The Zika Challenge
While Olympians are racing for gold in Rio, Zika virus is running rampant through the American tropics. A relatively mild illness that generally goes unnoticed even in those infected, Zika can be devastating to fetuses and expectant mothers, causing infants to be born with a severe developmental brain disorder known as microcephaly. Their plight makes us desperate: so desperate that Pope Francis has deemed contraceptives—usually condemned by the Catholic Church—acceptable to use in affected countries; so desperate that the World Health Organization is recommending that at-risk women delay pregnancy; so desperate that a massive diversion of resources has gone toward finding pharmaceuticals and vaccines to counter Zika, while efforts have redoubled to control its vector, the Aedes aegypti mosquito; so desperate, in fact, that some people are once again calling for the eradication of this species—and perhaps other mosquitoes as well.
While the world reacts in a desperate scramble to counter Zika’s spread, we may be missing the larger point. While efforts to curb, genetically modify, or permanently eradicate Ae. aegypti mosquitoes may be the most obvious course of action, they won’t get to the root cause of the problem—and they might inadvertently make things worse. With technologies to eradicate species emerging quickly, it is imperative that we carefully consider whether or not that strategy will actually solve fundamental challenges to human health and quality of life. As a microbiologist focused on the evolution and diversity of disease-causing viruses, I’m not convinced.
Originally from Africa, Ae. aegypti followed waves of European colonization to the New World, and then exploded through the rest of tropics and subtropics with shipping and commerce. A day-biting mosquito, it has a special fondness for humans: a predilection for our blood, an abiding attachment to our homes, and a dependency that curbs their will to wander farther than our gardens and backyards in search of pockets of water in which to lay their eggs. These adaptations, honed over millennia, have served to propel Ae. aegypti into a dominant position at the top of the urban food chain. As humans continue to spread, this mosquito follows, invading and thriving in villages and cities of all sizes. Because of the special human-centric niche it occupies, Ae. aegypti is the perfect vector for transmitting not only Zika, but also dengue, chikungunya, and yellow fever viruses among humans.
With vaccines and antivirals specific to Zika, dengue, and chikungunya still not available, one important approach to combatting the spread of these diseases is to control the mosquito itself. Several new tools are emerging that make this feasible—the most extreme of these approaches place within our grasp the power to eliminate an entire species, if not multiple species, and therefore require thoughtful consideration.
Traditionally, efforts to control Ae. aegypti have targeted their breeding grounds: small pockets of standing water—from barrels to bottle caps—where females lay their eggs. This process is laborious and requires constant upkeep, particularly in the rainy tropics. Sometimes insecticides are sprayed to kill adults, an expensive and arguably ineffective approach. But several breakthroughs represent big game changers in the battle against Ae. aegypti.
For example, the British company Oxitec releases “self-limiting” male mosquitoes that seek out wild females and pass on a genetic modification that renders the offspring incapable of surviving to adulthood. Currently being tested in Brazil, this approach will likely suppress populations much better than spraying insecticides, but still requires regular upkeep.
Another breakthrough control method targets a mosquito’s ability to carry and transmit viruses such as dengue and Zika. A group of scientists led by Scott O’Neill has discovered a strain of Wolbachia bacterium not normally found in Ae. aegypti that interferes with the mosquito’s ability to transmit the viruses to humans. Tests in Australia indicate that mosquitoes experimentally infected with Wolbachia, and rendered incapable of virus transmission, can persist and spread through the environment—certainly a more sustainable solution.
“The Zika epidemic has exposed deep societal inequalities, which differentially affect women in particular, including their rights and ability to control their reproduction.”
Despite these advances, some models predict that reducing mosquitoes or interfering with their ability to transmit disease may slow the development of our own “herd immunity.” This is the beneficial process by which individuals who have been exposed and are resistant to a disease help protect others in the population who are more vulnerable by breaking transmission cycles. By reducing the larger population’s exposure to Zika-carrying mosquitoes, we could, in fact, bring on more frequent epidemic waves of the virus.
In light of the imperfect results of our current control efforts, many people have proposed attempts to completely eradicate Ae. aegypti—and this strategy may soon be feasible. A new breakthrough called “gene drive” could allow us to insert genes carried by genetically modified male mosquitoes more efficiently into mosquito populations, ensuring that all offspring from these tinkered matings inherit and pass on the genetic defect. Predicted to spread rapidly and require little to no upkeep, gene drive has the potential to wipe out an entire species. And while it may be tempting to imagine the extinction of Ae. aegypti as a solution to all of our Zika problems, it most likely wouldn’t be the cure-all we’re hoping for—and it might make things worse
If we eliminate Aedes aegypti, other competitors will surely be waiting in the wings to take over its niche. (As Aristotle once wrote, “Nature abhors a vacuum.”). The yellow fever mosquito’s primary invasive competitor is the Asian tiger mosquito Ae. albopictus. Often, where Ae. aegypti is eliminated, Ae. albopictus moves in. Like its better-known relative, the tiger mosquito also transmits Zika, dengue, and chikungunya. Moreover, it generally spreads out farther beyond the urban setting, making it harder to control. We also know that Zika virus, like other rapidly evolving viruses, can easily adapt to new vectors. In fact, Zika has now been found in the common house mosquito, Culex quinquefasciatus. As much as we’ve come to malign Ae. aegypti mosquitoes, its elimination may be only a band-aid solution before others move in to take its place. Yes, we could potentially eradicate each new mosquito as it arises. But at what cost, and where do we stop?
As we run through the list of mosquito species on the chopping block, we must think more broadly about what the large-scale eradication of mosquitoes would mean for our ecosystems. There are about 3,800 mosquito species worldwide, and less than an estimated 3 percent of them transmit human diseases. Although we never come into contact with most of these insects, many are critical components of food webs that include other species we value.
For many species of birds, for example, mosquitoes are a dietary mainstay, and a dearth of the insects can lead directly to fewer hatchlings. Mosquitoes are also critically important to many species of bats, with single colonies capable of consuming literally tons of the insects each night. It’s unclear whether or not these bats would be capable of finding a substitute food source in a world without mosquitoes—but if they did, that switch could have severe and unpredictable effects, impacting moths and other important pollinators.
Additionally, many aquatic species, including a number of threatened frog species, rely on mosquito larvae for food. The mosquito larvae themselves feed on detritus and aquatic life, playing an important role in nutrient recycling, particularly in rarefied environments. Adult mosquitoes also act as pollinators. While the females ingest blood, males feed on nectar and other natural sugar sources, collecting and sharing pollen along the way. Although some people would counter that no important crops are pollinated this way— and in fact have argued that these examples provide little direct benefit to humans—time has shown that when species leave an ecosystem, their loss can have unanticipated ripple effects not only on us but on the environment as a whole, and its inherent resilience.
As a research scientist focused on disease-causing viruses, my explorations often take me to their roots in nature. My research in Thailand has shown that places with high overall mosquito diversity have fewer disease-carrying species. In 2008, my team collected more than 84,000 mosquitoes across a variety of habitats—from relatively undisturbed forests to much-altered suburban and urban communities—in central Thailand’s Nakhon Nayok province. We observed that mosquito diversity took a dive in human-altered landscapes, and troublesome invaders like Ae. aegypti became more common there. While it may seem counterintuitive, working to sustain biodiversity— including mosquito diversity—may be one of our best long-term defenses against the rise of mosquito-borne diseases
Maintaining biodiversity and immunizing mosquitoes against viruses—especially when we can do so using ingredients already found in nature, like Wolbachia bacteria—are certainly more sustainable and permanent solutions to combatting Zika virus than pursuing a revolving door of eradication efforts, targeting one species of mosquito after another. But the truth is, mosquitoes are only part of the problem facing us today. Consider that the impact of dengue and Zika continues to be most strongly felt in communities where humans are particularly exposed to mosquito bites. In these areas, access to running water is often limited, and mosquitoes have ample opportunities to breed in water-storage containers and in discarded objects near homes and communities. Dengue transmission in Mexico often stops dead at the Texan border, and it’s not because Ae. aegypti respects immigration laws, but rather because humans and mosquitoes come into contact far less in a typical air-conditioned Texas town than in neighborhoods and open-air markets in Mexico.
Not only does the rate of infection differ in different societies, but the degree of harm a disease might cause also varies widely. Because Zika is primarily a threat to unborn children, a major response has been to recommend delaying pregnancy in affected regions. And yet many of these same regions are culturally opposed to family planning or lack the infrastructure to support it either through education or supplies. Thus, the Zika epidemic has exposed deep societal inequalities, which differentially affect women in particular, including their rights and ability to control their reproduction. No amount of mosquito eradication will solve these disparities, but the Zika phenomenon has galvanized a watershed of much-needed social services that include support, education, and empowerment for women and their reproductive choices.
As we strive to meet the challenges of Zika and other mosquito-borne diseases, we can’t forget the human element—a vital part of the equation. In today’s global society, we move ourselves and other species around the planet on a regular basis, living cheek by jowl with invasive species. In doing so, we play a significant part in the emergence of infectious diseases. Thus, humans must be part of the solution. The answer lies in taking a hard look at not only how we live and structure our societies, but also how we live with nature; maintaining diversity, from microbes to mosquitoes and beyond, may be more important than we think when it comes to human health and well-being.