Resurgence of Malaria Following Discontinuation of Indoor Residual Spraying of Insecticide in an Area of Uganda With Previously High-Transmission Intensity

Abstract

Background: Indoor residual spraying (IRS) and long-lasting insecticidal nets (LLINs) are the primary tools for malaria prevention in Africa. It is not known whether reductions in malaria can be sustained after IRS is discontinued. Our aim in this study was to assess changes in malaria morbidity in an area of Uganda with historically high transmission where IRS was discontinued after a 4-year period followed by universal LLIN distribution.

Methods: Individual-level malaria surveillance data were collected from 1 outpatient department and 1 inpatient setting in Apac District, Uganda, from July 2009 through November 2015. Rounds of IRS were delivered approximately every 6 months from February 2010 through May 2014 followed by universal LLIN distribution in June 2014. Temporal changes in the malaria test positivity rate (TPR) were estimated during and after IRS using interrupted time series analyses, controlling for age, rainfall, and autocorrelation.

Results: Data include 65 421 outpatient visits and 13 955 pediatric inpatient admissions for which a diagnostic test for malaria was performed. In outpatients aged <5 years, baseline TPR was 60%-80% followed by a rapid and then sustained decrease to 15%-30%. During the 4-18 months following discontinuation of IRS, absolute TPR values increased by an average of 3.29% per month (95% confidence interval, 2.01%-4.57%), returning to baseline levels. Similar trends were seen in outpatients aged ≥5 years and pediatric admissions.

Conclusions: Discontinuation of IRS in an area with historically high transmission intensity was associated with a rapid increase in malaria morbidity to pre-IRS levels.

Keywords: IRS; LLIN; Uganda; malaria; resurgence.

Figure 1.

Map of Uganda showing study district (solid red) and other 9 districts where indoor residual spraying of insecticide was discontinued (red dots). Expanded view of study district showing location of outpatient (Aduku HC IV) and inpatient (Apac Hospital) health facilities where surveillance data were collected. Abbreviations: HC IV, Health Center Level 4; IRS, indoor residual spraying of insecticide.

Figure 2.

Temporal changes in malaria test positivity rates in relation to indoor residual spraying of insecticide (IRS). Vertical dashed lines separate the study period into the following 4 categories: a baseline period (July 2009–August 2010); an initial period of effective IRS (September 2010–February 2011); a sustained period of effective IRS (March 2011–August 2014), which included the first 3 months after the last round of IRS was completed; and the 4–18 month period following IRS discontinuation (September 2014–November 2015) when a resurgence of malaria was observed. Pink bars show the timing of IRS with a single round of alpha-cypermethrin, yellow bars indicate the timing of IRS with 9 rounds of bendiocarb, and blue bars show the mass deployment of long-lasting insecticidal nets.