Insecticide Resistance in East Africa — History, Distribution and Drawbacks on Malaria Vectors and Disease Control

Malaria is a major contributor to the global disease burden and a significant impediment to socio-economic development in resource-poor countries. In contrast to improved trends of malaria morbidity and mortality in some parts of the world, malaria has remained a life threatening disease in many other regions including East Africa because of factors such as weak health systems, growing drug and insecticide resistance, ecological change, climate anomalies, socio-economic factors and changes in land use patterns. Ongoing malaria vector control strategies rely mainly on the use of indoor residual spraying (IRS) and insecticide treated nets (ITNs) which are the primary intervention strategies to reduce malaria burden. The current success in reducing malaria related morbidity and mortality has led to the optimism that elimination of the disease as a public health problem may be a realistic objective. Efforts during the last decades enabled access to ITNs in sub-Saharan Africa protecting millions of people at risk of malaria. The number of countries that employed IRS as a vector control strategy increased almost by two fold and the percentage of households owing at least one ITN in sub-Saharan Africa is estimated to increase from time to time. Currently, all ITNs are treated with pyrethroids while IRS depends on pyrethroids, DDT and recently on carbamates. Despite IRS and ITNs are known in reducing malaria incidence, insecticide resistance in malaria vectors threatens the success of malaria control program. Resistance to insecticides has occurred in most arthropod vectors with different mechanisms. If the current trends of increased insecticide resistance continue, it may jeopardise the efficacy of current vector control tools. Given the limited choice of available insecticides, i.e., only 12 insecticides belonging to 4 classes of insecticides (organochlorines, organophosphates, pyrethroids and carbamates), resistance to these insecticides has become a limiting factor for current efforts to sustain control. Currently, no other insecticide class with similar efficacy has been approved by WHOPES. The development of insecticide resistance in malaria vectors has been attributed to the prolonged use of insecticides for IRS and high coverage of ITNs/LLINs. The recent use of pyrethroids for indoor residual spraying is likely to have enhanced the selection pressure for insecticide resistance alleles among East African vector populations. Moreover, mosquitoes breeding in agricultural habitats are exposed to sub lethal doses of pesticides used in agriculture. Since currently recommended insecticides for IRS or ITNs were developed with similar active ingredients of pesticides used for agricultural pest control, their extensive and widespread use to boost agricultural productivity is believed to foster insecticide resistance in mosquito populations. There is strong evidence on the emergence of resistance to DDT and pyrethroids in the major malaria vectors in East Africa however, current information on resistance status of the malaria vectors in different areas of the sub-region is scarce. Genes conferring resistance to malaria vectors, including kdr, super kdr and acetylcholinesterase mutations and metabolic resistance are not mapped. The frequency and spatial distribution of East and West African kdr mutations and their association with the phenotypic resistance in East Africa is less understood. The bioassay results after WHO diagnostic tests in different East African malaria vector populations against insecticides used in public health is not well documented. In conclusion, planning and implementing insecticide resistance monitoring and management strategy should be part of the vector control program either for pre-emptive action without waiting for the development of resistance or to slowdown the spread of resistance in malaria vectors in the sub-region