Papua New Guinea (PNG) has the highest malaria transmission outside of Africa. Longlasting insecticidal nets (LLINs) are believed to have helped to reduce average malaria prevalence in PNG from 16% in 2008 to 1% in 2014. Since 2015 malaria in PNG has resurged significantly. Here, we present observations documenting decreased bioefficacy of unused LLINs with manufacturing dates between 2013 and 2019 collected from villages and LLIN distributors in PNG. Specifically, we show that of n = 167 tested LLINs manufactured after 2013, only 17% are fulfilling the required World Health Organisation bioefficacy standards of ≥ 80% 24 h mortality or ≥ 95% 60 min knockdown in bioassays with pyrethroid susceptible Anopheles farauti mosquitoes. In contrast, all (100%, n = 25) LLINs with manufacturing dates prior to 2013 are meeting these bioefficacy standards. These results suggest that decreased bioefficacy of LLINs is contributing to the malaria resurgence in PNG and increased scrutiny of LLIN quality is warranted.

Background: Papua New Guinea (PNG) has the highest malaria transmission outside of Africa and long-lasting insecticidal nets (LLIN) are the only vector-control tool distributed country-wide. LLIN were introduced into PNG in about 2005 and have been attributed to have had a huge impact on malaria transmission, with reductions in malaria prevalence observed from 15.7% (2008) to 1% (2014). Since 2015, malaria indicators in PNG have risen significantly. Similar trends have been observed in several African nations. In the present study, we observed a drastic reduction in bioefficacy of LLIN collected both from communities (used nets) and prior to use (new nets). We hypothesise that decreased bioefficacy of LLINs is a major contributor to the observed malaria resurgence in PNG and possibly in other parts of the world. Methods: New LLIN in original and unopened packaging (n=192) with a range of manufacturing dates from 2007-2019 were collected either directly from consignments or from households in 15 PNG provinces. Used LLIN (n=40) were collected in 2 provinces, with manufacturing dates ranging from 2008-2017. LLIN were subjected to standard WHO cone bioassays using fully susceptible An. farauti mosquitoes. A subset of LLIN was re-tested using fully susceptible An. gambiae mosquitoes in order to ensure reproducibility of results. Results: Only 7% (95% CI 4-12%) of new LLIN manufactured between 2013-2019 exhibited 100% mortality. However, 84% (95%CI: 65-84%) new nets manufactured in 2012 or before exhibited 100% mortality. Only 29 % of used LLIN less than 3 years old exhibited > 80% 24h-mortality. Results obtained in tests using An. farauti corresponded well with confirmatory tests conducted using An. gambiae. Discussion: Bioefficacy of LLIN in PNG appears to have been highly variable since 2013, with few nets manufactured since 2013 meeting WHO standards. This time-frame coincides with malaria resurgence in the country. These results may have ramifications for LLIN-based malaria control that go beyond the local PNG scenario.

Insecticide resistance (IR) monitoring is an important component of vector-borne disease control. The last assessment of IR in Papua New Guinea (PNG) was conducted in 2010. Since then, vector populations have been exposed to higher levels of pyrethroids with the continued nation-wide distribution of insecticide-treated nets. Here, we provide an update on phenotypic IR in four highly malaria-endemic areas of PNG. IR against deltamethrin, lambda-cyhalothrin, and dichlorodiphenyltrichloroethane was assessed using World Health Organization bioassays. A total of 108 bioassays for each insecticide were conducted screening 2,290 adult female anopheline mosquitoes. No phenotypic resistance was observed. Bioassay parameters agreed well with those observed in other studies that used the same assays and insecticides. These results indicate that the three tested insecticides are still universally effective in PNG. Continued IR monitoring (every 1-2 years) in PNG is recommended to detect reduced susceptibility early and adjust guidelines to prevent widespread resistance.

Background Insecticide resistance (IR) monitoring is essential for evidence-based control of mosquito-borne diseases. While widespread pyrethroid resistance in Anopheles and Aedes species has been described in many countries, data for Papua New Guinea (PNG) are limited. Available data indicate that the local Anopheles populations in PNG remain pyrethroid-susceptible, making regular IR monitoring even more important. In addition, Aedes aegypti pyrethroid resistance has been described in PNG. Here, Anopheles and Aedes IR monitoring data generated from across PNG between 2017 and 2022 are presented. Methods Mosquito larvae were collected in larval habitat surveys and through ovitraps. Mosquitoes were reared to adults and tested using standard WHO susceptibility bioassays. DNA from a subset of Aedes mosquitoes was sequenced to analyse the voltage-sensitive sodium channel (Vssc) region for any resistance-related mutations. Results Approximately 20,000 adult female mosquitoes from nine PNG provinces were tested. Anopheles punctulatus sensu lato mosquitoes were susceptible to pyrethroids but there were signs of reduced mortality in some areas. Some Anopheles populations were also resistant to DDT. Tests also showed that Aedes. aegypti in PNG are resistant to pyrethroids and DDT and that there was also likelihood of bendiocarb resistance. A range of Vssc resistance mutations were identified. Aedesalbopictus were DDT resistant and were likely developing pyrethroid resistance, given a low frequency of Vssc mutations was observed. Conclusions Aedes aegypti is highly pyrethroid resistant and also shows signs of resistance against carbamates in PNG. Anopheles punctulatus s.l. and Ae. albopictus populations exhibit low levels of resistance against pyrethroids and DDT in some areas. Pyrethroid-only bed nets are currently the only programmatic vector control tool used in PNG. It is important to continue to monitor IR in PNG and develop proactive insecticide resistance management strategies in primary disease vectors to retain pyrethroid susceptibility especially in the malaria vectors for as long as possible. Graphic abstract

Background: Insecticide resistance monitoring is key for evidence-based control of Anopheles and Aedes disease vectors in particular, since the vast majority of insecticide-based public health adult vector control tools are reliant on pyrethroids. While widespread pyrethroid resistance in Anopheles species and Aedes aegypti has been described in many countries, data for Papua New Guinea are scarce. Available data indicate the local Anopheles populations remain pyrethroid-susceptible, making regular insecticide resistance monitoring even more important. Knowledge on Aedes insecticide resistance in PNG is very limited, however, high levels of Aedes aegypti resistance have been described. Here we present insecticide resistance monitoring data from across PNG generated between 2017 and 2022. Methods: Mosquito larvae were collected in larval habitat surveys and through ovitraps. Mosquitoes were reared to adults and subjected to insecticide treated filter papers in WHO insecticide susceptibility bioassays. Subsets of Aedes mosquitoes were subjected to sequencing of the voltage-sensitive sodium channel (Vssc) region to identify resistance mutations. Results: Overall, nearly 20,000 adult female mosquitoes from nine PNG provinces were used in the tests. We show that in general, Anopheline mosquitoes in PNG remain susceptible to pyrethroids but with worrying signs of reduced 24 h mortality in some areas. In addition, some Anopheles populations were indicated to be resistant against DDT. We show that Ae. aegypti in PNG are pyrethroid, DDT and likely bendiocarb resistant with a range of Vssc resistance mutations identified. We demonstrate that Ae. albopictus is DDT resistant and is likely developing pyrethroid resistance based on finding a low frequency of Vssc mutations. Conclusion: This study represents the largest overview of insecticide resistance in PNG. While Ae. aegypti is highly pyrethroid resistant, the Anopheline and Ae. albopictus populations exhibit low levels of resistance in some areas. It is important to continue to monitor insecticide resistance in PNG and prepare for the widespread emergence of pyrethroid resistance in major disease vectors.