You may think that having cerebral malaria is bad enough, but what if the parasite could manipulate your body odour and make you even more attractive to blood-feeding mosquitoes?
It’s a disturbing prospect both for the patient and the wider population: mosquitoes pick up malaria when they feed on infected human blood and in this way they can spread the disease when they bite again.
A multinational research team from the U.K, Kenya and the Netherlands have tried to answer this question in a study lately published in PNAS.
It’s already known that animals infected with malaria appear to be more attractive to mosquitoes but until now, no one had explored what happens to the chemical make-up of human odour when someone is naturally infected with malaria.
The researchers used odour samples collected via socks worn by Kenyan schoolchildren to see whether the mosquitoes had a preference between the smell of children who were infected with malaria and those who were not.
They found that children with any stage of malaria infection were more attractive to mosquitoes than their uninfected classmates.
The children, who were tested and treated if infected, were asked to wear socks for 20 hours. A few weeks later the same children, now confirmed free from malaria, provided another sample. These were used in a choice experiment with caged Anopheles gambiae mosquitoes, the insect responsible for the majority of malaria transmission in the region.
The cocktail of volatile compounds that rise from our skin contains only a small number the insects use to locate human prey so the researchers continued their study to identify whether the presence of these compounds were different between the infected and uninfected children.
They found that the samples from infected children contained more organic hydrocarbons in the form of three aldehydes, heptanal, octanal and nonanal, than their classmates.
“These are fairly common smells, which are described as fruity or grassy,” said Dr Jetske de Boer, a study author and researcher in entomology and chemical ecology at Wageningen University & Research in The Netherlands. “Now that we have identified and quantified the aldehydes associated with malaria infection, we understand more of the parasite’s infection route.”
To identify these compounds the researchers collected samples from the feet of 56 children using a method called air entrainment. They also measured how infected the children were by measuring the density of malaria parasites in blood samples.
Over three sampling time points, 117 foot odour samples were collected from these 56 children, including 16 samples from parasite-free children and 101 samples from individuals with different parasite densities. These odour samples were analysed by gas chromatography, a method that separates and quantifies different compounds, to reveal the differences in odour profiles according to the number of parasites the children had in their blood.
The researchers also studied how the mosquitoes responded to the different odours by connecting the mosquito the antennae to microelectrodes and used wind tunnel experiments to measure the behaviour of mosquitoes towards aldehydes.
This sequence of experiments showed that these aldehydes were both detected by mosquitoes, and the more malaria parasites a child had in their blood, the more of these compounds they produced.
“Our work provides evidence that human hosts become more attractive to malarial mosquitoes during infection,” said Dr Mike Birkett, one of the study authors and a chemical ecologist in the Department of Biointeractions and Crop Protection at Rothamsted.
He added: “Identification of the volatile human-derived compounds that cause this phenomenon provides opportunities to develop these compounds as biomarkers of malaria and as components of chemical lures to trap mosquitoes”.
This work can be seen in the wider context of malaria control as Dr Ailie Robinson, a researcher at the London School of Hygiene and Tropical Medicine and lead author or the paper, pointed out: “The more that we understand about the intricacies of vector-parasite-host interactions and specifically the transmission of disease, the better equipped we are to control it.
“You never know when or where the next major discovery will happen, that might enable new methods to prevent or treat disease.”
She explained that if this finding of attractive, malaria-associated compounds translates into more mosquito biting in areas where malaria is common, this could profoundly affect the way the disease spreads through the population, something that needs to be fully understood.
Categories: Peer reviewed research news