Performance of pirimiphos-methyl based Indoor Residual Spraying on entomological parameters of malaria transmission in the pyrethroid resistance region of Koulikoro, Mali.

Abstract

Background:

Vector control relies heavily on Long-lasting insecticidal nets ( (LLINs) and Indoor Residual Spraying (IRS) in selected districts in Mali including Koulikoro district. As part of strengthening vector control strategies in the district, IRS was initiated by the National Malaria Control Program (NMCP) with the support of the US President's Malaria Initiative (PMI) since 2008. LLINs coverage was 93.3% and 98.2% for IRS in Koulikoro, the only district of the region where IRS was supported by PMI. Following mosquito vector resistance to both pyrethroid and carbamates, organophosphate (pirimiphos-methyl) was used for the IRS campaigns of 2015 and 2016 in the district of Koulikoro. In this study, we assessed the effect of IRS on malaria transmission by comparing entomological indices in of the district of Koulikoro, where IRS was implemented and its neighbored district of Banamba, where IRS had never been implemented.

Methods:

The study was conducted in two villages of each district (Koulikoro and Banamba). Pyrethrum spray catches and entry window trapping were used to collect mosquitoes on a monthly basis. WHO tube tests were carried out to assess mosquito susceptibility to insecticides. Mosquitoes were identified to species level by PCR and their infection to P. falciparum was detected by ELISA.

Results:

Of the 527 specimens identified, An. coluzzii was the most frequent species (95%) followed by An. gambiae (4%) and An. arabiensis (1%). Its density was rainfall dependent in the no-IRS area, and almost independent in the IRS area. The infection rate (IR) in the no-IRS area was 0.96%, while it was null in the IRS area. In the no-IRS area, the entomological inoculation rates (EIR) was 0.21 infective bites /person month with a peak in September. High resistance to pyrethroids and carbamates and susceptibility to organophosphates was observed at all sites.

Conclusion:

The introduction of pirimiphos-methyl based IRS in the area resulted in a significant decrease in malaria transmission. An.gambiae s.l., the main malaria vector of the area, was resistant to pyrethroids and carbamates, and remained susceptible to the organophosphates.