Animal Waste, Water Quality and Human Health

286Animal Waste, Water Quality and Human Health(e.g. animal excreta or storm water runoff storage) with passive natural purification(e.g. reedbeds or constructed wetlands).8.2 A STEP-WISE APPROACH TO MEDIATINGWATERBORNE ZOONOSES8.2.1 Weight of evidenceThe disease cycle of certain microbial pathogens leaves no room for doubt as totheir mode of transmission. A good example of this is malaria, a commoninfectious disease caused by protozoans of the genus Plasmodium which aretransmitted to humans by females of mosquito vector species belonging to thegenus Anopheles. Intimate knowledge of the parasite’s life cycle providesevidence as to which interventions may prove successful in interruptingtransmission. Options include the reduction of mosquito population densities, areduction of the vectorial capacity or the prevention of human/vector contact. Areduction of mosquito population densities can be achieved by environmentalmanagement (drainage and other methods of eliminating mosquito breedingplaces), biological control (predators and parasites of anophelines) or chemicalmeasures, or a combination of these. This will only interrupt the malariatransmission in situations where there is a linear relationship between densityand transmission level. Such situations may exist in fringe areas (altitudeboundaries, forest fringes, latitudes where temperature becomes a limitingfactor) or man-made situations (urban areas or irrigation schemes). In large areaswhere malaria currently continues to be a public health problem, densities wouldhave to be reduced by several orders of magnitude in order to have an impact ontransmission levels.Spraying with residual insecticides, the intervention on which a partial successtowards malaria eradication was based in the 1950s and 1960s, reduced the lifespan of female mosquitoes so their capacity to complete a full cycle of theparasite is significantly curtailed and, hence, the associated entomologicalinoculation rate. This leads to sharp falls in levels of endemicity, but theapproach is highly vulnerable to the development of insecticide resistance.The use of barriers (mosquito nets, screening of windows and eaves) to separatemosquitoes from humans at peak biting times) has a history going back to thediscovery of the transmission of malaria by mosquitoes, but has gained newmomentum with the development of insecticide-impregnated nets in the 1980sand their large scale deployment since the late 1990s. In the context of thisbook, the use of cattle as a specific barrier separating human communities fromthe breeding places and resting sites of zoophilic anophelines (i.e. Anopheles