Influence of testing modality on bioefficacy for the evaluation of Interceptor® G2 mosquito nets to combat malaria mosquitoes in Tanzania

Bio-efficacy of Olyset® Plus, PermaNet® 3.0 and Interceptor® G2 on pyrethroid-resistant populations of Anopheles gambiae s.l. prior to the June 2023 net distribution campaign in Benin, West Africa

BACKGROUND

This study investigates the effectiveness of new-generation mosquito nets, like Olyset® Plus and PermaNet® 3.0, and dual-action nets such as Interceptor® G2, against pyrethroid-resistant Anopheles gambiae mosquitoes following the 2023 mass distribution of long-lasting insecticidal nets in Benin.

METHODS

We tested wild mosquito populations from six communes in Benin against various pyrethroid (permethrin 0.75%, alphacypermethrin 0.05%, and deltamethrin 0.05%) using WHO tube tests. Additionally, we exposed mosquitoes to chlorfenapyr 100 µg/ml using the CDC bottle bioassay method. A subset of mosquitoes underwent biochemical and PCR tests to check the overexpression of metabolic enzymes and the Kdr L1014F mutation. We evaluated the effectiveness of Olyset® Plus, PermaNet® 3.0, and Interceptor® G2 nets using cone and tunnel tests on both laboratory and field populations of An. gambiae.

RESULTS

Overall, the highest mortality rate was 60% with pyrethroid and 98 to100% with chlorfenapyr. In cone tests, all three types of nets induced mortality rates above 80% in the susceptible laboratory strain of An. gambiae. Notably, Olyset® Plus showed the highest mortality rates for pyrethroid-resistant mosquitoes in cone tests, ranging from 81.03% (95% CI: 68.59-90.13) in Djougou to 96.08% (95% CI: 86.54-99.52) in Akpro-Missérété. PermaNet® 3.0 had variable rates, from 42.5% (95% CI: 27.04-59.11) in Djougou to 58.54% (95% CI: 42.11-73.68) in Porto-Novo. However, revealed good results for Interceptor® G2, with 94% (95% CI: 87.40-97.77) mortality and 89.09% blood sampling inhibition in local populations of An. gambiae. In comparison, Interceptor® had lower rates of 17% (95% CI: 10.23-25.82) and 60%, respectively.

CONCLUSION

These results suggest that tunnel tests are effective for evaluating dual-active ingredient nets. Additionally, Interceptor® G2 and PBO nets like Olyset® Plus could be considered as alternatives against pyrethroid-resistant mosquitoes.

Video augmentation of the WHO cone assay to quantify mosquito behavioural responses to insecticide-treated nets

BACKGROUND

Insecticide-treated nets (ITNs) using pyrethroids have been the main vector control tools deployed in malaria endemic countries and are responsible for the dramatic reduction in African malaria cases in the early 2000s. The World Health Organization (WHO) cone test was designed to assess the rapid toxicity effects of pyrethroid exposure on mosquito vectors but has yielded no insights beyond 60-min knockdown and 24-h mortality. As dual-active-ingredient (AI) ITNs become more widespread, bioassays that can provide realistic assessment of single- and dual-treated ITNs (i.e. nets with more than one active ingredient) are urgently needed.

METHODS

We present an augmentation of the cone test that enables accurate quantification of vector behavioural responses (specifically movement, spatial and temporal occupancy) to ITNs using video recording and bespoke software that uses background segmentation methods to detect spatial changes in the movement of mosquitoes within the cone. Four strains of Anopheles gambiae sensu lato (s.l.) were exposed to four ITNs (PermaNet 2.0, PermaNet 3.0, Olyset Net, Interceptor G2) and untreated nets in these modified cone tests. Life history data (post-exposure blood-feeding, blood meal weight, longevity) for individual mosquitoes were recorded.

RESULTS

All mosquitoes responded to the presence of ITNs, spending from 1.48 to 3.67 times more time in the upper region of the cone, depending on the ITN type. Of all ITNs, PermaNet 2.0 provoked the smallest change in behavioural response. Activity in the cone influenced observed post-exposure longevity, and in resistant strains exposed to Interceptor G2, the higher the activity, the greater the risk of dying, as long as the proportion of activity at the net surface was less than 50%. All ITNs inhibited blood-feeding, and smaller blood meals were taken when mosquitoes fed.

CONCLUSIONS

The additional mosquito behaviour data obtained by using this modification to the WHO cone test provides unique insight into the innate responses of different mosquito strains on untreated nets and the entomological mode of action of ITNs, important evidence when evaluating ITN characteristics.

Sub-lethal exposure to chlorfenapyr reduces the probability of developing Plasmodium falciparum parasites in surviving Anopheles mosquitoes

BACKGROUND

Pyrethroid resistance in the key malaria vectors threatens the success of pyrethroid-treated nets. To overcome pyrethroid resistance, Interceptor® G2 (IG2), a 'first-in-class' dual insecticidal net that combines alpha-cypermethrin with chlorfenapyr, was developed. Chlorfenapyr is a pro-insecticide, requiring bio-activation by oxidative metabolism within the insect's mitochondria, constituting a mode of action preventing cross-resistance to pyrethroids. Recent epidemiological trials conducted in Benin and Tanzania confirm IG2's public health value in areas with pyrethroid-resistant Anopheles mosquitoes. As chlorfenapyr might also interfere with the metabolic mechanism of the Plasmodium parasite, we hypothesised that chlorfenapyr may provide additional transmission-reducing effects even if a mosquito survives a sub-lethal dose.

METHODS

We tested the effect of chlorfenapyr netting to reduce Plasmodium falciparum transmission using a modified WHO tunnel test with a dose yielding sub-lethal effects. Pyrethroid-resistant Anopheles gambiae s.s. with L1014F and L1014S knockdown resistance alleles and expression levels of pyrethroid metabolisers CYP6P3, CYP6M2, CYP4G16 and CYP6P1 confirmed by quantitative reverse transcription polymerase chain reaction (RT-qPCR) prior to conducting experiments were exposed to untreated netting and netting treated with 200 mg/m3 chlorfenapyr for 8 h overnight and then fed on gametocytemic blood meals from naturally infected individuals. Prevalence and intensity of oocysts and sporozoites were determined on day 8 and day 16 after feeding.

RESULTS

Both prevalence and intensity of P. falciparum infection in the surviving mosquitoes were substantially reduced in the chlorfenapyr-exposed mosquitoes compared to untreated nets. The odds ratios in the prevalence of oocysts and sporozoites were 0.33 (95% confidence interval; 95% CI 0.23-0.46) and 0.43 (95% CI 0.25-0.73), respectively, while only the incidence rate ratio for oocysts was 0.30 (95% CI 0.22-0.41).

CONCLUSION

We demonstrated that sub-lethal exposure of pyrethroid-resistant mosquitoes to chlorfenapyr substantially reduces the proportion of infected mosquitoes and the intensity of the P. falciparum infection. This will likely also contribute to the reduction of malaria in communities beyond the direct killing of mosquitoes.

Small-scale field evaluation of PermaNet® Dual (a long-lasting net coated with a mixture of chlorfenapyr and deltamethrin) against pyrethroid-resistant Anopheles gambiae mosquitoes from Tiassalé, Côte d'Ivoire

BACKGROUND

Due to the rapid expansion of pyrethroid-resistance in malaria vectors in Africa, Global Plan for Insecticide Resistance Management (GPIRM) has recommended the development of long-lasting insecticidal nets (LLINs), containing insecticide mixtures of active ingredients with different modes of action to mitigate resistance and improve LLIN efficacy. This good laboratory practice (GLP) study evaluated the efficacy of the chlorfenapyr and deltamethrin-coated PermaNet^® Dual, in comparison with the deltamethrin and synergist piperonyl butoxide (PBO)-treated PermaNet^® 3.0 and the deltamethrin-coated PermaNet^® 2.0, against wild free-flying pyrethroid-resistant Anopheles gambiae sensu lato (s.l.), in experimental huts in Tiassalé, Côte d'Ivoire (West Africa).

METHODS

PermaNet^® Dual, PermaNet^® 3.0 and PermaNet^® 2.0, unwashed and washed (20 washes), were tested against free-flying pyrethroid-resistant An. gambiae s.l. in the experimental huts in Tiassalé, Côte d'Ivoire from March to August 2020. Complementary laboratory cone bioassays (daytime and 3-min exposure) and tunnel tests (nightly and 15-h exposure) were performed against pyrethroid-susceptible An. gambiae sensu stricto (s.s.) (Kisumu strain) and pyrethroid-resistant An. gambiae s.l. (Tiassalé strain).

RESULTS

PermaNet^® Dual demonstrated significantly improved efficacy, compared to PermaNet^® 3.0 and PermaNet^® 2.0, against the pyrethroid-resistant An. gambiae s.l. Indeed, the experimental hut trial data showed that the mortality and blood-feeding inhibition in the wild pyrethroid-resistant An. gambiae s.l. were overall significantly higher with PermaNet^® Dual compared with PermaNet^® 3.0 and PermaNet^® 2.0, for both unwashed and washed samples. The mortality with unwashed and washed samples were 93.6 ± 0.2% and 83.2 ± 0.9% for PermaNet^® Dual, 37.5 ± 2.9% and 14.4 ± 3.9% for PermaNet^® 3.0, and 7.4 ± 5.1% and 11.7 ± 3.4% for PermaNet^® 2.0, respectively. Moreover, unwashed and washed samples produced the respective percentage blood-feeding inhibition of 41.4 ± 6.9% and 43.7 ± 4.8% with PermaNet^® Dual, 51.0 ± 5.7% and 9.8 ± 3.6% with PermaNet^® 3.0, and 12.8 ± 4.3% and - 13.0 ± 3.6% with PermaNet^® 2.0. Overall, PermaNet^® Dual also induced higher or similar deterrence, exophily and personal protection when compared with the standard PermaNet^® 3.0 and PermaNet^® 2.0 reference nets, with both unwashed and washed net samples. In contrast to cone bioassays, tunnel tests predicted the efficacy of PermaNet^® Dual seen in the current experimental hut trial.

CONCLUSION

The deltamethrin-chlorfenapyr-coated PermaNet^® Dual induced a high efficacy and performed better than the deltamethrin-PBO PermaNet^® 3.0 and the deltamethrin-only PermaNet^® 2.0, testing both unwashed and 20 times washed samples against the pyrethroid-susceptible and resistant strains of An. gambiae s.l. The inclusion of chlorfenapyr with deltamethrin in PermaNet^® Dual net greatly improved protection and control of pyrethroid-resistant An. gambiae populations. PermaNet^® Dual thus represents a promising tool, with a high potential to reduce malaria transmission and provide community protection in areas compromised by mosquito vector resistance to pyrethroids.

Multi-country evaluation of the durability of pyrethroid plus piperonyl-butoxide insecticide-treated nets: study protocol

BACKGROUND

Mass distributions of long-lasting insecticidal nets (LLINs) have contributed to large reductions in the malaria burden. However, this success is in jeopardy due in part to the increasing pyrethroid-resistant mosquito population as well as low LLINs coverage in various areas because the lifespan of LLINs is often shorter than the interval between replenishment campaigns. New insecticide-treated nets (ITNs) containing pyrethroid and piperonyl-butoxide (PBO) have shown a greater reduction in the incidence of malaria than pyrethroid LLINs in areas with pyrethroid-resistant mosquitoes. However, the durability (attrition, bio-efficacy, physical integrity and chemical retainment) of pyrethroid-PBO ITNs under operational settings has not been fully characterized. This study will measure the durability of pyrethroid-PBO ITNs to assess whether they meet the World Health Organization (WHO) three years of operational performance criteria required to be categorized as "long-lasting".

METHODS

A prospective household randomized controlled trial will be conducted simultaneously in Tanzania, India and Côte d'Ivoire to estimate the field durability of three pyrethroid-PBO ITNs (Veeralin®, Tsara® Boost, and Olyset® Plus) compared to a pyrethroid LLIN: MAGNet®. Durability monitoring will be conducted up to 36 months post-distribution and median survival in months will be calculated. The proportion of ITNs: (1) lost (attrition), (2) physical integrity, (3) resistance to damage score, (4) meeting WHO bio-efficacy (≥ 95% knockdown after 1 h or ≥ 80% mortality after 24 h for WHO cone bioassay, or ≥ 90% blood-feeding inhibition or ≥ 80% mortality after 24 h for WHO Tunnel tests) criteria against laboratory-reared resistant and susceptible mosquitoes, and insecticidal persistence over time will be estimated. The non-inferiority of Veeralin® and Tsara® Boost to the first-in-class, Olyset® Plus will additionally be assessed for mortality, and the equivalence of 20 times washed ITNs compared to field aged ITNs will be assessed for mortality and blood-feeding inhibition endpoints in the Ifakara Ambient Chamber Test, Tanzania.

CONCLUSION

This will be the first large-scale prospective household randomized controlled trial of pyrethroid-PBO ITNs in three different countries in East Africa, West Africa and South Asia, simultaneously. The study will generate information on the replenishment intervals for PBO nets.

Insecticides for Mosquito Control: Improving and Validating Methods to Strengthen the Evidence Base

Efforts to eliminate vector-borne diseases, for example malaria which caused an estimated 619,000 deaths in 2021 [...].

WHO cone bioassay boards with or without holes: relevance for bioassay outcomes in long-lasting insecticidal net studies

BACKGROUND

The World Health Organization (WHO) cone bioassay is a key method used to evaluate the bioefficacy of long-lasting insecticidal nets (LLINs) used for malaria control. These tests also play an important role in LLIN product prequalification and longitudinal monitoring. Standardization of these assays is therefore important. While many parameters for WHO cone bioassays are defined in the respective WHO guidelines, others are not. One of these undefined parameters is the exact configuration of the bioassay boards. In cone bioassays, LLIN samples are pinned onto a bioassay board for testing. Anecdotal evidence suggests that bioassay boards with holes behind the LLIN samples lead to greater exposure to insecticide, as the mosquitoes are 'forced to stand on the net material'. This may increase the key assay outcomes of 60 min knockdown (KD60) and 24 h mortality (M24). The present study tested this hypothesis in two facilities using two fully susceptible mosquito colonies.

METHODS

WHO cone bioassays were performed using bioassay boards with holes and boards without holes in parallel, following WHO guidelines. Five brands of LLINs with four new and unwashed whole net samples per brand were used (total of n = 20 whole nets). Five pieces per whole net sample were prepared in duplicate resulting in a total of n = 100 pairs. Knock-down (KD) was recorded in 10 min intervals within the first hour after exposure and mortality was recorded at 24 h. Assays with Anopheles farauti were done at the Papua New Guinea Institute of Medical Research (PNGIMR) and assays with Aedes aegypti were done at James Cook University, Australia.

RESULTS

Results varied not only with bioassay board configuration but also with mosquito colony. In particular, with An. farauti, a significantly higher M24 was observed when boards with holes were used, while this was not observed with Ae. aegypti. WHO cone bioassay results were systematically biased between the two facilities such that the use of An. farauti at PNGIMR predicted higher KD60 and M24.

CONCLUSION

The present study highlights the need for further harmonization of WHO cone bioassay methodology. Parameters such as bioassay board configuration and mosquito species systematically affect the observations, which impedes generalizability of WHO cone bioassay outcomes.

Modified World Health Organization (WHO) Tunnel Test for Higher Throughput Evaluation of Insecticide-Treated Nets (ITNs) Considering the Effect of Alternative Hosts, Exposure Time, and Mosquito Density

The standard World Health Organization (WHO) tunnel test is a reliable laboratory bioassay used for “free-flying” testing of insecticide-treated nets (ITNs) bio-efficacy where mosquitoes pass through a ITN sample to reach a live animal bait. Multiple parameters (i.e., bait, exposure time, and mosquito density) may affect the outcomes measured in tunnel tests. Therefore, a comparison was conducted of alternative hosts, exposure time, and lower mosquito density against the current gold standard test (100 mosquitoes, animal bait, and 12-h exposure) as outlined in the WHO ITN evaluation guideline. This was done with the aim to make the tunnel test cheaper and with higher throughput to meet the large sample sizes needed for bio-efficacy durability monitoring of chlorfenapyr ITNs that must be evaluated in “free-flying” bioassays. Methods: A series of experiments were conducted in the WHO tunnel test to evaluate the impact of the following factors on bio-efficacy endpoints of mosquito mortality at 24-h (M24) and 72-h (M72) and blood-feeding success (BFS): (1) baits (rabbit, membrane, human arm); (2) exposure time in the tunnel (1 h vs. 12 h); and (3) mosquito density (50 vs. 100). Finally, an alternative bioassay using a membrane with 50 mosquitoes (membrane-50) was compared to the gold standard bioassay (rabbit with 100 mosquitoes, rabbit-100). Pyrethroid-resistant Anopheles arabiensis and pyrethroid susceptible Anopheles gambiae were used to evaluate Interceptor® and Interceptor® G2 ITNs. Results: Using a human arm as bait gave a very different BFS, which impacted measurements of M24 and M72. The same trends in M24, M72 and BFS were observed for both Interceptor® ITN and Interceptor® G2 unwashed and washed 20 times measured using the gold standard WHO tunnel test (rabbit-100) or rabbit with 50 mosquitoes (rabbit-50). M24, M72 and BFS were not statistically different when either 50 or 100 mosquitoes were used with rabbit bait in the tunnel bioassay for either the susceptible or resistant strains. No systematic difference was observed between rabbit-50 and rabbit-100 in the agreement by the Bland and Altman method (B&A). The mean difference was 4.54% (−22.54–31.62) in BFS and 1.71% (−28.71–32.12) in M72 for rabbit-50 versus rabbit-100. Similar M24, M72 and lower BFS was measured by membrane-50 compared to rabbit-100. No systematic difference was observed in the agreement between membrane-50 and rabbit-100, by B&A. The mean difference was 9.06% (−11.42–29.64) for BSF and −5.44% (−50.3–39.45) for M72. Both membrane-50, rabbit-50 and rabbit-100 predicted the superiority of Interceptor® G2 over Interceptor® ITN for the resistant strain on M72. Conclusion: These results demonstrate that WHO tunnel tests using rabbit bait may be run with 50 mosquitoes to increase sample sizes needed for bio-efficacy durability monitoring of ITNs in “free-flying” bioassays. Using a membrane feeder with 50 mosquitoes is a potential replacement for the WHO tunnel bioassay with animal bait if control blood feeding rates can be improved to 50% because blood feeding impacts mosquito survival after exposure to insecticides.