Combining organophosphate-Treated wall linings and long-lasting insecticidal nets fails to provide additional control over long-lasting insecticidal nets alone against multiple insecticide-resistant Anopheles gambiae in Côte d'Ivoire: An experimental hut trial

Background: Insecticide-Treated wall lining (ITWL) is a new concept in malaria vector control. Some Anophele gambiae populations in West Africa have developed resistance to all the main classes of insecticides. It needs to b demonstrated whether vector control can be improved or resistance managed when non-pyrethroid ITWL is use alone or together with long-lasting insecticidal nets (LLINs) against multiple insecticide-resistant vector populations 

Methods: Two experimental hut trials were carried out as proofs of concept to evaluate pirimiphos methy (p-methyl)-Treated plastic wall lining (WL) and net wall hangings (NWH) used alone and in combination with LLIN against multiple insecticide-resistant An. gambiae in Tiassalé, Côte d'Ivoire. Comparison was made to commercia deltamethrin WL and genotypes for kdr and ace-1R resistance were monitored 

Results: The kdr and ace-1R allele frequencies were 0.83 and 0.44, respectively. Anopheles gambiae survivin discriminating concentrations of deltamethrin and p-methyl in WHO resistance tests were 57 and 96%, respectively Mortality of free-flying An. gambiae in huts with p-methyl WL and NWH (66 and 50%, respectively) was higher tha with pyrethroid WL (32%; P < 0.001). Mortality with LLIN was 63%. Mortality with the combination of LLIN plu p-methyl NWH (61%) or LLIN plus p-methyl WL (73%) did not significantly improve upon the LLIN alone o p-methyl WL or NWH alone. Mosquitoes bearing the ace-1R were more likely to survive exposure to p-methy WL and NWH. Selection of heterozygote and homozygote ace-1R or kdr genotypes was not less likely after exposur to combined LLIN and p-methyl treatments than to single p-methyl treatment. Blood-feeding rates were lower i huts with the pyrethroid LLIN (19%) than with p-methyl WL (72%) or NWH (76%); only LLIN contributed to persona protection 

Conclusions: Combining p-methyl WL or NWH with LLINs provided no improvement in An. gambiae control o personal protection over LLIN alone in southern Côte d'Ivoire; neither did the combination manage resistance Additional resistance mechanisms to kdr and ace-1R probably contributed to the survival of pyrethroid an organophophate-resistant mosquitoes. The study demonstrates the challenge that malaria control programmes wil face if resistance to multiple insecticides continues to spread.