The parasite that causes malaria can change the way you smell, making you more attractive to mosquitoes, according to a new study. The work may help explain why the disease is able to spread so effectively.
The new study adds important details about how human odor is influenced by malaria, says Audrey Odom John, a parasitologist at Washington University School of Medicine in St. Louis, Missouri who was not involved in the study. “This is very cool, and it’s been needed for some time.”
Earlier studies have found that infection with Plasmodium parasites, which cause malaria, can influence how animals smell and people’s attractiveness to mosquitoes. The new research, published today in the Proceedings of the National Academy of Sciences, expands on them by testing a larger number of people outside a lab, and by dissecting body odors to see which chemicals matter to mosquitoes.
The socks of 45 Kenyan schoolchildren, some infected by malaria, played a key role. To see whether mosquitoes preferred the smell of people with malaria, the scientists placed socks the children had worn in a contraption of two boxes joined by a tube. They put mosquitoes into the tube and tracked which sock they flew toward.
The bugs showed a preference for socks worn by infected children. When given a choice between those and socks worn by the same child 3 weeks later after medication had wiped out the infection, 60% of the mosquitoes flew to the infected socks. The mosquitoes showed no preference between two pairs of socks collected at different times from children who never showed a sign of malaria.
To learn what caused the different response, scientists sent foot odor samples from 56 kids through a device that analyzed which chemicals were present, then puffed each chemical one at a time over mosquito antennae attached to tiny electrodes. The test zeroed in on a handful of chemicals that activated the antennae and that also were found at higher levels in infected children. The chief ones were from a class of chemicals called aldehydes, including heptanal, octanal, and nonanal. Those chemicals easily vaporize and are common additives in perfumes. Several are described as having a “fruity” odor, including heptanal, which is found in allspice.
If infected people smell better to mosquitoes, that could increase the likelihood that the insect sucks up the parasite along with its blood meal then spreads the infection by biting someone else, says study co-leader James Logan, a medical entomologist at the London School of Hygiene & Tropical Medicine. “The malaria parasite is sort of manipulating the system both in the mosquito host and the human host,” he says. “It’s very clever.”
His team is also looking at whether Plasmodium changes mosquitoes’ antennae, making insects carrying the parasite more tuned to the smell of humans.
Scientists aren’t certain what drives the odor difference. It’s possible higher aldehyde levels come directly from the parasite or when fat in the person’s cells breaks down during an infection, according to the study. It’s also possible the different odor isn’t unique to malaria. “This is probably specific to malaria,” Logan says. “But it could be that other infections could cause the same effect.”
There is growing interest in the potential role odor plays in spreading the disease and how it could be used to help diagnose and reduce the spread of the illness. Researchers at Durham University in the United Kingdom are studying whether dogs can be trained to sniff out people with malaria. Odom John is investigating the potential for a breath test to identify malaria infections. The new body odor work could also help lead to a diagnostic tool, or improve mosquito traps, Logan says.
But getting the right chemical cocktail is tricky. The new paper found small tweaks in labmade scents influenced whether mosquitoes were enticed. In one seemingly paradoxical case, mosquitoes were drawn to a scent spiked with a small dose of heptanal, but not when the amount of the chemical was increased slightly. “It’s a little bit like Chanel,” says Odom John, referring to the famous perfume. “Sometimes a little is nice but a lot is not.”