You smell like chicken (to a mosquito)

Although swine flu is the virus of the year, there are plenty of other viruses out there, like West Nile virus. This new paper looks at the interplay between the virus, birds, humans, and the sensory systems of mosquitoes.

Insects live in a different sensory world than humans. Insects react to molecules (smells) the way we react to photons (vision). Female mosquitoes use their sense of smell to track down larger animals so that they can get a blood meal, which they need to reproduce. Syed and Leal look at the sense of smell in a mosquito (Culex pipiens; pictured).

Because it’s already known that these mosquitoes feed on both humans and birds, the authors extracted odourants from both. Strangely, they present data from five different ethnicities... but three of them are represented by only one or two subjects. I’m not convinced that sample sizes of one or two tell us anything useful. The authors claim that there is no difference, so I would rather have just seen the pooled data and leaving ethnicity out of the picture.

The antennae has three kinds of small sensory hairs (sensilla).

• A1 sensilla (sharp trichoid sensilla), which come in long and short varieties. Each sensillum has two different sensory neurons in it, which can usually be distinguished by the size of their spikes.
• A2 sensilla (blunt trichoid sensilla), which also have two different sensory neurons in them.
• A3 sensilla (grooved pegs).

Much of the paper consists of throwing chemical after chemical on the antennae and looking for neural responses to the chemicals. Their big finding is that one of the sensory neurons in A2 sensilla is highly sensitive to a chemical called nonanal. These sensory neurons respond to nonanal at concentrations about a hundred times lower than any other chemical tested.

That matters, because they show nonanal is the major chemical in the odours of the birds they tested, and it’s a major chemical in human odours, too. That these chemical are present in things mosquitoes feed on, and that a whole class of neurons are highly tuned to that chemical, strongly suggests nonanal is used in blood-seeking behaviour. It also neatly explains why this mosquito could be a major source of viruses jumping from bird to human populations, and vice versa.

Once you know a bias, you can exploit it. Syed and Leal also baited mosquito traps, which normally use carbon dioxide to lure in the mozzies, with nonanal. Nonanal alone didn’t work as well as carbon dioxide alone... but the combination worked better than either alone. Although trapping mosquitoes probably couldn’t be used for mosquito control, it could improve mosquito monitoring.

Unfortunately, the organization of the paper is slightly disjointed. For instance, alternating the names from “thichoid sensilla” to “A2 sensilla” causes more re-reading that would otherwise be needed. The paper uses the acronym “EAD” in the figures in the Results well before they spell it what it means a few pages later in the Discussion section. (It means “electroantennographic detection,” a needlessly fancy way of saying extracellular recording.) Not cool.

Reference

Syed, Z., & Leal, W. (2009). Acute olfactory response of Culex mosquitoes to a human- and bird-derived attractant Proceedings of the National Academy of Sciences, 106 (44), 18803-18808 DOI: 10.1073/pnas.0906932106