The fragrant waft of a black lentil dhal, the acrid perfume of newly laid bitumen, the delectable aroma of freshly ground coffee beans. Smells make the world a richer, safer and more memorable terrain than it would be without our olfaction in tact.
And we’re certainly not the only ones. Indeed, when it comes to olfactory fortitude, our abilities pale in comparison to some of nature’s aroma-detecting titans. Whereas human olfactory senses — and the number of olfactory receptor proteins in our noses — have been whittled away in favour of a large visual cortex, the truffle-foraging pig and the smell-a-rabbit-two-days-after-it’s-passed-by bloodhound have evolved extraordinarily keen senses of smell.
But for the Anopheles mosquito, there’s nothing quite like the smell of human flesh. And it seems that the mosquito’s penchant for Eau de humaine is heightened when infected with the malaria parasite (Plasmodium falciparum), according to a study published last week in PloS One.
In a simple yet effective bioassay, the team of entomologists from the London School of Hygiene and Tropical Medicine placed a worn sock (“human foot odor was collected on a nylon matrix”) into chambers containing either infected or uninfected mosquitoes. The infected mosquitoes were carrying sporozoite-stage parasites in their salivary glands — the stage at which they are ready for transmission to a human host if the mosquito feeds.
Unsurprisingly, all mosquitoes landed on the smelly sock (courtesy of study author Renate Smallegange) more often than an unworn control sock. But the worn sock attracted more infected mosquitoes to its enticing bouquet than uninfected mosquitoes. The malaria parasite had increased the foot odor–detecting ability of the mosquito, or else its attraction to it.
Parasites have previously been found to alter the sometimes complex behaviour of their hosts. Toxoplasma gondii can famously render a mouse undeterred by the smell of cat urine. This makes the hapless rodent more likely to be captured and consumed, thereby allowing the parasite to be transmitted to its definitive host, the cat.
In its quest to complete its life cycle, the malaria parasite must be transmitted from human to mosquito, and back again. Enhancing the human host–finding efficiency of the mosquito is one way of ensuring that the parasite makes the crucial journey into a human capillary.
The researchers haven’t yet identified how the parasite changes mosquito behavior, but predict that the mechanism lies in the mosquito olfactory system. They are currently investigating whether the olfactory receptor proteins on the insects’ antennae are altered during infection.
Reference: Smallegange, van Gemert, van de Vegte-Bolmer, Gezan, Takken, Sauerwein & Logan. (2013) Malaria infected mosquitoes express enhanced attraction to human odor. PLoS ONE 8(5): e63602. doi:10.1371/journal.pone.0063602