Studying their effects could lead to novel ways to fight malaria, which is transmitted by mosquitoes and kills a million people every year around the world, most of them children.

"Many researchers focus on the direct effects of Plasmodium on the human body but the mosquito is an equally important battleground in fighting the disease," said Fotis Kafatos, Director-General of the European Molecular Biology Laboratory in Heidelberg, Germany.

"We now see a way to potentially stop the parasite in its tracks," Kafatos, who led the study, added in a statement.

"These studies are the first to show the power of the mosquito's immune system, and give us some very real options for fighting the disease in the insect before it even has a chance to be passed to a human."

When female mosquitoes feed on an infected animal, they suck in Plasmodium parasites with the blood.

There they grow and develop, moving from the gut into the insect's salivary glands, where the parasites are injected into another animal as the mosquito feeds. In humans, malaria causes fever, an enlarged spleen and kidney damage.

But some mosquitoes do not transmit the parasites.

Colleagues of Kafatos identified two genes that control proteins made by the mosquitoes, called TEP1 and LRIM1, which kill the parasite in the gut.

"By enhancing these natural defenders, we may be able to block the parasite-mosquito cycle," Stephanie Blandin, a researcher who worked on the study, said in a statement.

This part of the research was published this month in the journal Cell.

Kafatos and colleagues found two other proteins, each controlled by a gene, called CTL4 and CTLMA2, that protect the parasite as it develops in the mosquito gut. If these proteins were eliminated, the parasites died.

"It is now clear that if we strip away protective proteins, the parasite becomes vulnerable to the mosquito's immune system," said George Christophides, another member of the research team.

"Developing novel chemicals to inhibit the ability of such proteins to protect the parasite is a promising avenue to decrease the prevalence of malaria."