**4. Possible causes of insecticide resistance**

was detected in four of the five sites tested and permethrin resistance in one of the two sites tested. However, insecticide resistance was not detected for bendiocarb, pirimiphos-methyl CS and DDT. *Anopheles gambiae* s.s showed reduced susceptibility to the carbamate insecticide, bendiocarb [81]. *An. arabiensis* collected from Lower Moshi showed complete susceptibility to pirimiphos-methyl and malathion, but reduced susceptibility to permethrin [82, 83]. In northwestern Tanzania, there was cross-resistance between pyrethroids and DDT. In Zanzibar, resistance is not homogeneously expressed across islands, and pyrethroid resistance is

West African leucine phenylalanine *kdr* mutation was detected in two heterozygous individ‐ uals field-collected *An. arabiensis* from Tanzania [84]. A study also showed that a low frequency of permethrin resistance mediated by mixed function oxidases and esterases are present in *An. arabiensis* from Lower Moshi. The permethrin resistance is probably caused by the agricultural use of insecticides, especially in the rice fields, as permethrin-treated nets were not widely used in Lower Moshi [76]. The *kdr*-eastern variant was present in homozygous form in 97% of *An. gambiae* s.s but was absent in *An. arabiensis*. Synergist assays with PBO showed to restore susceptibility to pyrethroids, indicating that the resistance is in part due to an oxidase enzyme mechanism. Knockdown resistance mutation (target site insensitivity) was also

The primary vector of malaria in Burundi is *Anopheles gambiae* s.s, while secondary vectors *An. funestus, An. arabiensis* and *An. nili*. The most predominant members of vector species complex in the highlands of Burundi are *An. gambiae* s.s and *An. funestus* s.s [86, 87]. Insecticide susceptibility study in Karusi for *An. gambiae* s.l. showed reduced mortality to permethrin, DDT and deltamethrin. There was complete susceptibility of *An. funestus* to DDT and pyreth‐ roids. A high frequency of East African *kdr* allele was detected in *An. gambiae* s.l., leading to cross resistance between DDT and permethrin in mosquito population. As there is little information on the frequency and distribution of insecticide resistance and the status of the susceptibility level of malaria vectors to insecticides used for vector control in the country, there is an urgent need for a nationwide and systematic evaluation of vector susceptibility level to current WHOPES-approved insecticides for malaria vector control, to inform ongoing

Earlier entomological studies indicate that *Anopheles gambiae* s.l. and *An. funestus* are the main vectors responsible for malaria transmission in Rwanda. *An. arabiensis* is also a locally important vector of malaria. The main malaria foci are in the east and southeast areas where

Insecticide susceptibility studies conducted in 2012 in several sites indicated signs of resistance to DDT in some areas, possible emergence of resistance to some pyrethroid compounds, and complete susceptibility to bendiocarb and fenitrothion. A similar insecticide susceptibility

the altitude is generally below 1,500 m and surrounded by marshy plains.

stronger in Pemba than Unguja.

detected in Pemba [84, 85].

interventions and control program.

**3.5. Burundi**

196 Insecticides Resistance

**3.6. Rwanda**

Emergence of resistance in disease vectors in particular mosquitoes have been associated with different factors and sources. One of the factors is the intensive use of some classes of insecti‐ cides such as pyrethroids both in public health and in agriculture, which led to its reduced efficacy of insecticides [88–90]. Agricultural use of pesticides plays a role on the development of resistance and cross-resistance in malaria vectors has been implicated in literature. Resist‐ ance of *An. arabiensis* to pyrethroids in Tanzania [76] and Ethiopia [72] was attributed to use of insecticides in agriculture and livestock.

Insecticide resistance selection pressure in malaria vectors in East Africa region has also been attributed to wide coverage of LLINs and/or IRS [91] and use of agricultural pesticides [92]. As the most commonly used pesticides in agriculture and IRS are pyrethroids, organophos‐ phate and organochlorides and for the treatments of LLINs are pyrethroids, cross-resistance is common between pyrethroids and DDT [85, 93].
