With mosquitoes against malaria?
Mosquito nets, insecticides, medicines - people are trying to get rid of malaria with all possible means. Does the key to fighting intermittent fever lie in the genes of the disease carrier?
Tourists have it good: For their vacation in the malaria region, they simply take the current malaria prophylactic drug for a few weeks and then return home – malaria-free, of course. The locals have it much harder. They live permanently with the disease and pay it a heavy toll: every year more than a million people die from the tropical disease worldwide - mostly children.
It is therefore the declared goal of the United Nations to stop the spread of malaria by 2015. However, this is proving to be difficult: the pathogen is becoming resistant to the drugs used, and even the simplest effective method of prevention – sleeping under a mosquito net – is still underused.
Disaster comes over humans with the bite of an Anopheles mosquito: the spores of the malaria pathogen are dormant in the salivary glands of infected animals. Four different species of the protozoa Plasmodium can infect humans – they get into the bite wound with the insect's saliva.
The spores then swim with the bloodstream to the liver, where they divide and finally penetrate the red blood cells. There they divide again and get back into the blood when the blood cell disintegrates. Since thousands of blood cells usually disintegrate at the same time, a fever flare occurs.
However, some pathogens develop into sex cells in the blood cells – they have to find their way back into a mosquito in order to be able to fertilize. The mosquito is kind enough to meet them and sucks up the protozoa with their blood meal. Finally, in the mosquito's gut, Plasmodium completes its life cycle and forms spores, which it sends to the insect's salivary glands so they can infect a new human victim on the next bite.
But by no means every mosquito takes on its intended role in this cycle of generations. Some mosquitoes simply get out of the game and kill the protozoa in their body without further ado. That alone is – from the human point of view – very gratifying. To make things even better, the rebellious insects pass their special ability on to their offspring. An international team led by Kenneth Vernick from the University of Minnesota has now investigated the genetic cause that gives these mosquitoes their resistance to Plasmodium.
In Mali, the scientists collected fertilized females of the Anopheles gambiae mosquito species, which transmits the particularly dangerous pathogen Plasmodium falciparum of the often fatal malaria tropica. The offspring of these progenitors fed them blood infected with Plasmodium falciparum. After seven to eight days, the fattened animals were taken to the slaughterhouse, where they gave up their insides.
The result was amazing: the intestines of 22 of the 101 offspring strains were completely free of parasites - resistance to the malaria pathogen seems to be quite widespread in nature.
A small region on one of the mosquito chromosomes was responsible for this resistance. With the help of precise genetic analyses, the scientists were finally able to identify the APL1 gene, which plays a particularly crucial role: when the researchers switched off this gene, the insects suddenly gave up their resistance and Plasmodium falciparum was able to invade the mosquitoes develop.
It may be a bit utopian, but it's a very interesting idea
(Willem Takken) Couldn't humans capitalize on mosquitoes' natural resistance and use them to fight malaria? The scientists think it is conceivable that natural selection could be given a little help and targeted eradication of malaria-sensitive insects. In fact, there is a fungus that could be of great service: it seems to primarily kill mosquitoes infected with Plasmodium. Release of this fungus could thus select for malaria-resistant Anopheles mosquitoes.
Willem Takken of Wageningen University, who was involved in research on the insecticidal fungi, thinks the idea is a bit utopian but interesting. If it can actually be implemented, it could possibly save the lives of many people living in malaria-prone areas.