Human modification of the natural environment continues to create habitats in which mosquitoes, vectors of a wide variety of human and animal pathogens, thrive if unabated with an enormous potential to negatively affect public health. Historic examples of these modifications include of impoundments, dams, and irrigation systems that create havens for the mosquitoes that transmit malaria, dengue, and filariasis. Additionally, contemporary deforestation appears to be associated with the expansion of mosquito distributions and the increase in mosquito-borne disease transmission. These observations are not unique to the developing world, as urban sprawl also contributes significantly to mosquito habitats and offers a sanctuary to some vector populations. With foresight and planning, most of these systems can be appropriately managed to control vector populations and pathogen transmission. The key to disease control is developing an understanding of the contribution of human landscape modification to vector-borne pathogen transmission and how a balance may be achieved between human development, public health, and responsible land use.
Five assessments covering less-developed countries have identified a ‘land balance’, available for future cultivation, using the approach of inventory and difference: assessment of the area cultivable, and subtraction of the area presently cultivated. All arrive at a balance of 1600–1900 Mha, about twice the present cultivated area. The supposed existence of this spare land is widely quoted in forecasts of capacity to meet the food requirements for future population increase. It is argued here that these estimates greatly exaggerate the land available, by over-estimating cultivable land, under-estimating present cultivation, and failing to take sufficient account of other essential uses for land. Personal observation suggests that the true remaining balance of cultivable land is very much smaller, in some regions virtually zero. An order-of-magnitude estimate reaches the conclusion that in a representative area with an estimated ‘land balance’ of 50%, the realistic area is some 3–25% of the cultivable land. This speculation could be tested by directly attempting to find such land in areas where it is supposed to exist. The impression given by current estimates, that a reserve of spare land exists, is misleading to world leaders and policy-makers.
Environmental effects on the transmission of many parasitic diseases are well recognized, but the role of specific factors like climate and agricultural practices in modulating transmission is seldom characterized quantitatively. Based on studies of transmission in irrigated agricultural environments in western China, a mathematical model was used to quantify environmental impacts on transmission intensity. The model was calibrated by using field data from intervention studies in three villages and simulated to predict the effects of alternative control options. Both the results of these interventions and earlier epidemiological findings confirm the central role of environmental factors, particularly those relating to snail habitat and agricultural and sanitation practices. Moreover, the findings indicate the inadequacy of current niclosamide-praziquantel strategies alone to achieve sustainable interruption of transmission in some endemic areas. More generally, the analysis suggests a village-specific index of transmission potential and how this potential is modulated by time-varying factors, including climatological variables, seasonal water-contact patterns, and irrigation practices. These time-variable factors, a village's internal potential, and its connectedness to its neighbors provide a framework for evaluating the likelihood of sustained schistosomiasis transmission and suggest an approach to quantifying the role of environmental factors for other parasitic diseases.