Wolbachia

Wolbachia is a bacterium that lives inside insect cells rather than in the environment around them. It occurs naturally in a large share of insect species, but not in Aedes aegypti, the mosquito that spreads dengue, Zika and chikungunya in most of the world's cities. Researchers therefore introduce it deliberately: mosquitoes carrying the bacterium are bred in a laboratory and released into a city, where they breed with the wild population.

The point of doing this is that a mosquito carrying Wolbachia is much worse at transmitting disease. Studies of released populations report that the bacterium significantly reduces the mosquitoes' capacity to pass on dengue and other viruses. The block is not total. Laboratory work shows that at very high viral loads some mosquitoes still develop infectious saliva, which researchers call a breakthrough infection.

What makes the approach unusual in public health is that it can maintain itself. An infected female passes the bacterium to her offspring through the cytoplasm of the egg, so once enough of a local population carries it the trait spreads and persists without further releases. Field monitoring in Niteroi, Brazil found the bacterium in at least 95 percent of sampled mosquitoes four years after release, and still high seven years on.

The strain used in most city releases is called wMel, originally isolated from Australian fruit flies and then bred into local mosquito lines so that it carries the antiviral effect alongside genetics suited to the place it is released.