Why can mosquitoes carry deadly viruses without succumbing to them and live on to give humans West Nile, dengue fever, and a host of other fatal illnesses. According to new research, the insects' primitive immune systems recognize that the viruses are dangerous and slice the microbes' genetic material into harmless pieces.

But while fighting the viruses, mosquitoes pass them to people (and in some cases other animals), who do not seem to have the capacity to chop them up. The finding, which was published in today's Proceedings of the National Academy of Sciences, could lead to therapies to beef up human immunity against such microbes or ways to remove mosquitoes' defenses so that they kick the bucket when invaded by viruses instead of surviving to spread them far and wide.

The prevailing theory had been that these viruses and mosquitoes lived in harmony. But entomologists found quite the opposite to be true. "We were shocked," lead author Kevin Myles told ScientificAmerican.com, "This had long been viewed as a very benign relationship."

Myles and his colleagues at Virginia Polytechnic Institute and State University in Blacksburg, Va., discovered the mosquito's secret when they infected the insects with a virus that is a close relative of disease-causing microbes. They found that the mosquito's immune system cut up the virus' genetic material so that the insect didn't get ill. The team then infected the mosquitoes with a genetically modified virus that blocked their chopping mechanism. Those mosquitoes were unable to mount an attack against the invader and died off two to five times more quickly than did mosquitoes infected with the normal virus.

Understanding how mosquitoes fend off viral attacks might improve human health in the future, says study co-author Zach Adelman.

"If you could genetically modify a mosquito, then the mosquito would die upon becoming infected," he says. "This work hints that that's a possible application." He adds that future antiviral therapies for people may be fashioned to mimic the mosquito's virus-killing tricks. "If we could understand how the mosquito is doing such a good job," he says "maybe we could learn something about how to treat this disease in humans."

(Malaria is a parasite, not a virus, and so it's unclear whether these techniques would work for this particular mosquito-borne disease.)

(Image courtesy of the U.S. Centers for Disease Control.)