Malaria parasite makes mosquitoes more likely to suck your blood
Mosquitoes seem to fancy human blood when they’re carrying malaria, in an apparent case of parasites directing their hosts’ behaviour.
Nature abounds with stories of parasites manipulating the organisms they live in to suit their own ends. There are eye flukes that make fish swim close to the surface so they’ll get caught by birds. There are wasp larvae that live inside other wasps and turn them into zombies. And there’s the notorious Toxoplasma parasite that makes rodents fearless so they get eaten by cats – and may also steer humans towards risky behaviour.
The malaria parasite, the single-celled organism Plasmodium falciparum, relies on mosquitoes for transmission, but it can only reproduce and develop in humans, and to a lesser extent other great apes. If the mosquito carries it into a cow or sheep, it reaches a dead end.
So Plasmodium has tricks up its sleeve to facilitate its life cycle. Infected humans become more attractive to mosquitoes. Mosquitoes infected with the immature stage of the parasite feed less to stay out of danger, and those infected with the transmissible stage feed more.
Amélie Vantaux at the Research Institute for Health Sciences in Bobo-Dioulasso, Burkina Faso and colleagues wondered if the infectious-stage parasite might also drive mosquitoes to seek human hosts over other sources of food.
They set up traps in three villages in Burkina Faso, some baited with human odour and some baited with calf odour. They analysed the blood in the mosquitoes’ guts to test for the presence of malaria parasites and determine what animals they had fed on.
The infection status of the mosquitoes didn’t affect which traps they were attracted to by smell. But there did seem to be a relationship with which animals the mosquitoes had bitten.
I fancy human tonight
Among mosquitoes infected with the infectious stage, 77 per cent had fed on humans. In mosquitoes infected with the uninfectious stage, the figure was 64 per cent. Uninfected mosquitoes were roughly the same, at 61 per cent.
Since the tests did not find an effect on mosquitoes’ odour preferences, some other effect must account for the increased propensity to bite humans. Perhaps the parasite influences the mosquitoes’ behaviour at close range, when they rely on vision or other sensory information to find a target, says Vantaux. Alternatively, it might change their choice of habitat or their activity patterns. “They could stay longer in houses after feeding on humans, or it could be that infected mosquitoes change their peak of activity to when humans are more available,” says Vantaux.
Using a mathematical model, the team estimate that this bias towards feeding on humans results in a 123 per cent increase in transmission potential when the mosquito population is roughly equal to the human population, and a smaller effect if the mosquito population is higher.
Jetske de Boer of the Netherlands Institute of Ecology says the study is important as it tests the hypothesis under field conditions. But it’s hard to be sure that malaria parasites are responsible for the effect. “It could be possible that the patterns can be explained by previous feeding on humans and therefore infectious mosquitoes are more likely to feed on humans again,” she says.