We came incredibly close to eradicating the mosquito that carries Zika from the Americas - here's why it came back

The ongoing outbreak of Zika virus in the Americas can be traced to a single species of mosquito called Aedes aegypti, which we once almost eradicated from these areas.

In the 1940s and 1950s, Central and South American countries launched massive campaigns to eradicate the pesky mosquito, which also carries yellow fever, dengue fever, and Chikungunya virus. And these efforts were largely successful:

As of 1964, the mosquito had reportedly been eradicated from all mainland countries of the Western Hemisphere between the United States and the southern tip of South America except for Venezuela, Colombia, and French Guiana.

According to the World Health Organization, "Aedes aegypti was virtually eliminated from the Americas. By the late 1960s, most mosquito-borne diseases were no longer considered to be major public health problems outside Africa."

But as the threat diminished, so did the resources available for battling the problem, and the mosquito came surging back.

We asked Gregory Lanzaro, a professor of pathology, microbiology and immunology at the University of California at Davis why these eradication efforts ultimately failed.

The main reason, he said, was that the programs weren't sustainable.

"When the numbers of mosquitoes and level of disease drops to a very low level, the people who are funding those efforts withdraw the money, the control goes away, and the mosquitoes come back," Lanzaro told Business Insider.

Why the approach used back then won't work today

1. The primary method of controlling mosquitoes back then involved spraying them with insecticides like DDT, but this wouldn't be practical today, Lanzaro said. Apart from the fact that DDT is incredibly toxic - as Rachel Carson's famous book "Silent Spring" made clear - many A. aegypti mosquitoes have evolved resistance to the pesticide.

2. Some of the early efforts to eradicate A. aegypti involved inspectors forcing their way into people's homes to ferret out places where the insects might breed. "Whereas in the past you could pass regulations that required homeowners to give access to mosquito control people, today in most societies that's not possible," Lanzaro said.

On the other hand, today's scientists have some techniques at their disposal that we didn't have in the 1960s: namely, genetic engineering.

New genetic tools in the fight against disease

In a promising new development, mosquitoes can be modified to be sterile or resistant to carrying parasites like Zika.

In fact, a British company called Oxitec is doing exactly that, by creating male mosquitoes with a gene that causes their offspring to die before they reach adulthood. Brazil has already begun doing field tests of these genetically modified mosquitoes, and the results are promising.

However, this approach has its problems too. Making these mosquitoes requires producing them in large quantities and releasing them regularly, which is expensive and may not be very sustainable.

Another promising approach involves creating mosquitoes that are guaranteed to pass on a trait like viral resistance or sterility to all of their offspring, a strategy known as gene drive. Compared with other genetic approaches, gene drive would require far fewer mosquitoes and less frequent releases.

But it poses some ethical issues. Some environmentalists worry that altering or completely eradicating a mosquito species would have unforeseen effects on an ecosystem.

A combined approach is needed

While Lanzaro is skeptical of these concerns, he's still cautious. "To say there absolutely would be zero risk would not be fair," he said, but "we have to balance that with the benefits."

Genetic engineering may be our best bet, but we need to combine it with other approaches, such as educating people about how to get rid of the mosquito's breeding spots.

"We're going to continually be faced with the introduction of these kinds of disease into the US. Being better able to manage them is something that really needs serious thought," Lanzaro said.