**Table 5.**

*Distribution of entomological transmission indices according to periods.*

#### *Effect of the Mass Distribution of ITNs in an Endemic Area with a High Entomological Index… DOI: http://dx.doi.org/10.5772/intechopen.105021*

gambiae s.l. resting indoors would have fed on humans. Thirdly, as mosquito collections for pre- and post-net distribution were carried out in different months of the dry and rainy seasons, seasonal variations cannot be excluded from the interpretation of the collection and bioassay data.

Metelo et al. reported seasonal differences in An. gambiae s.l. populations in Bandundu between the dry and rainy seasons [49]. It should be noted that in many settings, nets are not used immediately upon receipt, but rather after the old nets have been torn and are no longer usable. The use of newly distributed nets in homes was not quantified, although the presence of Dawa Plus nets was observed in most households during the implementation of CHPs and HLCs. Large-scale use of DHS data has shown that ITN use is always associated with reduced malaria transmission, especially when community use is high, however, insecticide resistance may reduce this effect, Ferrari et al. found that sleeping under an ITN the previous night was associated with a reduced risk of Plasmodium infection [61]. In the present study, however, no significant impact on entomological measures of transmission was observed immediately after ITN distribution. In an area where insecticide resistance levels are already high, the distribution of new ITNs no longer has an immediate or strong effect on key entomological measures of malaria transmission. This may be due to increased resistance in the study area, compromising both new nets with a full dose of insecticide, and old nets, which will have lost some of their insecticide. This may also mean that the old nets remained effective for the full 3 years of the net's life expectancy, and therefore the distribution of new nets did not improve control. However, the presence of sporozoite-positive mosquitoes in both periods indicates that a better control measure is needed to reduce transmission in this area.

Resistance to pyrethroids and DDT: With the exception of bendiocarb which caused 100% mortality of Anopheles mosquitoes, the other insecticides tested were ineffective against An. gambiae s.l. collected before and after ITN distribution. An. gambiae s.l. was resistant to pyrethroids (deltamethrin and permethrin) and DDT in both periods. Mortality of Anopheles to insecticides varied according to the period (before and after mass ITN distribution). Mortality was limited to deltamethrin, 52% before the mass distribution and was reduced to 34% afterwards, which reduced the effectiveness of this product. After pre-exposure to PBO, the efficacy of deltamethrin was fully restored during both study periods. For permethrin (31–36%) and DDT (4–16%), Anopheles mosquitoes were also resistant to varying degrees depending on the period (pre-post). It can be observed that after the mass ITN distribution, permethrin and DDT increased their efficacy somewhat. These two molecules have not been used for a decade. This could be explained by the fact that the distributed ITNs were impregnated with deltamethrin, which increased the selective pressure and served as a basis for the emergence of resistance in *An. gambiae* s.l. this resistance poses fundamental and operational problems as the behaviour of the Anopheles is altered, resulting in a significant decrease in the efficacy of these products. The endemicity of malaria and the high number of infected An. gambiae in the city of Bandundu are of concern and must be taken into account.

#### **3. Conclusions**

The ITNs deployed in Bandundu-city in 2018 are still effective on A. gambiae sl strain Kisumu but are ineffective on the wild *An*. *gambiae* s.l. wild strain found in the city. The wild *An*. *gambiae* s.l. were resistant to both pyrethroids and DDT.

An increase in the sensitivity of *An*. *gambiae* s.l to pyrethroids was observed after pre-exposure to 5% PBO suggesting the resistance was partly of metabolic origin, i.e. related to P450 mono-oxygenases. The high entological indices in Bandundu-city throughout the year indicate intense malaria transmission. This reflects the ineffectiveness of the vector control strategy which has been based solely on the mass distribution of ITNs for several years. We find that mass distribution of ITNs in Bandundu has not had a significant effect on malaria transmission. Given the intensity of transmission and the levels of resistance observed, it is necessary to consider a new alternative to curb the emergence of resistance and maintain the gains of mass distribution.

## **Acknowledgements**

The research team would like to thank the CREC teams for their support and assistance in setting up our insectarium, in particular Prof Martin AGKOBETO and Dr. Osée RAZZAK. Our sincere thanks to Marianne Sinka of Oxford University for her contribution to the correction of this paper, especially its translation into English.

The thanks also go postum to the late Professor Paul MANSIANGI, the coordinator of this project where death brutally snatched away our affection, eternal felicity to your soul. The field supervisor Ladius MBAYA for his dedication and perfect collaboration.
