Scientists infecting mosquitoes before they infect humans

In a laboratory in downtown Medellín, Colombia, it is lunchtime: A technician in a white coat carries a loaded tray into a steamy nursery. She walks between rows of white mesh cages, each the size of a minifridge, and slides a thin tray of blood into every one. In response, her charges, all 100,000 of them, begin to whir and emit an excited hum.

This is a mosquito factory. Each week, it churns out more than 30 million adult Aedes aegypti mosquitoes, with their distinctive white polka dots on their wiry black legs. The brood stock of females is fed on discarded blood bank donations, and horse blood. Eventually, some of their progeny will be released into Medellín, Cali, and cities and towns in Colombia’s verdant river valleys. Other insects will be chilled into a stupor for a journey up to Honduras.

The elaborate effort is part of an experiment that is making encouraging progress in the long fight against mosquito-borne disease.

Aedes aegypti spreads arboviruses, including dengue and yellow fever, which can severely sicken or kill people. But these are special Aedes aegyptis: They carry a type of bacteria that can neutralize those deadly viruses.

Five decades ago, entomologists confronting the many kinds of suffering that mosquitoes inflict on humans began to consider a new idea: What if, instead of killing the mosquitoes (a losing proposition in most places), you could disarm them? Even if you couldn’t keep them from biting people, what if you could block them from passing on disease? What if, in fact, you could use one infectious microbe to stop another?

These scientists began to consider a parasitic bacteria called Wolbachia, which lives quietly in all kinds of insect species. A female mosquito with Wolbachia passes it on in her eggs to all of her offspring, who eventually pass it on to the next generation.

But Wolbachia isn’t naturally found in the mosquito species that cause humans the most problems — the Aedes aegypti, the virus carrier, and the Anopheles subspecies, which carry malaria. If it were, it might eventually render those species essentially harmless.

So, how do you infect a mosquito with Wolbachia?

Researchers found, after painstaking trial and error, that they could insert the bacteria into mosquito eggs using minute needles. The mosquitoes that grew from those eggs were infected.

The Aedes aegypti mosquitoes that hatched and lived with Wolbachia did just fine. And as hoped, the Wolbachia mostly blocked the viruses: The mosquito who bit someone with dengue, and picked up the virus, didn’t pass it on to the next person it bit.

That got the researchers thinking: If they could infect all the mosquitoes in a village or city, they might stop the disease. Unlike truckloads of insecticides, sprayed down every street and running off into water systems, this method would not harm the ecosystem.

But how do you get Wolbachia into all the mosquitoes in a city the size of Medellín?

Once they were confident they could infect generations of mosquitoes in the lab, the scientists needed to know if their theory would work in the wild. The method was first tested in small towns in northern Australia, where females with Wolbachia released in the field mated with wild males and did, indeed, spread Wolbachia through the mosquito population.