Why you can’t just wipe out mosquitoes to get rid of the Zika virus
On Saturday Colombia’s national health institute confirmed that more than 2,100 pregnant Colombian women are infected with the Zika virus, which has been linked to a devastating birth defect that affects a fetus’ brain.
The mosquito-borne disease, for which there is no vaccine or treatment, has been quickly spreading across the Americas since May, when the first confirmed Zika infection was reported in Brazil. More than 4 million people could be infected by the end of the year, according to the World Health Organization.
In response to the outbreak, governments have urged women to delay getting pregnant. The CDC has cautioned pregnant women against traveling in Zika-affected areas. But the disease’s catastrophic impact, together with the suffering caused by malaria and other mosquito-borne diseases, raises an obvious question: Why not kill all mosquitoes?
That question has two answers. One relates to the potential downside of wiping out an entire species: both the ethics of doing so, as well as the environmental impact, since mosquitoes are a food source for all kinds of creatures. The other involves the challenge of actually making it happen.
A few hundred types of mosquito bite humans, out of 3,500 known species. The females do the biting; they need amino acids from blood to develop and lay their eggs. (Male mosquitoes feed on nectar.) Mosquitoes live and breed in water ranging from ponds to flower pots, in most parts of the world.
Only three species are primarily responsible for transmitting human diseases, but they’re deadly efficient at doing so. When female mosquitoes bite an infected animal or person, they become infected — and then transmit the virus through saliva when they bite their next victim. The same virus that can kill humans can have no impact on mosquitoes, making them frustratingly resilient.
Over one million people die annually from mosquito-borne illnesses, which include malaria, dengue, Japanese encephalitis and chikugunya virus. Nearly half of the world’s population is at risk of malaria; in 2015 an estimated 438,000 people died from the disease.
The species responsible for spreading Zika is the Aedes aegypti, the same mosquito that transmits yellow fever, dengue and chikungunya. According to the Associated Press, after countries sprayed insecticide in the early 1900s, Aedes aegypti disappeared from most of South America, including Brazil, where it is wreaking havoc today. But it returned after many of these programs were scaled back, often because of worries over insecticide-associated health risks.
Insecticide spraying was the solution of choice for many decades. In the 1930s the Brazilian government used the Paris Green larvicide to kill off the malaria-carrying Anopheles gambiae species; Brazil was malaria-free by 1940. In the mid-1940s the United States used DDT to combat malaria and typhus; the EPA banned it domestically in 1972. In June the World Health Organization said that DDT “probably causes cancer,” with scientific evidence linking it to non-Hodgkin lymphoma, testicular cancer and liver cancer.
More on the Zika virus
* WHO meets to decide whether to declare Zika a global emergency
* Zika virus spreads fear among pregnant Brazilians
* Slideshow: the Zika babies
* Opinion: Why Zika may be as tough to beat as Ebola
As an alternative to insecticides, scientists are now turning to genetics. The British biotech firm Oxitec has created genetically modified males of the Zika- and dengue-carrying Aedes aegypti species. When these genetically modified males mate with wild females of the same species, they create offspring that don’t live to become adults.
Another effort, funded partly by the Bill & Melinda Gates Foundation, transfers a natural bacteria called Wolbachia into the Aedes aegypti species — where it otherwise doesn’t occur. The Wolbachia mosquitoes are less likely to transmit dengue; researchers believe the method has the potential to work on other insect-transmitted diseases as well.
The challenge is scaling these efforts. Spraying tons of insecticide in a targeted area is a more straightforward approach than getting every female mosquito to breed with genetically modified males, in the case of Oxitec’s strategy.
And despite the millions of people affected by mosquito-borne diseases, they remain, to many, the developing world’s problem. For drug companies balancing R&D budgets with projected profits, there’s a financial disincentive to conducting research on diseases of the poor.
There’s also a risk that wiping out an entire species could have unintended consequences, and a negative impact up and down the food chain. Mosquitoes are a food source for birds and bats; frogs, fish and turtles consume their larvae. If they’re eradicated, some scientists fear, they could be replaced by an even worse species. Additionally, by helping to make some regions — like rain forests — inhabitable for humans, mosquitoes have actually helped to protect and conserve some fragile parts of the world.
So rather than focus efforts on killing off all mosquitoes, R&D funding directed at vaccines, treatment and other interventions is likely to produce greater returns. On Tuesday President Barack Obama called for the rapid development of tests, vaccines and treatment to fight Zika — a move that came after a recent study estimated the virus could affect areas where 60 percent of the U.S. population lives.
Now is the time to act. It’s fine to spare the mosquitoes — as long as we invest in sparing human lives as well.