**8. Insecticide monitoring and insecticide resistance management option**

#### **8.1. Mixture**

In resistance management strategies, four tools (rotation, mosaic, mixture and combination) have been suggested [117] either to slow down resistance or reduce the rate of insecticide selection pressure. In control programs, simultaneous use of two or more insecticide com‐ pounds with different modes of action within a single product or formulation is preferred to manage resistance in insects. Two mixed insecticides with different modes of action can lead to reduced chance of double resistance by killing an insect which is resistant to one of the insecticide compounds [71, 82]. The use of mixture of insecticides relies on the assumption that the number of insects carrying a resistant allele at both loci is rare if the frequency of resistant allele at two loci is low [23, 118]. This approach may have a reduced efficacy if resistance in insects is at detected level to one of the mixed insecticide compounds. The major aim of insecticide mixture is to overcome the resistance selection pressure rather than maintaining the high susceptibility status of the insect population. The mixture should be up to the standard application concentration ratio of two insecticide compounds for effective control. Mixture of insecticides has usually high cost implication which may not be affordable in community protection against malaria. The mixture of insecticide compounds has practically shown to be effective when applied in small scale [82].

### **8.2. Mosaic**

development of resistance in vector species to various classes of insecticides. These have been documented in Kenya [21], Tanzania [79, 113], Uganda [114] and Ethiopia [70, 73, 115].

After the successful reduction of the malaria vector and disease transmission, the increased resistance among potential vector populations has been witnessed across East Africa [1, 21, 116]. The current status of pyrethroid resistance in malaria vectors and an increase in malaria incidence shows the compromised vector control system due to insecticide resistance, which calls the need for the development of new tools for malaria control. Insecticide resistance has shown to compromise the effectiveness of malaria control efforts in Kenya and other West African countries. The use of non-pyrethroid insecticides for IRS is a potential option as the ITN are mainly pyrethroid-based. It has been observed that pyrethroid resistance mosquitoes are entering and surviving exposure to LLINs, which may quantify the indoor transmission

**7. Prospects of prevention on development and spread of IR in malaria**

In vector control, constant use of the same insecticide induces resistance selection pressure in small vector population which subsequently spread to the large population. The spread of resistance depends on the frequency of the resistance genes within the vector population. In operational programs, the coverage of LLINs and IRS are most critical to be considered in the prevention of the development and spread of the insecticide resistance. Further, insecticideresistant monitoring plan and management strategy should be developed and implemented

**8. Insecticide monitoring and insecticide resistance management option**

In resistance management strategies, four tools (rotation, mosaic, mixture and combination) have been suggested [117] either to slow down resistance or reduce the rate of insecticide selection pressure. In control programs, simultaneous use of two or more insecticide com‐ pounds with different modes of action within a single product or formulation is preferred to manage resistance in insects. Two mixed insecticides with different modes of action can lead to reduced chance of double resistance by killing an insect which is resistant to one of the insecticide compounds [71, 82]. The use of mixture of insecticides relies on the assumption that the number of insects carrying a resistant allele at both loci is rare if the frequency of resistant allele at two loci is low [23, 118]. This approach may have a reduced efficacy if resistance in insects is at detected level to one of the mixed insecticide compounds. The major aim of insecticide mixture is to overcome the resistance selection pressure rather than maintaining the high susceptibility status of the insect population. The mixture should be up to the standard

resurgence in areas with high level of pyrethroid resistance.

to delay the development or spread of resistance.

**vectors in East Africa**

200 Insecticides Resistance

**8.1. Mixture**

This approach is the use or application of two different classes of insecticides to control the same disease vector in the same area [119]. The mosaic approach is effective if application takes into consideration the spatial pattern. This technique helps in restoring the susceptibility status of the vector to an insecticide. It is a method for control of resistance secured to be working if properly done and monitored [120]. In some malaria endemic countries, large-scale mosaic application has shown to effectively control resistant populations of *An. albimanus* [119]. It has been observed that resistance developed fast in areas with pyrethroid alone than in areas with mosaic application along organophosphate, pyrethroids and carbamates [119]. Recently, industries have developed mosaic LLINs (PermaNet 3.0 and OlysetPlus) containing a pyreth‐ roid insecticide and a synergist (piperonyl butoxide), an oxidase inhibitor on the fabric to increase the bio-efficacy against pyrethroid-resistant vectors [73, 114, 121]. Further research is needed in the future to use mosaic in LLINs and IRS.

#### **8.3. Rotation**

This is employing two or more insecticide compounds of different insecticide classes with different modes of action by switching the insecticide of choice each round or in alternating sequences. This approach is based on the assumption that resistance genes have a selective disadvantage in the absence of an insecticide used in operational program. If vector resistance to each insecticide is low, then the occurrence of multiple insecticide resistance is minimal or practically impossible [122]. The rotational use of insecticides plays a major role in killing resistant insects when the switch is made to a second insecticide. The defined rotation time should be as short as possible to reduce the risk of resistance development against the insecticide in use. It also slows down the evolution of the resistance [119]. For LLINs, it is difficult to implement rotation technique as only pyrethroids are used for the treatment of nets [123]. This method has higher financial implications for the implementation in vector control.

### **8.4. Combination of tools**

In monitoring and management of resistance, the use of two or more tools or combinations of interventions simultaneously is an option in insecticide resistance management. The use of tools targeting adults such as LLINs and implementation of IRS or vise versa or combined with larviciding or larval source reduction is shown to have effects on vector control in Kenya [26, 124], Tanzania [43, 79, 113], Uganda [125] and Ethiopia [126].

Combination of tools is appreciated as it is cost-effective, prohibits mosquito feeding and causes mortality instead of reducing resistance alone. In this approach, using insecticides which share the same resistance mechanism should be avoided as resistance in malaria vectors develops faster. Combination tools have shown to increase the protection efficiency against vectors and maintain reduced susceptibility status of the vectors for longer period.
