**5. Frequency and mechanisms of resistance in malaria vectors in East Africa**

There is variation in the frequency of resistance in malaria vectors and the mechanisms conferring resistance in different sites of different countries in East Africa (Table 1). The frequency and mechanism of resistance in insects depend on the degree of selection pressure and the mode of action of the insecticide, respectively. Insecticides target the nervous system of an insect. Organophosphate insecticides are cholinesterase inhibitors. Cyclodienes insecti‐ cides affect the chloride channel by inhibiting the gamma amino butyric acid (GABA) receptor. Pyrethroids and DDT act on the sodium channel preventing those channels from closing, resulting in continual nerve impulse transmission which eventually leads to the death of an insect [94].

Target site insensitivity is the most frequently reported mechanism conferring resistance to several insecticides used for vector control by altering the target site of the insecticides. The mode of action of each insecticide on insects is site-specific. For instance, the mode of action of organophosphate and carbamate insecticides is mainly by inhibition of the enzyme acetyl‐


**Table 1.** Insecticide resistance mechanisms conferring resistance to different insecticide families in the major malaria vectors and other mosquitoes in East Africa

cholinesterase (ACHE). Insects develop resistance to these insecticides through structural modification of ACHE due to large number of point mutations that occurs in gene encoding the protein for acetyl cholinesterase (ACHE), an active target site for carbamates and organo‐ phosphates which operates in the nerve cell synapses. These mutations result in altered ACHE, which reduces the sensitivity of target site to an insecticide. Another common site insensitivity mechanism is referred as knockdown resistance (*kdr*): insects usually get paralyzed rapidly following exposure to DDT and pyrethroids, and this is expressed as 'knockdown resistance' (*kdr*). However, knockdown is absent in insects exposed to DDT and pyrethroids due to mutations in the para-gated sodium channel gene, whose protein sub-units make up the voltage-sensitive sodium channels on the nerve membranes. Voltage-gated sodium channels are the target for both pyrethroid insecticides and DDT by which insecticides alter the function of the sodium channels in nerve membranes. Knockdown resistance mutation results from a single nucleotide polymorphism in the domain II, segment 6 of the sodium channel gene. Lucine (TTA) to serine (TTT) and leucine (TTT) to phenylalanine (TCA) amino acid substitu‐ tions at this position result in West and East African *kdr* mutations, respectively, which confer resistance to DDT and/or pyrethroids in the East African malaria vectors *An.gambiae* s.s and *An. arabiensis* [95, 96]. Another mutation of methionine to threonine, known as the super-*kdr* mutation, occurs between segment 4 and segment 5 of domain II of the sodium channel gene and results in a much higher resistance than *kdr*. The super-*kdr* mutation is mostly occurring together with the *kdr* mutation. The *kdr* resistance mechanism produces cross-resistance between DDT and pyrethroids and it is a genetically recessive mechanism.

Metabolic resistance is another important mechanism conferring resistance to insect vectors which is associated with the production of increased quantities of families of enzymes involved in insecticide metabolism. It resulted from structural change in the enzyme molecule that enhances its ability to detoxify or bind the insecticides which alter the affinity of the enzyme to insecticides. In the latter case, this mechanism enhances insecticide tolerance status in insects. Some of the common enzymes involved in detoxifying or sequestrating insecticides in insects are monooxygenases which include the cytochrome P450 enzymes. These large groups of enzymes confer resistance mainly to pyrethroids and carbamates and to a lesser extent to organochlorines and organophosphates. Another extremely important group of enzymes which confer resistance to organophosphate, carbamates and to some extent pyrethroid insecticides are esterases. Elevated level of esterases results in sequestration and metabolism of the target insecticides. Elevated glutathione S-transferases (GSTs) also play a role in the detoxification and excretion of organophosphates and DDT in insects. Cross-resistance between DDT and organophosphates is often caused by GSTs.

Behavioral resistance in mosquito vectors tends to change their behavior due to long-term exposure to insecticide-treated surfaces such as walls and LLINs. This behavior has been found to be associated with avoidance of exposure to lethal doses of insecticides due to reduced contact with the insecticide [97, 98]. The behavior is known to increase the longevity of insects in an environment where there is insecticide application through IRS, LLINs or both for vector control. Insects show limited tendency to enter sprayed houses or in houses with LLINs. For example, the evaluation of LLINs or IRS compounds in East African experimental huts have shown avoidance behavior by *An. gambiae*, *An. arabiensis* and *An. funestus*[98]. This also results in irritancy and excito-repellency, which keeps the mosquitoes away from different treated surfaces before contact with the host [99–102]. Shifting of vector species composition (from *An. gambiae* to *An. arabiensis*) due to implementation of LLINs or IRS has also been observed in Tanzania and Kenya [61, 103, 104]. In Africa, there is high proportion of *An. gambiae*s.l and *An. funestus* in areas with high coverage of LLINs [105]. The host-seeking behavior of vectors have changed from endophagic to exophagic due to intensive LLINs coverage [106].

cholinesterase (ACHE). Insects develop resistance to these insecticides through structural modification of ACHE due to large number of point mutations that occurs in gene encoding the protein for acetyl cholinesterase (ACHE), an active target site for carbamates and organo‐ phosphates which operates in the nerve cell synapses. These mutations result in altered ACHE, which reduces the sensitivity of target site to an insecticide. Another common site insensitivity mechanism is referred as knockdown resistance (*kdr*): insects usually get paralyzed rapidly following exposure to DDT and pyrethroids, and this is expressed as 'knockdown resistance' (*kdr*). However, knockdown is absent in insects exposed to DDT and pyrethroids due to mutations in the para-gated sodium channel gene, whose protein sub-units make up the voltage-sensitive sodium channels on the nerve membranes. Voltage-gated sodium channels are the target for both pyrethroid insecticides and DDT by which insecticides alter the function of the sodium channels in nerve membranes. Knockdown resistance mutation results from a single nucleotide polymorphism in the domain II, segment 6 of the sodium channel gene. Lucine (TTA) to serine (TTT) and leucine (TTT) to phenylalanine (TCA) amino acid substitu‐ tions at this position result in West and East African *kdr* mutations, respectively, which confer resistance to DDT and/or pyrethroids in the East African malaria vectors *An.gambiae* s.s and *An. arabiensis* [95, 96]. Another mutation of methionine to threonine, known as the super-*kdr* mutation, occurs between segment 4 and segment 5 of domain II of the sodium channel gene

sulfotransferase

**Country Insecticide Mosquito species Mechanism Reference(s)**

Cytochrome

Pyrethroids *An. arabiensis* Cytochrome P450s monooxygenases [73]

pyrethroids *An. gambiae* s.l. *Kdr* (L1014S) [133]

Pyrethroids *An. gambiae* s.l. *Kdr* [137]

**Table 1.** Insecticide resistance mechanisms conferring resistance to different insecticide families in the major malaria

*arabiensis , An. funestus* cytochrome P450s, GSTs, pNPA [53, 55, 59, 91]

*An. arabiensis kdr* (L1014F) [67, 70, 72, 134]

Mixed function oxidases, b-esterases, P450s, cuticle proteins, GABA,

*Kdr* (L1014S, L1014F) [21, 52, 54, 104, 105, 116,

*Kdr* (L1014S, L1014F) [55, 57, 58, 62, 114, 133]

P450s monooxygenases, esterases [53, 61, 130, 132]

*Kdr* (L1014F, L1014S), *rdl* [76, 84, 92]

130–132]

[76, 92, 135, 136]

*An. gambiae, An. arabiensis*

*An. gambiae, An. arabiensis*

*An. gambiae, An.*

*An. gambiae, An. arabiensis*

*An. arabiensis, Cx. quinquefasciatus*

*An. gambiae, An.arabiensis, An.funestus*

Kenya

198 Insecticides Resistance

Uganda

Ethiopia

Tanzania

Burundi

Rwanda

DDT & pyrethroids

Pyrethroids

DDT & pyrethroids

DDT & pyrethroids

DDT & Pyrethroids

DDT & pyrethroids

DDT & Pyrethroids

DDT &

DDT &

vectors and other mosquitoes in East Africa

Moreover, mutation in the GABA-gated chloride channel, which leads to dieldrin resistance other than DDT, has been described in different species of mosquitoes. The role of cuticular resistance mechanism is not yet known in the phenotypic resistance in the East African malaria vectors.
