Personal Protection with PBO-Pyrethroid Synergist Treated Nets After Two Years of Household Use Against Pyrethroid-Resistant Anopheles in Tanzania

Abstract

Background: The spread of pyrethroid resistance in malaria vectors threatens the effectiveness of standard long-lasting insecticidal nets (LLIN). Synergist nets combine pyrethroid (Py) and piperonyl-butoxide (PBO) to enhance potency against resistance mediated by mono-oxygenase mechanisms. Our project assessed personal protection of WHO first-in-class PBO-Py treated nets (Olyset Plus) versus standard LLIN (Olyset net) against pyrethroid resistant Anopheles gambiae and An. funestus in North West Tanzania after 20 months of household use.Methods: From household survey, 39 standard Olyset net and 39 Olyset Plus houses were selected, physical integrity and hole index (HI) of nets assessed, resting mosquitoes collected from inside nets and from room walls, indoor abundance estimated using CDC light traps and species identified using PCR. Residual bio-efficacy of PBO and standard LLINs was assessed using 30 min cylinder bioassays.Results: Of 2397 Anopheles collected, 8.9% (n=213) were resting inside standard Olyset nets while none were found inside Olyset Plus PBO-Py nets of any HI category. Resting density of blood fed mosquitoes was higher on walls of sleeping rooms with Olyset net compared to Olyset Plus (0.62 vs 0.10, density ratio: 0.03, 95% CI: 0.01-0.13, p<0.001). Mosquitoes were found inside Olyset nets of all WHO HI categories but more were collected inside the more damaged (HI≥ 643) nets than in less damaged (HI 0-64) nets (DR: 6.4, 95% CI: 1.1-36.0, p=0.037). In residual bioassay, mortality of An. gambiae s.l was higher with Olyset Plus than with Olyset net for new nets (76.8% vs 27.5%) and 20 months’ nets (56.8% vs 12.8%); similar trends were observed with An. funestus. Conclusion: The PBO-Py treated net provided improved protection after 20 months of household use, as demonstrated by the higher bioassay mortality and absence of pyrethroid resistant An. gambiae s.s. and An. funestus collected from inside Olyset Plus, irrespective of hole index category, as compared to Olyset nets.

Impact of textile durability on the efficacy of three types of dual-active-ingredient long- lasting insecticidal nets: A secondary analysis on malaria prevalence and incidence from a cluster-randomized trial in north-west Tanzania

“…Enhancing the bottom part might remedy nets from early hole development and further enlargement, although other studies have shown that dense knitting in this part of the net did not necessarily make it more durable. 33 Similarities in physical properties ( bers, denier, and integrity) between some nets [Chlorfenapyr-PY LLIN and PY-LLIN], but differences in protective e cacy against malaria infection emphasize the pivotal role of non-pyrethroid insecticide and synergists (chlorfenapyr and PBO 34 ) in malaria vector control. In addition, this study demonstrates that the resilient physical integrity of the nets on its own is not enough to provide sustained protection to users of intact (physically durable but insecticide-free) nets against bites of malaria-transmitting mosquitoes even in settings of high net coverage, as it was in the cohort children where we observed > 80% use.…”