Malaria control: the power of integrated action
Vector control tools within an IVM framework
In integrated vector management, appropriate vector control solutions are highly dependent on the local behaviour of malaria vectors, on local ecological, hydrological and environmental conditions, and on patterns of disease transmission. Continuous feedback on vector breeding requirements and behaviour and on disease incidence, and subsequent fine-tuning of strategies, is a prerequisite to applying IVM principles to more effective vector control. Indeed, the early history of environmental management offers ample evidence of projects that failed because the local vector behaviour was poorly understood, i.e. brush-clearance programmes carried out for a vector that thrives in sunlight. Below are brief summaries of key issues that may arise.
Better engineering design of dams, irrigation schemes that allow for alterations in level and flow of water, and flushing of reservoirs can help reduce the availability of vector habitats (15). In addition, irrigation schemes that permit intermittent irrigation of fields, as well as alternation between cycles of irrigated and non-irrigated crops, have proven very successful in controlling Anopheles mosquitoes in rice-growing regions of China, India, and other parts of Asia (16,17). Such schemes control vectors by disrupting breeding cycles (18,19). (See Section 6, Case-studies.) However, improved design or redesign of water resource systems, irrigation systems, and dams is most likely to occur when major infrastructure investments are being planned, and thus it is critical that health and environment issues be considered by development actors at the outset of design processes through effective health impact assessment (see also Section 4, Intersectoral cooperation). For links to resources on health impact assessment, search the Impact Assessment page of the HELI portal. Anne Marie, can we create an interactive link here?
In specific settings, time-limited changes in local vegetation, shade, and drainage patterns provide an effective way to reduce vector habitats. For instance, the creation of shade over the breeding grounds of malaria vectors that prefer sunny locales can help reduce vector propagation. Conversely, for malaria vectors that thrive in shadier habitats, the removal of overgrowth, weeds, and aquatic plants may significantly reduce breeding potential and thus vector abundance. In Oaxaca, Mexico, the clearance of algae from rivers, in a sustained community action programme, has been an important component in an integrated nationwide malaria control programme that has reduced malaria incidence from 15 121 cases in 1998, to 4996 cases in 2001 (20).
Human settlement siting and management
More strategic placement of new human settlements away from potential malaria- breeding areas can also reduce transmission, since malaria vectors typically do not travel more than a few kilometres from their breeding grounds. Better management and control of man-made sites where malarial mosquitoes may easily reproduce – such as water wells and bore holes – may help reduce malaria breeding close to human settlements (15,21). Some vectors prefer to take blood meals from animals – instead of, or along with, human hosts. Therefore, strategic placement of animal pens and corrals may potentially divert vectors from human to animal hosts (zoo prophylaxis). Ongoing research in Kenya on the interactions between malaria, livestock, and agriculture is examining if and how different patterns of livestock management may actually help reduce malaria disease transmission to humans (22). (See Section 6, Case-studies.)
Natural predators for larval control
Along with environmental management of malaria, a range of biological controls have been identified. This approach may kill mosquitoes or their larvae in a more targeted manner than chemicals, averting certain impacts on health and local environments. The best known of those controls include various species of larvivorous fish and biolarvicides, such as Bacillus thuringiensis israelensis and Bacillus sphaericus (15). Maintenance of larvivorous fish stocks and repeated applications of biolarvicides, however, require substantial community awareness and involvement – an issue addressed in Section 4, Challenges and opportunities for IVM. Neem oil, extracted from the seeds of the neem tree, also has been successfully tested as a biolarvicide..
Chemical larvicides, adulticides, and itns
When other measures are ineffective or not cost effective, IVM makes judicious use of chemical control methods. Indeed, chemical tools remain particularly important in areas of intense disease transmission where environmental management strategies cannot reduce vector densities sufficiently to actually make an impact on disease incidence. As noted, chemical methods, including indoor residual sprays and space spraying may thus play a very important role in reducing disease transmission, by shortening or interrupt the lifespan of adult vectors (2,3). In urban settings, application of chemicals to breeding places (larvicides) plays an important role in keeping vector populations down. Use of ITNs, e.g. protective nets impregnated with insecticides and covering beds, living quarters, or water containers, represents a recent innovation, combining the synergies of judicious chemical use with physical barrier approaches.