Wednesday, 3 March 2010

Dengue fever

Dengue is a mosquito-borne virus which affects some 50 million people a year. A report by the BBC blames climate change for the growth in Dengue:

"Some scientists say climate change is to blame, with rising temperatures and increased rainfall boosting the number of mosquitoes, which carry the disease."

However, another piece by the BBC says it is overpopulation, slum conditions and air travel that are to blame. A much more plausible explanation. It is far too easy to point the finger at climate change without a shred of evidence. However.

It was actually a third BBC report on Dengue that I was searching for when I came across those two articles. Entitled 'Lame' mosquitoes to stop Dengue, the piece explains how a team of scientists has genetically-engineered male mosquitoes to carry a dominant gene that prevents flight muscles from forming in the female. Original article: [Female-specific flightless phenotype for mosquito control, Fu et al, Proceedings of the National Academy of Sciences, February 22nd 2010] (free access to pdf of article)

Now, will it work? Will releasing sterile males into an outbreak population (or an endemic one) reduce the numbers of mosquitoes? At first sight, the plan seems flawless. We can release thousands of males (they don't bite, remember - only the females do), each carrying a little time bomb - a genetic one. Each female that mates with one of our males will produce two kinds of offspring: males with the inhibitor gene but that are unaffected by it, and females that can't fly, and thus can't feed, or reproduce. What's not to like?

The plan does have potential, but it is most likely of short-term effect. Natural selection is a wonderful thing, and there will be very strong selection against our males (because they produce half as many viable offspring as a non-modified male). The question really is whether such hi-tech solutions are really of any point, when cheap environmental solutions could be put in place much more easily and with permament effect.

For instance, putting lids on water butts might be an effective way to reduce standing water for mosquitoes to breed in (if this sounds naive, it shouldn't - Dengue is present in built-up areas, including relatively affluent ones). Clearly such an approach is laughable where waterways are little more than open sewers... but building new sewage systems not only combats Dengue, it combats a bunch of other stuff too.