Mosquitoes &Surveillance Bill Brieger | 03 Oct 2009 08:44 am
It takes a village to understand malaria transmission
Malaria researchers in Niger have been observing malaria transmission and mosquito populations in two villages over two years and discovered major differences even though the settlements are only 30 km apart. Finding a 10-fold difference in mosquito density, the authors conclude that, “The highly focal nature of malaria in the Sahel makes detailed representation necessary to evaluate village-level risks associated with hydrology-related vector population variability.”
Other factors affecting transmission differentials include seasonal availability of nutrients and predators. Ultimately the authors recommend, “Topography, vegetation, soil type differences as well as shallow groundwater behaviour must all be incorporated at appropriate scales in order to accurately evaluate malaria transmission at the village scale using coarse resolution climate models.” These issues should inform ‘malaria early warning systems’ at a more focused level, the village.
The researchers were concerned about, “The sudden appearance of widespread but temporary water pools gives rise to a rapid increase in mosquito populations clustered around human habitation, resulting in the highly focal malaria transmission that is a characteristic of Sahel villages.” While these local variations are known, the problem arises when computer models lump together areas of tens to hundreds of kilometers in size together, missing the local differences that imply different control strategies.While the overall climate and vector behavior were not different, local hydrology was. The researchers specifically modeled differences in “local topography, distributed land cover type, and subsurface hydrology environment.”
Such differences in transmission dynamics have also been reported for urban malaria. For example, in Luanda, Angola transmission was more than four times greater in areas 15 km or more beyond the city core, compared with the central areas. In Dakar, Senegal transmission was highly focal, and “no mean figures for transmission would provide a comprehensive picture of the situation; risk evaluations should be conducted on a local scale,” even for areas only a few kilometers apart. Urban agriculture provides another example of very focal transmission.
A the present moment when countries are focusing on large efforts to ‘scale up for impact’ the small scale or focal differences might be ignored. As we move along the pathway toward elimination, more focal surveillance and intervention will be essential for mopping up transmission and monitoring against reinfestation. Now is the time to build those surveillance systems so that countries and communities will not be caught unaware when malaria attempts to make a comeback some years hence.