Introduction, transmission and aggravation of malaria in desert ecosystem of Rajasthan, India.

BACKGROUND & OBJECTIVES: Malaria is an important public health problem in northwestern desert part of Rajasthan. Since desert malaria is seasonal or unstable, there is a need to study its epidemiology in the totality to address factors like how malaria is introduced into desert every year and what are desert-specific transmission risk factors leading to epidemics? METHODS: Twenty-six villages in irrigated, semi-irrigated and non-irrigated settings of the desert have been selected. Periodic investigations were undertaken in all the seasons from 2001 through 2002 to determine causes of introduction of disease, the factors involved in its transmission and the epidemic risk factors. Standard methods/procedures for mosquito collection, preservation and identification and for parasitological studies were employed. Rainfall data were analysed and correlated. Results: Importation of malaria infection through individuals was the major cause of introduction of disease in all the 12 villages showing some initial load of infection. Areas having higher population of cattle in relation to humans (human to cattle ratio of 1:25) had less transmission of disease (ratio of primary to secondary cases 1:3) as compared to relatively less cattle (human to cattle ratio 1:3) resulting into high transmission of disease with 1:47 ratio of primary to secondary cases. Introduced through migration, prospective transmission of malaria was observed as a resultant interaction of density of three components namely malaria cases, vector density and cattle to human ratio. Rainfall more than average when coupled with more cases of imported malaria leads to the eruption of malaria epidemics. INTERPRETATION & CONCLUSION: Importation of malaria cases was the major cause of seasonal introduction of malaria in the desert. This initial or primary parasitic load available in a village leads to a particular transmission quantum depending upon the density of vectors, human and cattle population at a particular time. Areas with more imported cases when coupled with profuse vector population lead to epidemic malaria situations in the desert.

Malaria outbreak in the Indira Gandhi Nahar Pariyojna command area in Jaisalmer district, Thar Desert, India.

A focal outbreak of malaria occurred in the villages situated close to the main Indira Gandhi canal near Ramgarh in Jaisalmer district, western Rajasthan. Stagnation of water over a month's period in the main canal as well as long standing rain water in the form of expansive lakes near these villages formed vast breeding grounds for the vectors like Anopheles culicifacies, along with A. stephensi already breeding in the 'tanka' and 'beri' in the epidemichit villages. Rapid mass blood surveys along with other entomological and parasitological investigations were conducted in four of the ten affected villages, viz., Seuva, Raghwa, Raimala and Sadhna. A total of 992 specimens belonging to four vector species were sampled, namely, A. stephensi (47.4%), A. culicifacies (41.0%), A. subpictus (11.2%) and A. annularis (0.4%). Epidemiologically, about one-fourth of the examined persons were positive (SPR 25.5%), although Plasmodium falciparum dominated the parasitaemia (49.5%). Available data are indicative of changed malariological scenario in the Indira Gandhi Nahar Pariyojna command area, where epidemics are regular features every year.