Bio-Efficacy Evaluation Of Long-Lasting Insecticidal Nets After Five Years Of Storage: Implications For Malaria Control Programmes

ABSTRACT

Long Lasting Insecticidal Nets (LLINs) are the most sustainable and effective malaria control tool currently available. Global targets are for ≥80% of the population living in malaria endemic areas to have access to and use a LLIN. However, current access to LLINs in endemic areas is 56% due to system inefficiencies and budget limitations. Thus, cost-effective approaches to maximize access of effective LLINs in endemic areas are required. This study evaluated whether LLINs that had been stored for more than five years under manufacturer recommended conditions may be optimally effective against Anopheles mosquitoes, to inform malaria control programs and governments on the periods over which LLINs may be stored between distributions, in an effort to maximise use of available LLINs. Standard World Health Organization (WHO) bioassays (cone and tunnel tests) were used to evaluate the bio-efficacy and wash resistance of Olyset® and DawaPlus® 2.0 (rebranded Tsara® Soft) LLINs after five years of storage at 25C - 33.4C and 40%-100% relative humidity. In addition, a small scale Ifakara Ambient Chamber test (I-ACT) was conducted to compare bio-efficacy of one long stored LLIN to one new LLIN of the same brand, unwashed and washed. Long-lasting insecticidal nets were evaluated using laboratory reared fully susceptible Anopheles gambiae sensu stricto (s.s) (Ifakara strain) and pyrethroid resistant Anopheles arabiensis (Kingani strain). After five years of storage, both unwashed and washed Olyset® and DawaPlus® 2.0 LLINs passed WHO efficacy criteria on Knockdown (KD60) ≥95%, 24-hr mortality ≥ 80% and ≥90% blood-feeding inhibition as per conducted WHO bioassays against susceptible An. gambiae s.s. The DawaPlus® 2.0 LLINs also passed combined WHO bio-assay criteria against resistant An. arabiensis. Confirmatory I-ACT test using whole nets demonstrated that long stored LLINs showed similar efficacy to new LLINs on both feeding inhibition and mortality endpoints against susceptible and resistant strains. Therefore, even after long storage of around 5 years, both Olyset® and DawaPlus® 2.0 remain efficacious against susceptible Anopheles mosquitoes at optimal storage range of 25C - 33.4C and 40%-100% relative humidity measured by standard WHO methods. DawaPlus® 2.0 remained efficacious against pyrethroid-resistant strain.