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

Delenasaw Yewhalaw and Eliningaya J. Kweka

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/61570

#### **Abstract**

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 re‐ mained 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. On‐ going 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 prob‐ lem 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 coun‐ tries 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 de‐ pends on pyrethroids, DDT and recently on carbamates. Despite IRS and ITNs are known in reducing malaria incidence, insecticide resistance in malaria vectors threatens the suc‐ cess of malaria control program. Resistance to insecticides has occurred in most arthro‐ pod 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), resist‐ ance to these insecticides has become a limiting factor for current efforts to sustain con‐ trol. Currently, no other insecticide class with similar efficacy has been approved by WHOPES. The development of insecticide resistance in malaria vectors has been attribut‐ ed to the prolonged use of insecticides for IRS and high coverage of ITNs/LLINs. The re‐ cent use of pyrethroids for indoor residual spraying is likely to have enhanced the selection pressure for insecticide resistance alleles among East African vector popula‐ tions. Moreover, mosquitoes breeding in agricultural habitats are exposed to sub lethal

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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 be‐ lieved 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* muta‐ tions and their association with the phenotypic resistance in East Africa is less under‐ stood. 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 manage‐ ment 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 resist‐ ance in malaria vectors in the sub-region.

**Keywords:** malaria vectors, insecticide resistance, resistant management, vector control, East Africa

#### **1. Introduction**

East Africa is a region encompassing six countries which include Kenya, Uganda, Ethio‐ pia, Tanzania, Rwanda and Burundi, and all these countries are prone to malaria transmis‐ sion with known efficient vectors. The main malaria vectors in the region are *Anopheles gambiae* s.s, *An. arabiensis* and *An. funestus* [1–4]. These vectors breed in different habitats ranging from temporary rain pools to permanent water bodies [5–8]. Vector species distribution in East Africa are governed by several factors which include anthropogenic activities [4, 5, 7], such as development projects [9–12]. Also, climate, particularly tempera‐ ture and rainfall, has been regarded as the function of habitats for vector abundance and distribution between low- and high-altitude areas [13, 14]. Human migration and move‐ ment from high land to low land have facilitated the distribution of parasites [15]. Topogra‐ phy has influenced the abundance and distribution of vector in all areas [16–19]. Thus, the abundance and distribution of efficient vectors have led to the wide use of control tools and intensive interventions across the sub-region. The main tools used for the control of malaria vectors are long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) [20]. The pyrethroids are the only insecticides which have been used for treating LLINs while organochlorides, organophosphates, carbamates and pyrethroids are used for IRS [1, 21]. Currently, organochlorides (especially DDT) are banned in most of the East African countries for IRS use due to resistance developed by the major malaria vectors and environmental concern [20]. The development of resistance is influenced by many factors [22]. These include genetic factors including the number and frequency of resistance alleles in the insect population, fitness cost and relative dominance of the characters; biological factors including the insect life history parameters, the fitness of the heterozygous and

homozygous resistant phenotypes and initial population size; reproductive factors includ‐ ing the rate of increase and fluctuations in population size; and operational factors includ‐ ing application methods of the insecticide and properties of an insecticide in use, previous selection with other insecticides, proportion of population exposed to selective doses, dosage of insecticide taken up by exposed insects and the life stage of the mosquito selected [22, 23].

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 be‐ lieved 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* muta‐ tions and their association with the phenotypic resistance in East Africa is less under‐ stood. 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 manage‐ ment 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 resist‐

**Keywords:** malaria vectors, insecticide resistance, resistant management, vector control,

East Africa is a region encompassing six countries which include Kenya, Uganda, Ethio‐ pia, Tanzania, Rwanda and Burundi, and all these countries are prone to malaria transmis‐ sion with known efficient vectors. The main malaria vectors in the region are *Anopheles gambiae* s.s, *An. arabiensis* and *An. funestus* [1–4]. These vectors breed in different habitats ranging from temporary rain pools to permanent water bodies [5–8]. Vector species distribution in East Africa are governed by several factors which include anthropogenic activities [4, 5, 7], such as development projects [9–12]. Also, climate, particularly tempera‐ ture and rainfall, has been regarded as the function of habitats for vector abundance and distribution between low- and high-altitude areas [13, 14]. Human migration and move‐ ment from high land to low land have facilitated the distribution of parasites [15]. Topogra‐ phy has influenced the abundance and distribution of vector in all areas [16–19]. Thus, the abundance and distribution of efficient vectors have led to the wide use of control tools and intensive interventions across the sub-region. The main tools used for the control of malaria vectors are long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) [20]. The pyrethroids are the only insecticides which have been used for treating LLINs while organochlorides, organophosphates, carbamates and pyrethroids are used for IRS [1, 21]. Currently, organochlorides (especially DDT) are banned in most of the East African countries for IRS use due to resistance developed by the major malaria vectors and environmental concern [20]. The development of resistance is influenced by many factors [22]. These include genetic factors including the number and frequency of resistance alleles in the insect population, fitness cost and relative dominance of the characters; biological factors including the insect life history parameters, the fitness of the heterozygous and

ance in malaria vectors in the sub-region.

East Africa

190 Insecticides Resistance

**1. Introduction**

Insecticide resistance is not new in insect vectors, and it is a genetically inherited characteristic which increases in the populations of vectors as a result of increased resistance selection pressure and also a trait capable of rapid spread. Malaria vector control in East Africa relies principally on the use of insecticides that can be applied either as an indoor residual deposit or can be used to treat mosquito nets and curtains. However, the long-term vector control program based on prolonged and frequent insecticide application faced the problem of resistance. Vector control subjects mosquito populations to selection and survival of the fittest. Evolution of insecticide resistance in an insect population arises when there is an increase in the frequency of one or more resistance genes in the population following exposure to insecticides. Attempts to kill the tolerant individuals lead to ever increasing doses and eventually resistant pest populations. This is an inevitable limitation in the use of any new or old class of insecticides. Malaria control initiatives introduced DDT during the Second World War from 1945 to 1948 to eradicate malaria since that time DDT showed to be an effective malaria vector control, but resistance has emerged throughout endemic countries including East Africa.
