Performance of IRS on malaria prevalence and incidence using pirimiphos-methyl in the context of pyrethroids resistance in Koulikoro region, Mali

Abstract Background: Koulikoro Health District is one of three districts of Mali where the indoor residual spray (IRS) has been implemented from 2008 to 2016. With widespread of resistance to pyrothroid, IRS was shifted from pyrethroid to pirimiphos-methyl from 2014 to 2016. We assessed the added value of IRS to LLINs on the prevalence of parasitemia and malaria incidence among children under 10 years old. Methods: A comparative study was carried out to assess the effects of pirimiphos-methyl based IRS on malaria prevalence and incidence among children from 6 months to 10 years old in selected pyrethroid resistance villages of two health districts in Mali: one where IRS was implemented in combination with LLINs (intervention area) and one with LLINs-only (control area). Two cross-sectional surveys were carried out at the beginning (June) and end of the rainy season (October) to assess seasonal changes in malaria parasitemia by microscopy. A passive detection case (PCD) was set-up in each study village for 9 months to estimate the incidence of malaria using RDT. Results: There was an increase of 220% in malaria prevalence from June to October in the control area (14% to 42%) versus only 53% in the IRS area (9.2% to 13.2%).Thus, the proportional rise in malaria prevalence from the dry to the rainy season in 2016 was 4 times greater in the control area compared to the IRS area. The overall malaria incidence rate was 2.7 per 100 person-months in the IRS area compared with 6.8 per 100 person-month in the control areas. The Log-rank test of Kaplan-Meier survival analysis showed that children living in IRS area remain much longer free from malaria (Hazard ratio (HR)=0.45, CI 95% = 0.37-0.54) than children of the control area (P < 0.0001).Conclusions: IRS using pirimiphos-methyl has been successful in reducing substantially both the prevalence and the incidence of malaria in children under 10 years old in the area of pyrethroid resistance of Koulikoro, Mali. Pirimiphos-methyl is a better alternative than pyrethroids for IRS in areas with widespread of pyrethroid resistance.