Comparing the new Ifakara Ambient Chamber Test with WHO cone and tunnel tests for bioefficacy and non-inferiority testing of insecticide-treated nets

Abstract

Background Insecticide-treated net (ITN) durability, measured through physical integrity and bioefficacy, must be accurately assessed in order to plan the timely replacement of worn out nets and guide procurement of longer-lasting, cost-effective nets. World Health Organization (WHO) guidance advises that new intervention class ITNs be assessed 3 years after distribution, in experimental huts. In order to obtain information on whole-net efficacy cost-effectively and with adequate replication, a new bioassay, the Ifakara Ambient Chamber Test (I-ACT), a semi-field whole net assay baited with human host, was compared to established WHO durability testing methods. Methods Two experiments were conducted using pyrethroid-susceptible female adult Anopheles gambiae sensu stricto comparing bioefficacy of Olyset ® , PermaNet ® 2.0 and NetProtect ® evaluated by I-ACT and WHO cone and tunnel tests. In total, 432 nets (144/brand) were evaluated using I-ACT and cone test. Olyset ® nets (132/144) that did not meet the WHO cone test threshold criteria (≥ 80% mortality or ≥ 95% knockdown) were evaluated using tunnel tests with threshold criteria of ≥ 80% mortality or ≥ 90% feeding inhibition for WHO tunnel and I-ACT. Pass rate of nets tested by WHO combined standard WHO bioassays (cone/tunnel tests) was compared to pass in I-ACT only by net brand and time after distribution. Results Overall, more nets passed WHO threshold criteria when tested with I-ACT than with standard WHO bioassays 92% vs 69%, (OR: 4.1, 95% CI 3.5–4.7, p < 0.0001). The proportion of Olyset ® nets that passed differed if WHO 2005 or WHO 2013 LN testing guidelines were followed: 77% vs 71%, respectively. Based on I-ACT results, PermaNet ® 2.0 and NetProtect ® demonstrated superior mortality and non-inferior feeding inhibition to Olyset ® over 3 years of field use in Tanzania. Conclusion Ifakara Ambient Chamber Test may have use for durability studies and non-inferiority testing of new ITN products. It measures composite bioefficacy and physical integrity with both mortality and feeding inhibition endpoints, using fewer mosquitoes than standard WHO bioassays (cone and tunnel tests). The I-ACT is a high-throughput assay to evaluate ITN products that work through either contact toxicity or feeding inhibition. I-ACT allows mosquitoes to interact with a host sleeping underneath a net as encountered in the field, without risk to human participants. Electronic supplementary material The online version of this article (10.1186/s12936-019-2741-y) contains supplementary material, which is available to authorized users.

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

“…This mode of action reduces the probability of mosquito dying from exposure to the insecticide following multiple contacts with net, but also gives Olyset ® its feeding inhibition properties that were observed in the I-ACT (Table 3), allowing protection of human volunteers sleeping beneath them even after five years and two months of storage. Similar results were observed by Massue et al [22]. It was again observed, by Jaramillo et al, on which permethrin treated net (Olyset ® LLIN) reduced contacts of Anopheles albimanus to net surface in the cone test [31].…”

“…DawaPlus ® 2.0 LLIN, met the WHO criteria in the standard WHO cone assay without the need to conduct a WHO tunnel test, while Olyset ® LLIN failed to meet the criteria based on the cone assay but passed based on WHO tunnel test ( Table 2). This is because Olyset ® is a high density polyethylene net, meaning that migration of permethrin is very slow with short wash intervals, hence surface concentrations are very low, sufficient to induce KD60 effect, but insufficient to induce mortality [22,29,30]. This mode of action reduces the probability of mosquito dying from exposure to the insecticide following multiple contacts with net, but also gives Olyset ® its feeding inhibition properties that were observed in the I-ACT (Table 3), allowing protection of human volunteers sleeping beneath them even after five years and two months of storage.…”

“…First, through a randomized double blinded, bio-efficacy evaluation of LLINs using standard WHO assays [8]. This was followed by partially randomized double blinded semi-field tests to compare the bioefficacy of long stored LLINs against the new LLINs of the same brand using the Ifakara Ambient Chamber tests (I-ACT) [22]. Untreated Safi Net was used as a negative control in all tests to monitor the quality of the experiment.…”

“…The I-ACT was used as an intermediate between laboratory and experimental hut tests [22]. One LLIN per condition (unwashed or washed twenty times) was tested as a confirmation of the WHO laboratory bioassay findings.…”

Use of alternative bioassays to explore the impact of pyrethroid resistance on LLIN efficacy

“…The cone test, which exposes groups of five mosquitoes directly to an LLIN, also dictates the use of young, nonblood-fed mosquitoes, and forces exposure through a very confined area instead of allowing the mosquito to naturally contact the net. The tunnel test, on the other hand, allows the mosquito to host-seek and blood-feed, though it uses a rodent host, which is not the preferred host of the anthropophilic Anopheles malaria vectors [32]. At present, there are no existing tools that can fully evaluate the response of resistant mosquitoes as they naturally contact a LLIN.…”