Effectiveness of three-year old piperonyl butoxide and pyrethroid-treated long-lasting insecticidal nets (LLINs) versus pyrethroid-only LLINs against malaria infection: results of a cluster randomised trial in Tanzania
Authors: Protopopoff Natacha , Mosha Jacklin F. , Messenger Louisa A. , Lukole Eliud , Charlwood Jacques D. , Wright Alexandra , Kessy Enock , Manjurano Alphaxard , Mosha Franklin W. , Kleinschmidt Immo , Rowland Mark 
Published: July 2022

Background

After decades of success in reducing malaria through the scale-up of pyrethroid long-lasting insecticidal nets (LLINs), malaria decline has stalled, coinciding with the rapid spread of pyrethroid resistance. A new class of net, treated with a mixture of a pyrethroid and a synergist, piperonyl butoxide (PBO), demonstrated superior efficacy compared to standard (std) pyrethroid LLINs against malaria in an area of intense pyrethroid resistance, reducing malaria prevalence by 44% over 2 years in the present trial. However, an important question is left unanswered regarding the performance of this PBO-LLIN over the World Health Organization recommended lifespan of 3 years for LLINs.

Methods and Findings

We conducted a four-arm randomized controlled trial using a two-by-two factorial design that evaluated the effectiveness of PBO-LLIN arms (12 clusters PBO-LLIN alone and 12 clusters PBO-LLIN + Indoor Residual Spraying; IRS) compared to std-LLIN (12 clusters std-LLIN alone and 12 clusters std-LLIN + IRS) and IRS arms versus no IRS arms from January 2014 to December 2017 in Muleba, Tanzania. Malaria infection prevalence in 80 children, 6 months to 14 years, per cluster was measured twice a year and analysed in an intention to treat (ITT) and per protocol (PP) approach. Density of malaria mosquito vectors and entomological inoculation rate (EIR) were assessed monthly in 7 houses per cluster. Logistic regression allowing for within cluster correlation of responses was used to compare malaria prevalence between PBO-LLIN groups vs std-LLIN groups and IRS groups vs no IRS groups during the third-year follow-up at 28- and 33-months post-intervention. No further IRS was conducted after the first spray round in 2015; as yearly IRS is recommended by WHO, results need to be interpreted in light of this limitation. Vector density and EIR were analysed using negative binomial regression. Malaria results were available for 7471 children. At 28 months, malaria infection prevalence was lower in the PBO-LLIN groups (69.3%) compared to the std-LLIN groups (80.9%, Odds Ratio: 0.45, 95% Confidence Interval: 0.21-0.95, p value: 0.0364). The effect was weaker at 33 months post-intervention (OR: 0.60, 95%CI:0.32-1.13, p value: 0.1131), in the ITT analysis but still evident in the PP analysis (OR: 0.34, 95%CI: 0.16-0.71, p value: 0.0051). At this time point, net usage in household participants was 31% and PBO concentration in PBO-LLINs was reduced by 96% compared to those of new nets. A total of 17,451 Anopheles mosquitoes were collected during the 3150 collection nights done in the third year. There was no reduction in EIR (DR: 0.63, 95%CI: 0.25-1.61, p value: 0.3296) between the PBO groups and std-LLIN groups or between IRS and no IRS groups (DR: 0.7, 95%CI: 0.41-2.28, p value: 0.9426).

Conclusions

PBO-LLINs no longer provided community protection from malaria infection, compared to std-LLINs by the third year of use due to low net usage. Children still sleeping under PBO-LLINs had lower odds of infection than those sleeping under a std-LLIN, however prevalence remained unacceptably high. It is urgent that net distribution frequencies and effective lifespan of this class of LLIN are aligned for maximum impact.

Trial registration

ClinicalTrials.govNCT02288637

Author summary

Why was the study done?

Widespread insecticide resistance among major malaria mosquito populations threatens control efforts worldwide.

A new class of long-lasting insecticidal net (LLIN), containing a pyrethroid insecticide and a synergist, piperonyl butoxide (PBO), improves insecticide toxicity by inhibiting metabolic enzyme activity, responsible for insecticide resistance.

PBO-LLINs reduced malaria prevalence by 44% in Tanzania and 27% in Uganda, compared to standard pyrethroid-only LLINs, in two 24-month cluster randomised controlled trials (CRTs), as conducted per World Health Organization (WHO) recommendations.

However, LLIN deployment regimens are currently based on 3 years assumed functional survival for nets, with questions remaining, regarding the effectiveness of PBO-LLINs to prevent malaria after 3 years of continuous field use.

What did the researchers do and find?

A 24-month CRT in Muleba, Tanzania, an area of high pyrethroid resistance, was extended for one year to assess the impact of PBO-LLINs, compared to pyrethroid-only LLINs, on malaria infection, after 3 years of use, corresponding to their expected lifespan.

After 28 months, malaria infection prevalence was still lower in users of PBO-LLINs, compared to standard LLINs, but this effect was lost by 33 months in the intention to treat (ITT) analysis but was still evident in the per protocol (PP) analysis.

No reduction in vector density or entomological inoculation rate was evident after 3 years of use.

Reasons underlying the poorer PBO-LLIN performance after 36 months, included declining net usage, poor netting durability and diminished PBO synergist content.

What do these findings mean?

By 3 years of continuous field use, PBO-LLINs no longer provided community protection from malaria infection, compared to pyrethroid-only LLINs, however children sleeping under a PBO-LLIN still retained a level of protection compared to those sleeping under a standard pyrethroid-only LLIN.

To maximise the impact of PBO-LLINs, net procurement and replacement strategies, must be modified to maintain high coverage.

Future CRTs and additional community studies are required to determine the effective lifespan of new classes of LLINs for appropriate incorporation into resistance management schemes, to preserve gains made in malaria control over the past two decades.