Niko McCarty.@NikoMcCarty
Here are more details on the Singapore mosquito paper.
The key result: Releasing bacteria-infected mosquitoes over large swaths of Singapore, twice weekly, led to a huge drop in dengue cases after 3 months.
The bacteria, called Wolbachia, naturally infect insects like bees and beetles. They do NOT naturally infect Aedes aegypti mosquitoes, which transmit Zika, dengue, and chikungunya.
A Wolbachia bacterium was found in a house mosquito (Culex pipiens) in 1924, though, and by the 1970s researchers figured out that these bacteria interfere with mosquito reproduction. Wolbachia were deliberately introduced into Aedes aegypti in 2006, with the first field releases of Wolbachia-infected mosquitoes happening in Australia in 2011. This Singapore paper isn’t a new idea.
Wolbachia-infected mosquitoes become infertile because the microbes enter cell nuclei and use proteins, called CifA and CifB, to mess with histones during sperm development. The sperm never develop properly so, when these infected males later mate, the female lays eggs that never hatch.
(Importantly, females only mate once, and it is also only females that bite humans. This is why the study releases MALE mosquitoes; so people don't complain about the government airdropping biting mosquitoes over their houses.)
For this paper, researchers divided Singapore into 15 clusters totaling ~724,000 people. Eight of those clusters had twice-weekly releases of Wolbachia-infected mosquitoes, and the other seven had no releases. They then tracked dengue infections.
After 6 months of releases, only 354 of 5,722 Dengue tests (6%) came back as positive in clusters with releases, compared to 1,519 of 7,080 tests coming back positive (21%) in the control clusters.
You could just irradiate males and release them (without the Wolbachia), and the U.S. government already does this to fight against screwworm invasions in Central America. X-rays alone are sufficient to make males sterile, but X-rays also damage somatic cells, meaning you have a higher likelihood that irradiated males will not “compete” as effectively for females. Wolbachia is better because it leaves fitness alone and targets sperm more directly.
It's also really expensive to make Wolbachia mosquitoes; about $5 per person for an urban study like this. This is because you not only need to separate mosquitoes by sex, but then you need to infect them with Wolbachia AND irradiate them, using x-rays, to make sure you sterilize any females that accidentally pass through the filter. Still, this is cost-effective at reducing disease transmission.
Another option is to release mosquitoes carrying gene drives. But that seems socially unpopular, and it’s tricky to guarantee the gene drive will never leak into the wild. So for now, Wolbachia seems like our best option, even though they need to be released constantly to keep mosquito populations low.
This paper is a very important, large-scale demonstration of bioengineering at scale!
