Read about our...
In response to the increased insecticide selection pressures, the major malaria vectors have evolved resistance to these insecticides and the resistance alleles are spreading at an exceptionally rapid rate throughout Africa.
The Liverpool Insect Testing Establishment (LITE) at the Liverpool School of Tropical Medicine (LSTM), which is core funded by IVCC, currently maintains seven well-characterised African Anopheline strains with a range of insecticide resistance genotypes and phenotypes. These resistant mosquito strains are maintained under selection pressure to preserve their resistance and all strains are profiled annually against 6 insecticides, representing the major classes of insecticide currently used for mosquito control, to monitor the stability of their resistance phenotype. Resistance intensity assays and synergist bioassays are also conducted to further profile the resistance in these strains. Each strain is genotyped approximately every 5th generation to determine species and the frequency of known target site resistance alleles.
Three different kdr alleles, mutations in the insect’s sodium channel linked with target site resistance to pyrethroids and DDT, are screened for (1014S, 1014F and 1575Y) plus the ace-1 119S allele, which is linked with target site resistance to organophosphate and carbamate insecticide. A glutathione transferase, GSTE2, that has also been linked with resistance to pyrethroid insecticides, has also been detected by molecular assay in one of the resistant strains maintained by LITE.
Pyrethroid resistance is widespread in the malaria vector Anopheles gambiae leading to concerns about the future efficacy of insecticidal treated nets with pyrethroids as the sole Active Ingredient (AI). The incorporation of pyriproxyfen (PPF), a juvenile hormone analogue, into pyrethroid treated insecticidal treated nets is being trialed in Africa.
Pyrethroid resistance is commonly associated with elevated levels of P450 expression including CYPs 6M2, 6P2, 6P3, 6P4, 6P5, 6Z2 and 9J5. Having expressed these P450s in E. coli we find all are capable of metabolizing PPF. Inhibition of these P450s by permethrin, deltamethrin and PPF was also examined. Deltamethrin and permethrin were moderate inhibitors (IC50 1–10 μM) of diethoxyfluorescein (DEF) activity for all P450s apart from CYP6Z2 (IC50 > 10 μM), while PPF displayed weaker inhibition of all P450s (IC50 > 10 μM) except CYP’s 6Z2 and 6P2 (IC50 1–10 μM).
Evidence of low levels of cross-resistance between PPF and other insecticide classes was established by comparing the efficacy of PPF in inhibiting metamorphosis and inducing female sterility in an insecticide susceptible strain of An. gambiae and a multiple resistant strain from Cote d’Ivoire.
IVCC has been working closely with Dr Mark Paine and his research group in the Vector Biology department of LSTM to understand the vulnerability of insecticides not previously used in vector control products to cross-resistance through the cytochrome P450-based mechanism.
This, together with cross-resistance bioassays using fully characterised strains of Anopheline mosquito vectors maintained by LITE at LSTM, gives us a much better understanding of the vulnerability of these insecticides to cross-resistance mechanisms already present in field populations of Anopheles in Africa.
