Female mosquitoes need to drink blood from vertebrate animals for their eggs to develop. The mosquito species Aedes aegypti can, and does, feed on the blood of many animals. But it prefers human blood. Mosquitoes that feed on human blood have better health and produce more offspring than mosquitoes fed on other kinds of animal blood, but the reason for this is unknown.
New research from scientists at the University of Queensland in Australia has found that when these mosquitoes are infected with a bacteria called Wolbachia, they can only make babies after drinking human blood. Guinea pigs, mice, sheep, pigs and chickens just aren’t good enough.
Aedes aegypti is the main species of mosquito that transmits the dengue virus to humans. Dengue virus causes dengue fever, which infects up to 100 million people a year and can be fatal. The most promising way to prevent Dengue is to reduce the numbers of mosquitoes that carry the virus.
Mosquitoes are going to keep having sex, so how can we keep the mosquitoes from breeding?
When the scientists infected the mosquitoes with Wolbachia, a bacteria that infects fruit flies and other insects, they had a surprising result: the mosquitoes were effectively sterile. They laid few eggs, and almost none of those eggs hatched.
The catch? This was only true when mosquitoes fed on non-human blood (like from mice or chickens). If they were given a nutritious human volunteer’s arm to nibble on, they went right back to laying lots of perfectly healthy eggs (though still less than females that were not infected with Wolbachia).
So what’s the secret ingredient in human blood?
No one knows for sure, but there have been some guesses made. One clue is that human blood is rich in an amino acid called threonine. Threonine is a key amino acid needed for the production of mosquito egg proteins. Threonine is also an nutrient that Wolbachia can’t make by itself–it has to get it from its host. Wolbachia may be depriving the mosquito’s eggs of threonine, leading to the loss in fertility when fed on non-human blood, which is lower in the amino acid.
Another possibility is cholesterol. Human blood tends to have more of it than that of say mice or chickens. Wolbachia needs cholesterol to make part of its cell wall, and the mosquito needs it to replenish its fat stores, creating another potential conflict between Wolbachia and the host mosquito.