Long-lasting insecticidal nets retain bio-efficacy after 5 years of storage: implications for malaria control programmes

Abstract

Background 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 (own) 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 to effective LLINs in endemic areas are required. This study evaluated whether LLINs that had been stored for 5 years under manufacturer’s recommended conditions may be optimally effective against Anopheles mosquitoes, to inform malaria control programmes and governments on the periods over which LLINs may be stored between distributions, in an effort to maximize use of available LLINs. Methods Standard World Health Organization (WHO) bioassays (cone and tunnel test) were used to evaluate the bio-efficacy and wash resistance of Olyset® and DawaPlus® 2.0 (rebranded Tsara® Soft) LLINs after 5 years of storage at 25 °C to 33.4 °C and 40% to 100% relative humidity. In addition, a small scale Ifakara Ambient Chamber test (I-ACT) was conducted to compare the bio-efficacy of one long stored LLINs to one new LLIN of the same brand, washed or unwashed. LLINs were evaluated using laboratory reared fully susceptible Anopheles gambiae sensu stricto (s.s.) (Ifakara strain) and pyrethroid resistant Anopheles arabiensis (Kingani strain). Results After 5 years of storage, both unwashed and washed, Olyset® and DawaPlus® 2.0 (Tsara® Soft) LLINs passed WHO bio-efficacy criteria on knockdown (KD60) ≥ 95%, 24-h mortality ≥ 80% and ≥ 90% blood-feeding inhibition in WHO assays against susceptible An. gambiae s.s. DawaPlus® 2.0 LLINs also passed combined WHO bioassay criteria against resistant An. arabiensis. Confirmatory I-ACT tests using whole nets demonstrated that long-stored LLINs showed higher efficacy than new LLINs on both feeding inhibition and mortality endpoints against resistant strains. Conclusions Even after long-term storage of around 5 years, both Olyset® and DawaPlus® 2.0 LLINs remain efficacious against susceptible Anopheles mosquitoes at optimal storage range of 25 °C to 33.4 °C for temperature and 40% to 100% relative humidity measured by standard WHO methods. DawaPlus® 2.0 (Tsara® Soft) remained efficacious against resistant strain.

Use of novel lab assays to examine the effect of pyrethroid-treated bed nets on blood-feeding success and longevity of highly insecticide-resistant Anopheles gambiae s.l. mosquitoes

“…The experiments also highlight that the standard WHO test procedures for evaluating resistance and measuring the bio-efficacy of ITNs [ 17 , 44 ] are weak indicators of how ITNs ultimately determine malaria transmission risk and, hence, of understanding the functional significance of insecticide resistance. The WHO resistance assay uses tubes (as used in the initial assays here) to force mosquitoes into contact with filter paper treated with diagnostic doses of insecticide for 1 h [ 45 – 47 ], and the WHO ITN bioefficacy assay uses cones to force mosquitoes into contact with ITNs for 3 min [ 48 – 50 ]. Neither assay simulates how mosquitoes contact ITNs during host-searching and blood-feeding in nature [ 27 ].…”

Use of Novel Lab-Assays to Examine the Effect of Pyrethroid-Treated Bed Nets on Blood Feeding Success and Longevity of Highly Insecticide-Resistant Anopheles Gambiae S.I. Mosquitoes.

“…The experiments also highlight the fact that the standard WHO test procedures for evaluating resistance and measuring the bio-e cacy of ITNs [16,43] are weak indicators of how ITNs ultimately determine malaria transmission risk, and hence for understanding the functional signi cance of insecticide resistance. The WHO resistance assay uses tubes (as used in the initial assays here) to force mosquitoes into contact with lter paper treated with diagnostic doses of insecticide for 1h [44][45][46].The WHO ITN bioe cacy assay uses cones to force mosquitoes into contact with ITNs for 3 minutes [47][48][49]. Neither assay simulates how mosquitoes contact ITNs during host searching and blood feeding in nature [26].…”

A Colorimetric Test for the Evaluation of the Insecticide Content of LLINs Used on Bioko Island, Equatorial Guinea

“…The e cacy of the insecticide residue is mostly determined by bioassay and/or chemical analysis, with chemical analysis being the only way to quantify the amount of insecticide on the net. Cone bioassays are the most commonly used method that exposes insecticide susceptible mosquito strains to impregnated netting material for determining the residual insecticidal activity of the net [12]. However, bioassay testing requires skilled staff with access to laboratory and insectary facilities.…”