing mosquitoes as very pesky pests, not sentinels of death. That’s all changed.
“When it comes to killing humans, no other animal even comes close,” wrote Bill Gates in a blog post two years ago. The Gates Foundation has worked extensively to curb mosquito-borne diseases, which cause more than 700,000 deaths a year and many millions of cases of serious, often-disabling illness.
“Ultimately, there seem to be few things that mosquitoes do that other organisms can’t do just as well—except perhaps for one,” reported Nature magazine in a 2010 article. “They are lethally efficient at sucking blood from one individual and mainlining it into another, providing an ideal route for the spread of pathogenic microbes.” (The Nature article, called “A World Without Mosquitoes,” concluded that wiping out the monsters, were it feasible, wouldn’t be a bad thing.)
The emergence of West Nile virus in the United States almost two decades ago made it clear that mosquito bites can have long-term and sometimes fatal consequences. More recent outbreaks of dengue fever in Florida have delivered a similar lesson. In another development of concern, mosquitoes carrying the chikungunya virus were reported in the Caribbean in 2013—their first appearance in the Americas. Although rarely fatal, chikungunya can cause fever, severe joint and muscle pain, and other symptoms.
And that was all before the current Zika virus outbreak, which has pushed domestic concern about mosquitoes to unprecedented levels. In particular, heartbreaking images of infants with microcephaly in Brazil and elsewhere have sparked anguished, wide-ranging discussions about how to prevent the South American epidemic from catching hold in the United States and other regions. And those discussions revolve around mosquito control.
Not surprisingly, my knee-jerk childhood wish—‘Let’s get rid of mosquitoes!’—is a popular one. But it represents a thorny proposition that incorporates a multitude of questions, some of them unanswerable. Overall, these can be divided into two categories. One is scientific: Is eradication possible? The second is ethical and ecological: If eradication is possible, is it advisable?
To start with, the expert consensus is that full eradication is not a viable possibility. Nor is it necessary for disease control. There are more than 3,000 identified varieties of mosquito, and the vast majority don’t mess with humans. “With thousands of species, if you could get rid of the ones that transmit dengue and Zika virus and other diseases, you’d still leave lots of mosquitoes,” said Arthur Reingold, an infectious disease doctor and the head of epidemiology at UC Berkeley.
In fact, only a couple of hundred species like to suck our blood. Of those, said Vincent Racaniello, a virologist at Columbia University, there are “dozens” of mosquito species that can serve as vectors for pathogens that make humans sick. The Zika virus is spread mostly by the species Aedes aegypti, which also transmits dengue and chikungunya. Species of the Anopheles genus spread malaria. Strategies for mosquito control vary with the species involved and the disease being targeted.
“Mosquitoes probably control other populations, and if you take them all out that’s going to be a problem, so it’s not a good idea,” says Dr. Racaniello. “But you can reduce the populations or target a particular species that’s confined to a local area.” He has covered the Zika virus outbreak extensively on his site, Virology Blog. (Full disclosure: I have been a frequent contributor to Virology Blog.)
Mosquitoes have been around for more than 100 million years. They thrive in a vast range of environmental niches around the globe, serving as predators and food sources, not to mention pollinators. Putting aside the fantasy of eradicating all of them, the future effects of eliminating even a single species in a specific regions can never be fully known. Other insects would invariably move in to fill the gaps. Many experts believe this would happen without severe ecosystem disruptions—and that the human benefits outweigh other factors.
The old-fashioned approach to mosquito control focuses on elimination of local or regional populations, rather than full-scale eradication, using two main strategies. In recent decades, pesticide use has lost much popular support, especially the once-common practice of widespread and indiscriminate spraying. Yet the targeted deployment of chemicals, in ways that create less collateral ecological damage, remains a common mitigation technique. The second approach, involving the aggressive removal of standing pools of water—in fountains, old tires, flower pots, old pipes, and all varieties of discarded junk—is also a critical weapon in controlling mosquito populations.
But the most recent addition to the anti-mosquito arsenal—genetically enhanced versions of the organisms themselves—presents a fresh set of dilemmas. Scientists are pursuing several methods to produce these novel creatures.
“And then there’s all the whiz-bang new scientific stuff that people are doing, breeding mosquitoes that in theory will mate with wild mosquitoes,” says Dr. Reingold. “That’s really interesting in the lab, but to what extent it works in the wild remains to be tested. And lots of communities rise up in arms when you talk about releasing genetically modified mosquitoes or Frankenstein-mosquitoes. Certainly many people will be jumping up and down about it.”
They’re already jumping up and down, of course. Last year, the British company Oxitec proposed releasing millions of Aedes aegypti mosquitoes in the Florida Keys. The mosquitoes were modified to transmit genes designed to kill their own offspring in the larval stage. The company engineered the bugs to combat dengue, but the technique could also be used against Zika virus and other pathogens spread by the same species.
Community foes and regulator roadblocks have stalled the Florida plan. Although the plan was to release only males, which don’t bite, residents worried that genetically engineered females would be released accidentally and transmit the synthetic gene to humans.
Oxitec has already released millions of its larvae-killing mosquitoes elsewhere, in field tests in the Cayman Islands and the Brazilian city of Piracicaba, for example. The company reported a more than 80 percent drop in wild Aedes aegypti larvae in the Piracicaba neighborhood where the mosquitoes were released. It is now expanding the effort, with the permission of local government officials.
The Oxitec strategy is just one of three methods currently being explored to genetically engineer mosquitoes. A second involves inserting into the mosquitoes a bacterium called Wolbachia, which can prevent the host from being infected with certain pathogens, thus sparing humans bitten by these mosquitoes. Researchers are testing whether this could prevent dengue in Vietnam and Indonesia, and in another region of Brazil.
Finally, and perhaps most controversially, other scientists are investigating what are called “gene drives”—the insertion of genes for a desired trait, along with a mechanism that propels them through an entire population. Unlike the engineered Oxitec mosquitoes, which will die off after they stop being released, the genes in a gene drive will survive, allowing them to mutate in unknown ways—with ultimately unpredictable results.
This technology is still in its early stages. Researchers have proposed deploying mosquito gene drives that would guarantee the dissemination of an infertility trait. Mary Power, a professor of integrative biology at Berkeley, said the technique could prove effective in disease control—but then what happens next? Scientists are also investigating follow-up gene drives that would counteract those previously released, in the event the first gene drive needed to be stopped.
Ultimately, every mosquito strategy hinges on weighing the potential human benefits of effective disease control against the unknown, and perhaps unknowable, future impacts. “It’s scary to have something that’s going to spread, we think fairly inexorably, through wild populations,” says Dr. Power. “There’s concern about subsequent evolution after it’s been released. The question is, can it be recalled?”
Posted on February 16, 2016 - 1:30pm