Malaria parasites can manipulate the scent of their mammal hosts to attract more mosquitoes -- and at a time when the blood is especially infectious. According to a new study published in the Proceedings of the National Academy of Sciences this week, malarial mice release a different odor profile from that of uninfected mice, rendering them more attractive to mosquitoes.
Mosquitoes acquire the malaria parasite, Plasmodium, when they feed on the blood of infected individuals. Then they become a disease vector, transmitting the parasites to subsequent hosts they take blood meals from. Previous work have shown that people and animals who are already infected with malaria are more attractive to mosquitoes than uninfected individuals. That’s terrifying because it suggests that the parasite coaxes the host to produce cues that are attractive to mosquitoes -- enhancing the likelihood of spreading the infection more. (A parasite that manipulates the victim and its vector... sounds like zombifying behavior.)
So, how do the mosquitoes know when someone is infected? A team led by Mark Mescher from the Swiss Federal Institute of Technology investigated the mechanism in a mouse model of malaria infection. They placed mice infected with rodent malaria, Plasmodium chabaudii, in a wind chamber, and Anopheles stephensi mosquitoes downwind thronged to their odor. The team observed the infection for at least 45 days.
Mosquitoes were much more attracted to the scent of infected mice, especially ones who’ve been infected for 10 to 20 days. That’s when the malaria gametocytes -- or sex cells, the transmissible stage -- are near their highest concentration in the mouse’s blood, making the blood is highly infectious. That’s a crucial time in the life cycle of the parasite, when they need to be passed back to a mosquito to reproduce.
The team used gas chromatography to separate out and identify the volatile compounds released by and collected from infected and uninfected mice in glass chambers (pictured). They found that the parasite modified the levels of most of the odorous compounds. The levels of compounds that are attractive to mosquitoes increased during the infection. Some levels were more elevated than others, and if we can somehow mask the corresponding compounds in people, maybe we could fend off Plasmodium-carrying mosquitoes.
Another interesting thing about this phenomenon: The blood is highly infectious when all the acute symptoms of the infection have subsided, showing again how it isn’t the outward symptoms or behavior that draws the mosquitoes. “One of the major potential values of this research is if it can help us identify people who do not show symptoms of the disease,” Mescher tells BBC. "Without symptoms people carry the disease without treatment and still transmit it.”
[Via Scientific American]
Images: Nick Sloff, De Moraes & Mescher Research Group
