Reduction in Malaria Burden Following the Introduction of Indoor Residual Spraying in Areas Protected by Long Lasting Insecticidal Nets in Western Kenya, 2016 – 2018
Authors: Dulacha Diba , Were Vincent , Oyugi Elvis , Kiptui Rebecca , Owiny Maurice , Boru Waqo , Gura Zeinab , Perry Robert T. 
Published: October 2021

Background: Long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) are the main malaria vector control measures. Kenya has employed multiple approaches, including free mass net distribution campaigns, for distributing LLINs to the community that resulted in increased household ownership of one or more nets from 44% in 2010 to 63 % in 2015. Migori and Homa Bay Counties are among the malaria endemic counties in Western Kenya that benefitted from three free mass net distribution campaigns between 2012 and 2018.. Widespread pyrethroid resistance among the primary vectors in Western Kenya has necessitated the re-introduction of IRS using an organophosphate insecticide, pirimiphos-methyl (Actellic® 300CS), as part of a strategy to manage insecticide-resistance. Evaluation of the effectiveness of the combined use of non-pyrethroid IRS and LLINs have yielded varied results. We aimed to evaluate malaria indicators before and after the introduction of IRS in a high malaria transmission area compared with an adjacent area where IRS was not introduced until one year later to estimate the effect of the intervention in an area with pyrethroid resistance.Methods: We reviewed records (MoH 405 A, 405 B, and 706) and tallied monthly aggregate of outpatient department (OPD) attendance, suspected malaria cases, those tested for malaria and those testing positive for malaria at two health facilities, one from Nyatike, an intervention sub-county, and one from Suba, a comparison sub-county, both located in Western Kenya, from February 1, 2016, through March 31, 2018. The first round of IRS was conducted in February – March 2017 in Nyatike sub-county and the second round one year later in both Nyatike and Suba sub-counties while the mass distribution of LLINs has been conducted in both locations. We performed a descriptive analysis and estimated the effect of the interventions and temporal changes of malaria indicators using Poisson regression for a period before and after implementation of the first round of IRS.Results: A higher reduction in the intervention area in total OPD, the proportion of OPD visits due to suspected malaria, testing positivity rate and annual malaria incidences except for the total OPD visits among the under 5 children where 59% decrease was observed in the comparison area while 33% decrease was observed in the intervention area (net change -27%, P <0.001). The percentage decline in annual malaria incidence observed in the intervention area was more than twice the observed percentage decline in the comparison area across all the age groups. A marked decline in the monthly testing positivity rate (TPR) was noticed in the intervention area, while no major changes were observed in the comparison area upon introduction of the non-pyrethroid IRS in the intervention area. The monthly TPR reduced from 46% in February 2016 (start of review period) to 11% in February 2018 (end of review period), representing a 76% absolute decrease in TPR among all ages (RR=0.24, 95% CI 0.12–0.46). In the comparison area, TPR was 16% in both February 2016 and February 2018 (RR=1.0, 95% CI 0.52–2.09). A month-by-month comparison revealed that the TPR in Year 2 remained lower than in Year 1 in the intervention area for most of the one year after the introduction of the IRS.Conclusions: Our findings demonstrated a reduced malaria burden among population protected by both non-pyrethroid IRS and LLINs implying a possible additional benefit afforded by the combined intervention in malaria endemic zone.