Integrated malaria vector control in different agro-ecosystems in western Kenya

Malaria is a complex disease and its transmission is a function of the interaction between the Anopheles mosquito vector, the Plasmodium parasite, the hosts and the environment. Malaria control has mainly targeted the Plasmodium parasite or the adult anopheline mosquitoes. However, development of resistant parasites and mosquito vectors requires the development of other strategies, such as larval control, which can be integrated in the current control programmes. To develop a larval control programme, the local vector species and their breeding characteristics need to be known.
This thesis describes the results of a study on mosquito ecology, with an emphasis on malaria vectors in different agricultural settings within two highland sites (Fort Ternan and Lunyerere) and a peri-urban area (Nyalenda) in western Kenya, and the development of practical and effective mosquito larval control strategies. To provide information about the ecology of local vectors, a longitudinal two-year study on the population dynamics and breeding characteristics of local mosquito species and malaria prevalence was undertaken in the respective study sites. In addition, community perception and knowledge on malaria, causes of malaria and the control of mosquito vectors were established through a questionnaire. This information allowed for the development of small-scale mosquito larval control strategies combining source reduction, environmental manipulation through provision of shade and biological control using predatory fish (Gambusia affinis) and application of the bio-larvicide, Bacillus thuringiensis var israelensis (Bti).
The main malaria vector species, Anopheles gambiae Giles sensu stricto and An. arabiensis Patton, were both present in all sites as larvae, while An. funestus Giles was only recorded in the highland villages. The majority (86%) of mosquito breeding sites were a result of human activities. Anopheles arabiensis was the main vector species in Nyalenda, while Anopheles gambiae s.s. was dominant in Lunyerere. Lunyerere had the highest percentage (12.5%) adult indoor resting An. arabiensis mosquitoes ever recorded in the western Kenyan highlands. Fort Ternan had the highest percentage (71%) of larval An. arabiensis ever recorded at such high altitude. Plasmodium falciparum, the main malaria parasite in the region, was present in the schoolchildren cohort examined and no significant differences in malaria prevalence were observed among the study sites. The inhabitants of the respective communities regarded malaria as a burden and they expressed a willingness to take part in mosquito control, although they did not know how this could be done. A pilot study of larval control strategies conducted in Nyalenda demonstrated the feasibility of environmental and biological control methods in man-made, mostly agricultural, mosquito habitats. The larval control strategies that were applied in the highland villages led to complete elimination of both early and late instar mosquito populations and compared well with the effects of Bti application.
The results suggest that the larval control strategies developed in this study will contribute significantly to a reduction in adult mosquitoes and hence, malaria transmission. Larval control strategies need to be developed that take into account the breeding habits of the local vectors as well as the suitability of habitats for a given control strategy. An integrated approach using various larval control strategies that are locally available, can easily be adopted by the communities concerned. Community involvement in disease control will lead to knowledge on how the peoples’ activities affect their health and this can empower them to take charge of their health.