A review of the emergence of Plasmodium falciparum-dominated malaria in irrigated areas of the Thar Desert, India.

Recently, there has been a resurgence of malaria in several parts of India, and the Thar Desert in north-western India, is currently suffering from the impact of repeated annual epidemics. Nearly all malaria epidemics in the Thar Desert have come about with the progression of canal-irrigation work, particularly the massive Indira Gandhi Nahar Pariyojana (IGNP). Therefore, the Thar Desert provides an excellent model for understanding the underlying factors responsible for the exacerbation of malaria, pathways of evolution of the epidemics, succession in anopheline fauna, changes in the vector breeding and feeding preferences and, most importantly, the possible repercussions of mismanagement of irrigation systems. Before the initiation of canalised irrigation only Anopheles stephensi, breeding exclusively in household and community-based underground water reservoirs, and transmitting malaria at a low level, was prevalent in the interior of the Thar Desert. Since the 1980s, extensive irrigation with water from three different canal systems has altered the desert physiography, vector preponderance, distribution and vectorial capacity, whilst triggering the emergence of Plasmodium falciparum-dominated malaria in the virgin levees of the Thar Desert. The major objective of bringing the Himalayan waters to the xeric environment of the Thar was to transform it into verdure through growing irrigation-intensive crops like paddy, groundnut, cotton, mustard, wheat and sugarcane, besides providing drinking water to the desert dwellers. The change in crop pattern, retention of high surface moisture, and excessive canalisation rife with mismanagement of irrigation water have attracted several anophelines, including Anopheles culicifacies, which were earlier unknown in the desert. Thus, A. culicifacies has penetrated into the interior of the Thar Desert, along with irrigation and is now established in vast areas covered by the IGNP project. The distribution of P. falciparum-dominated malaria in the Thar Desert is more or less synchronous with the spread of IGNP-related irrigated agriculture and of A. culicifacies.