Scientists know that humans infected with malaria are more likely to be bitten again by disease-carrying mosquitoes – and now they think they know why.
Malaria may alter the way people smell to make them more alluring to mosquitoes, a new study has revealed.
Experiments with mice suggest that the malaria parasite acts like a perfumier in the body by mixing fragrances to produce an irresistible cocktail of odours.
Scientists believe it is likely to have the same effect on humans and are studying volunteers in Africa to test the theory.
As well as aiding the search for new treatments, the work could lead to a method of screening symptom-free malaria carriers at risk of spreading the infection.
In order to complete its complex life cycle, the malaria parasite – the single-celled organism Plasmodium – must both be transmitted and ingested by the Anopheles mosquito via blood hosts.
Plasmodium in the blood is able to alter the host’s odour profile to attract the hungry insects and it even times the changes to coincide with when the host is highly infectious.
‘There appears to be an overall elevation of several compounds that are attractive to mosquitoes,’ said Professor Consuelo De Moraes, one of the scientists from the Swiss research institute ETH Zurich.
Tests showed that mosquitoes were most drawn to mice with large numbers of gametocytes – the parasite’s reproductive cells – in their blood.
When the mosquito consumes these cells along with the host’s blood, a new development cycle starts in the insect’s gut.
The scientists used a technique called gas chromatography to analyse Plasmodium’s effect on body odour.
They found no evidence that it generated unique odour components in the host. Instead, the parasite altered levels of compounds already naturally present, increasing most of them and raising concentrations of several more than others.
The resulting blend of odours was especially tempting to Anopheles mosquitoes.
Dr Mark Mescher, the lead author of the study who is also from ETH Zurich, said: ‘Since mosquitoes probably don’t benefit from feeding on infected people, it may make sense for the pathogen to exaggerate existing odour cues that the insects are already using for host location.’
Malaria infection appeared to leave its mark on body odour for life. Even when infected mice no longer displayed any symptoms, their smell showed that they still carried the parasite.
Not all stages of the disease smelled the same, however. Early and late stages of infection had different odour profiles.
Writing in the journal Proceedings of the National Academy of Sciences, the scientists concluded: ‘These findings…provide an important proof of concept regarding the identification of volatile biomarkers of malarial infection and the specific compounds identified should be viewed as promising candidates for further investigation, with work on malaria-induced changes in the odours of human subjects being an obvious priority for further research.’
