Every year, mosquitoes kill more humans than any other animal—including our fellow humans—due to the deadly diseases they carry, with Zika being merely the latest high-profile threat.

Now, a new multipurpose strategy could one day control mosquito disease transmission as a whole. And one of the hottest frontiers in this effort is, strangely enough, Marlon Brando’s private island.

The late actor fell in love with and secured ownership of the Polynesian atoll of Tetiaroa in 1967. Now the area is home to one of the most successful mosquito-control efforts on Earth.

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In just six months, without the use of chemical pesticides or genetic modification, researchers have freed one of the atoll’s islets from Aedes polynesiensis, an invasive mosquito species that is the vector of diseases such as dengue, chikungunya, and Zika. (Find out which Central American country has women with Zika afraid of going to prison.)

It’s a simple process: Introduce certain male mosquitoes that will mate with and sterilize the wild females in a particular locale, rendering their eggs nonviable. After a few rounds of such treatment, the population is unable to reproduce itself and collapses.

Related methods are being tried in Southeast Asia, Europe, and the Americas. In French Polynesia, they're led by Hervé Bossin of the Institut Louis Malardé, who's working with the nonprofit Tetiaroa Society, based on the atoll now owned by Brando's estate.

The society carries out Brando’s vision of researching and conserving the atoll's natural environment. It's supported in part by The Brando, a high-end eco-friendly resort that was another part of the actor's vision.

A Pest You Can’t Refuse

Out of the 3,500 species of mosquitoes, only two that are known vectors for disease are found in French Polynesia, and neither is likely to have made it there if it had not been for humans.

Aedes polynesiensis is the older of the two, since it probably arrived with Polynesian voyagers who first reached the area a thousand years ago. The other species, Aedes aegypti, originated in Africa and came to the Pacific with European trade in the late 19th and early 20th centuries.

It's likely that, from the beginning, the mosquitoes brought diseases such as dengue with them. Now the spread of Zika is giving rise to new questions and challenges for global efforts in disease control.

For instance, these mosquito species can pass on viral infections to their offspring, and their eggs can lie dry and dormant for extended periods—eggs of the Ae. aegypti mosquito can last a year before getting rehydrated and hatching.

That means any truly successful eradication program is going to require diverse approaches.

Last Tango in Tetiaroa

Up to 60 percent of insect species carry a harmless type of bacteria called Wolbachia in their cytoplasm, the thick mixture of water, salts, and protein that fills every cell. The key is that different insect populations have different Wolbachia strains.

When a male mosquito infected with Wolbachia A mates with a female infected with Wolbachia B, the fertilized eggs fail to develop as a result of what's called cytoplasmic incompatibility. The adults may be perfectly healthy—and could even mate successfully with other individuals if their bacterial strains matched—but mismatched insects won’t produce any offspring.

This is what Bossin is using to control Ae. polynesiensis on Brando’s island, which he describes as a perfect natural lab. Tetiaroa is an atoll made up of several small islets, with only a tight range of plant and animal life present. That makes it much easier to control variables and isolate the effects of slight changes.

At the very beginning of the project, the team on Tetiaroa set baited traps and caught large amounts of wild mosquitoes. The bugs were given an antibiotic to wipe out the naturally occurring Wolbachia in their systems, and then they were infected with another strain of Wolbachia from a different population. The team let the treated mosquitoes breed, then worked solely from that population.

As new broods reached the pupa state, a mechanical sorter separated them by size to pull out the males, which are smaller. Samples were verified under a microscope to ensure no females had snuck into the bunch—at less than one per 10,000, the system’s record is strong.

The scientists then set the females aside with some males so they could get started making the next generation of pupae carrying the foreign strain of Wolbachia.

View Images Fully emerged and ready to track down females and mate, these Wolbachia-incompatible males await release on the islet of Onetahi in Tetiaroa. Photograph by J. Marie, ILM

The team releases its lab-born males in large numbers every week. Since these males are incompatible with the wild females, none of the fertilized eggs will hatch.

From egg to adult, the process takes just over a week. In terms of animal husbandry, this is about as good as it gets. In September 2015, when Bossin’s project began, the lab would produce 40,000 males every week. As of October 2016, they’re up to 100,000 per week.

Six months after the project began, the team had nearly eradicated Ae. polynesiensis from the atoll's Onetahi islet. Ae. aegypti is one of the species that doesn’t already harbor any Wolbachia, so it’s being controlled there by conventional, nonchemical methods, such as employing best practices for eliminating mosquito breeding sites.

Now Bossin and the Tetiaroa Society have their sights set on eliminating Ae. polynesiensis from the rest of the atoll.

Bossin and his colleagues note that getting rid of the atoll’s mosquitoes won’t have a negative effect on the natural environment. Neither species has been there long, and both were brought in by humans. They're also not an essential food source for any Tetiaroan species.

“Reptiles and spiders have plenty of other insects to prey on,” he says.

On Other Waterfronts

In Australia, a similar program uses Wolbachia for disease control but not mosquito eradication. Scientists there are introducing the bacteria into Ae. aegypti. Rather than creating infertile insects, the bacteria inhibit the growth of viruses and render the adults incapable of transmitting them to their bite victims. The team releases both males and females to mate freely and spread the bacteria throughout the wild population.

And in the U.S., Stephen Dobson of the University of Kentucky has been working with Wolbachia in the lab and in limited field trials, including an experiment that brought the technique to the outskirts of Los Angeles.

View Images Once Aedes polynesiensis males with the foreign Wolbachia strain have been isolated, the pupae are kept floating in water before being packaged and transported to the release site. Photograph by H. Bossin, ILM

There, Joseph Wakoli Wekesa and Susanne Kluh of the Greater Los Angeles County Vector Control District released some 10,000 Wolbachia-incompatible males into a six-acre plot.

While they saw an encouraging 67-percent drop in the adult population, it was a much shorter test than the Polynesian one. Also, the effect of the released males was less dramatic because the test area is less isolated, and nontreated males from surrounding regions quickly filled the void.

Dobson’s team is now awaiting regulatory approval for commercial use of Wolbachia to control Ae. albopictus, the Asian tiger mosquito. Next on their list is Ae. aegypti.

Both Wekesa and Bossin stress that this isn't a technique that can instantly combat an epidemic. If an area already faces a mosquito-driven disease outbreak, immediate response with conventional techniques, such as pesticides and breeding-area removal, remains the best hope for minimizing the impact on public health.

Instead, the innovative work being done with Wolbachia bacteria and sterilization is about long-term control and prevention. A few more field seasons over a few more years, and this technique could be the key to ending one of our greatest natural threats.

It’s got class. It’s a contender. Brando would probably like that.

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