The Importance of Understanding Evolution for Public Health

While lots of people are scrambling to make [good] phylogenetic trees of the new swine flu sequences in the context of other flu viruses, another paper came out in this week's PLoS Biology that presents a really powerful argument for incorporating evolution into public health. Andrew Read, of Penn State University, and others both at Penn State and at the Open University in the U.K. just published the results of their work on late-life acting (LLA) insecticides, arguing that if you understand a little bit about natural selection, you just might be able to profoundly stack the deck in the global battle against malaria. Mosquitoes as a whole, suffer a high mortality rate - according to Read et al., the figure is around 10% per day or 20-40% per gonotrophic, or egg-laying, cycle. In order to transmit malaria, a mosquito must take one blood meal from an infected person and then survive long enough to need a second one, with the interval between these events being long enough for the parasite to develop to an infective stage. Because development to the infective stage typically takes 10-14 days in malaria-endemic regions, very few infected mosquitoes live long enough to actually vector the disease. The paper argues that conventional insecticides such as DDT and pyrethroids, classes of which are "early-acting" insecticides that kill 80% of mosquitoes they come into contact with, exert tremendous selection pressure on mosquitoes to evolve resistance, because they are robbing a large proportion of the whole population of all of their fitness. Conversely, LLA insecticides, which kill mosquitoes after their first gonotrophic cycle, impose far less natural selection and thus the corresponding selection for resistance is slower to evolve. Crunching some numbers allowed Read et al. to show that an insecticide that killed mosquitoes after 2 or more gonotrophic cycles could reduce the number of malaria-infectious bites by 99.2%. Even the insecticides that took longer to kill mosquitoes still showed drastic reductions: the 4-cycle killers showed a 94.2% drop in deadly bloodmeals. Because evolution of resistance itself bears fitness costs to mosquitoes, LLA could allow for some insecticides to be "evolution proof" - i.e. the time it would take for mosquitoes to evolve resistance to these sprays would be so long as to essentially be immortal.

Hopefully this study and others like it (e.g. Wargo et al., 2007) will be carefully read by those making public health decisions - because thinking long-term - in an evolutionary sense, not just a medical sense - is absolutely critical. And if you're not a public health official, read these studies anyway - they make great examples for teaching the importance of evolution to everyday life.