How mosquitoes sense us from afar -- and why we get West Nile

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Ever wonder how mosquitoes know where we are -- and that, furthermore, our skin is something they’d like to bite? The answer, for the Southern house mosquito, Culex quinquefasciatus -- which transmits West Nile virus -- seems to be a chemical called nonanal.

The odor we emit is loaded with nonanal, scientists at UC Davis reported in the journal Proceedings of the National Academy of Sciences. (They ascertained this by collecting odors, or ‘odorant bouquet,’ as they put it, emitted from the forearms of 16 men and women.) So, it turns out, are the odors emitted by birds (as ascertained from collecting gases emitted from a pigeon and chicken sitting in a glass chamber). This explains why birds (which are reservoirs for West Nile virus) and humans get bit by the same bug.

The mosquitoes sense nonanal by virtue of nerves in their antennaeand maxillary palps (the links can explain far better than I what a maxillary palp is). Moreover, this response to nonanal is ‘remarkably sensitive,’ the authors write, ‘even higher than the gold standard pheromone-detecting neurons in moths.’ The scientists suggest that this sensitivity allows mosquitoes to sense a bird or human from far away, then buzz in for the bite.

The researchers also showed that when mosquito traps were baited with nonanal, many more mosquitoes of the Culex family were collected. Mosquitoes also are attracted to the carbon dioxide we exhale -- and if you bait a trap with both chemicals, you get an even more crowded mosquito trap.

-- Rosie Mestel