Effectiveness of a national mass distribution campaign of long-lasting insecticide-treated nets and indoor residual spraying on clinical malaria in Malawi, 2018-2020

Abstract

Introduction: Malawi's malaria burden is primarily assessed via cross-sectional national household surveys. However, malaria is spatially and temporally heterogenous and no analyses have been performed at a subdistrict level throughout the course of a year. The WHO recommends mass distribution of long-lasting insecticide-treated bed nets (LLINs) every 3 years, but a national longitudinal evaluation has never been conducted in Malawi to determine LLIN effectiveness lifespans.

Methods: Using District Health Information Software 2 (DHIS2) health facility data, available from January 2018 to June 2020, we assessed malaria risk before and after a mass distribution campaign, stratifying by age group and comparing risk differences (RDs) by LLIN type or annual application of indoor residual spraying (IRS).

Results: 711 health facilities contributed 20 962 facility reports over 30 months. After national distribution of 10.7 million LLINs and IRS in limited settings, malaria risk decreased from 25.6 to 16.7 cases per 100 people from 2018 to 2019 high transmission seasons, and rebounded to 23.2 in 2020, resulting in significant RDs of -8.9 in 2019 and -2.4 in 2020 as compared with 2018. Piperonyl butoxide (PBO)-treated LLINs were more effective than pyrethroid-treated LLINs, with adjusted RDs of -2.3 (95% CI -2.7 to -1.9) and -1.5 (95% CI -2.0 to -1.0) comparing 2019 and 2020 high transmission seasons to 2018. Use of IRS sustained protection with adjusted RDs of -1.4 (95% CI -2.0 to -0.9) and -2.8% (95% CI -3.5 to -2.2) relative to pyrethroid-treated LLINs. Overall, 12 of 28 districts (42.9%) experienced increases in malaria risk in from 2018 to 2020.

Conclusion: LLINs in Malawi have a limited effectiveness lifespan and IRS and PBO-treated LLINs perform better than pyrethroid-treated LLINs, perhaps due to net repurposing and insecticide-resistance. DHIS2 provides a compelling framework in which to examine localised malaria trends and evaluate ongoing interventions.

Keywords: PCR; epidemiology; malaria.

Figure 1

Health facility catchment areas and distribution of long-lasting insecticide-treated nets from September to December 2018 during Malawi’s mass distribution campaign, and implementation of IRS in October and November 2018 and in November and December 2019. (A) Health facility catchment areas for health facilities with data in Malawi’s District Health Information Software 2 in 2018–2020 (n=711), calculated by Euclidean distance using Thiessen polygons. (B) Assigned intervention and insecticide type by district. *Mchinji distributed 69.4% pyrethroid-treated nets and 30.6% PBO-treated nets; †IRS in 2019 only. (C) Density of nets (number of nets per person) by health facility catchment area. *Values >1 set to 1. IRS, indoor residual spraying; PBO, piperonyl butoxide.

Figure 2

Median 1-month risk of malaria (cases per 100 people) by high (January to May) and low (June to December) malaria transmission season and health facility catchment area, 2018–2020, among (A) the total population, (B) children <5 years and (C) individuals ≥5 years of age. Confirmed case data are taken from Malawi’s District Health Information Software 2 and population denominators from adapted WorldPop 2018 estimates. *Risk values >50 are set at 50 cases per 100 people.

Figure 3

One-month risk of malaria (cases per 100 people) from January 2018 to June 2020, stratified (A) by age group and (B) by insecticide type. ‘No data’ refers to health facilities which did not have any bed net distribution or IRS information. Dark grey colour blocks represent months where the mass distribution campaign occurred (September to December 2018) and light grey blocks represent months which fall during the yearly high malaria transmission season (January to May). Risk was measured monthly and curves are smoothed using X-splines. IRS: 1 district; 22 health facilities; 387 523 people; PBO: 9 districts; 127 health facilities; 3 150 863 people; Pyrethroid: 17 districts; 423 health facilities; 10 854 018 people; Pyrethroid and PBS: 1 district; 48 health facilities; 1 141 817 people; No data: 91 health facilities; 2 221 396 people. IRS, indoor residual spraying; PBO, piperonyl butoxide.

Figure 4

Risk of malaria (cases per 100 people) and risk differences from 2018 to 2019 and 2020, stratified by district and type of intervention. Data are shown for months falling within the high malaria transmission season (January to May), and estimates represent 5-month risks. Risk difference point values and 95% CIs are shown by black dots and lines underlying each coloured point, although CIs are not readily visible because of the narrow range. Mchinji’s net distribution was comprised of 69.4% pyrethroid-treated nets and 30.6% PBO-treated nets. IRS, indoor residual spraying; PBO, piperonyl butoxide.