Reference
Haque, U., Hashizume, M., Glass, G.E., Dewan, A.M., Overgaard, H.J. and Yamamoto, T. 2010. The role of climate variability in the spread of malaria in Bangladeshi highlands. PLoS ONE 5: 10.1371/journal.pone.0014341.
Background
The authors write that "malaria is the most important tropical and parasitic disease in the world," noting that in 2008 there were an estimated 243 million cases that accounted for an estimated 863,000 deaths, according to the World Health Organization. And in Bangladesh, where malaria is endemic, they report that in that same year, malaria morbidity and mortality totals were 84,690 and 154, respectively; yet they indicate that no study had ever examined the relationship between malaria epidemics and climatic factors in Bangladesh. So they decided to do it.
What was done
Working with monthly malaria case data pertaining to the malaria endemic district of Chittagong Hill Tracts from January 1989 to December 2008, Haque et al. looked for potential relationships between malaria incidence and various climatic parameters (rainfall, temperature, humidity, sea surface temperature and the El Niņo-Southern Oscillation), as well as the normalized difference vegetation index (NDVI), which is a satellite-derived measure of surface vegetation greenness.
What was learned
The six scientists report that "after adjusting for potential mutual confounding between climatic factors there was no evidence for any association between the number of malaria cases and temperature, rainfall and humidity," and they say that "there was no evidence of an association between malaria cases and sea surface temperatures in the Bay of Bengal and NINO3." Instead, they found that "the best leading indicator of the number of malaria cases was NDVI at a lag of 0-3 months, and that NDVI was negatively associated with malaria cases," such that "each 0.1 increase in monthly NDVI was associated with a 30.4% decrease in malaria cases."
What it means
Haque et al. say "it seems counterintuitive that a low NDVI, an indicator of low vegetation greenness, is associated with increases in malaria cases," since the primary vectors of the disease in Bangladesh are associated with forests. And in light of this surprising result, they state that their study "draws attention again to the complex nature of the relationship between malaria and climate," which in the case of the highlands of Bangladesh appears to be non-existent.