In the semi-arid region of northwest India, malaria is at the edge of its geographic distribution and therefore under an epidemic regime. Here, sporadic large populations of the mosquito vectors are triggered by rainfall events during the monsoon season. In these same areas, large scale irrigation projects have changed, and continue to change, the landscape and also the coupling between rainfall and epidemics. Concerns have been raised about the consequences of such change for the ecology of A. culecifacies, the main malaria vector in the region. At the same time, however, these developments have contributed to the alleviation of poverty and the improvement of socioeconomic conditions, and may therefore enable better disease prevention and control.
Extensive time series data of confirmed Plasmodium falciparum cases at the sub-district (‘taluka’) scale provide an opportunity to explore how land-use changes, resulting from irrigation, have influenced the temporal dynamics of malaria, its spatial distribution, and efforts to control the disease. Here, we examine (1) whether the spatial pattern of irrigation is characterized by distinct regions with similar spatio-temporal disease dynamics; (2) how control efforts and agricultural productivity, presumably enabled by land-use, affect the temporal dynamics of the disease.
Results/Conclusions
With remote sensing techniques and a Bayesian statistical method specifically developed to identify spatial groups with similar spatio-temporal dynamics, we identify two distinct groups of talukas corresponding respectively to irrigated and poorly irrigated areas. Large epidemics occur primarily in the driest and most poorly irrigated talukas. Talukas that have experienced irrigation for decades exhibit the lowest levels of the disease. Those that have experienced the most pronounced increase in irrigation in the last decade also exhibit low incidence due to high control efforts, but the risk of malaria remains high. Moreover, inter-annual variability in cases shows a strong signature of a ‘reactive’ control strategy that responds to past epidemics, with surprises arising when climatic determinants, socioeconomic conditions, and control efforts vary in opposite directions. These results support the view that in the long term, the benefits of irrigation outweigh its ecological costs on malaria in semi-arid environments. During transitions from low to moderate levels of irrigation, some evidence suggests that the malaria risk can be exacerbated, necessitating high levels of control. We discuss the importance of including climatic information in the implementation of control efforts.