Background:

The spread of pyrethroid resistance in malaria vectors threatens the effectiveness of standardlong-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 assessedpersonal protection of WHO st-in-class PBO-Py treated nets (Olyset Plus) versus standard LLIN (Olysetnet) against pyrethroid resistant Anopheles gambiae and An.funestus in North West Tanzania after 20months of household use.

Methods:

From household survey, 39 standard Olyset net and 39 Olyset Plus houses were selected, physicalintegrity and hole index (HI) of nets assessed, resting mosquitoes collected from inside nets and from roomwalls, indoor abundance estimated using CDC light traps and species identified using PCR.

Results:

Of 2397 Anopheles collected, 8.9% (n=213) were resting inside standard Olyset nets while nonewere found inside Olyset Plus PBO-Py nets of any HI category. Resting density of blood fed mosquitoes washigher 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 morewere 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 thanwith Olyset net for new nets (76.8% vs 27.5%) and 20 months’ nets (56.8% vs 12.8%); similar trends wereobserved with An. funestus.

Conclusion:

The PBO-Py treated net provided improved protection after 20 months of household use, asdemonstrated by the higher bioassay mortality and absence of pyrethroid resistant An. gambiae s.s. andAn.funestus collected from inside Olyset Plus, irrespective of hole index category, as compared to Olysetnets.