How wind lures mosquitoes

Spread of malaria depends on the direction of wind

 
By Ishita Das
Last Updated: Saturday 04 July 2015 | 02:50:09 AM

imageA STUDY conducted in coastal Kenya has found a fascinating link between wind direction and transmission of malaria in nearby households. Geographical features, other than distance from water bodies or potential larval sites, that lead to formation of malaria hotspots are not fully understood. This study was initiated to explore additional factors that may be significant contributors to the formation of such hotspots.

The female anopheles needs to take blood meals from human hosts to nourish her eggs and then find a suitable water body to lay them. It is during blood meal that the mosquito may acquire and consequently transmit malarial parasites. The distance between the larval site and the blood meal also has a ‘fitness cost’ (common term in evolutionary biology, where the cost in performing an activity, in terms of energy, is calculated versus the benefit to the organism) that affects the survival of mosquitoes.

For the study, the team from Imperial College and University of Oxford in the UK analysed blood samples of 642 children in Kilifi district to look for the malarial parasite Plasmodium falciparum.

As expected, the study found an inverse relation between the distance of larval sites to households and incidence of malaria. But the spatial distribution of hotspots had a loophole. The most affected households lay north of the larval sites, instead of surrounding them in all directions in a circular pattern.

This made the team believe something else was also guiding the formation of the hotspots. Female mosquitoes locate hosts by a variety of olfactory cues. For locating any source of food any animal or insect travels the direction of the smell. Since the odours can only be carried downwind, it is obvious that the source of the odour lies upwind. “I had been interested in wind because I’ve been trying to develop models of how mosquitoes search and find blood meal hosts starting from larval habitat,” says David Smith, professor at the department of epidemiology at John Hopkins School of Public Health in the US, and co-author of the study.

They found the location of the hotspots in the Kilfi District coincided with this theory, the wind direction reports, indicated the direction to be southerly about 90 per cent of the time. The hotspots were mainly found upwind (to the north) of the larval sites.

So, can the same rule be applied to other diseases spread by mosquitoes? “We still need to find out if this is a general phenomenon. It may vary between different Anopheles species and even more likely between different genera (like Aedes),” says Philip Bejon of Center for Vaccinology and Tropical diseases, University of Oxford.

The study was published in the February 14 issue of Nature Communications. Bejon adds that the study results can be used by malaria control programmes.

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  • Interesting article from

    Interesting article from Ishita Das. Defining 'upwind' as meaning downwind did nothing to add clarity to the conclusions I managed to draw from it. Upwind in a southerly is south, Ishita.

    If the malarial hot spots are to the north of the larval sites, as stated, they suggest that the mozzies keep flying upwind before laying their eggs after feeding on blood. If so, newly hatched ones must fly downwind to mate before they search for their prey when flying back upwind on their way to lay their eggs in the area they pupated.

    Clearly the 'fitness cost' is least, if this is their cycle, because the immature females would get a free ride on the prevailing wind so that more of them would survive the trip to mate and return. One wonders how much of the blood feed helps them tackle the head wind and how much goes towards developing their eggs.

    With climate change shifting the range of malarial mosquitoes south down the east coast of Australia and probably the US, this sort of research should receive a great deal more funding from the West than it has in the past, to everyone's benefit. However, all this concentration of the behaviour of the messenger (the mosquito) rather than the message (the malarial parasites) seems rather out of date.

    Posted by: Anonymous | 7 years ago | Reply
  • Dear Confused, Upwind did not

    Dear Confused,

    Upwind did not mean downwind. Odors are carried downwind, as clearly any vapors or particles that travel with the wind cannot travel opposite the wind direction. The direction at which the wind is flowing is traditionally called 'downwind'. Whereas mosquitoes have wings and can travel in any direction they want ( unless the wind is overpowering, which is only during storms or strong gusts of wind). Once human odors are carried downwind, mosquitoes trace their origin by flying 'upwind' . This is quite logical, but would require a moment of thought.

    Posted by: Anonymous | 7 years ago | Reply