Absence of knockdown resistance suggests metabolic resistance in the main malaria vectors of the Mekong region.
BACKGROUND
As insecticide resistance may jeopardize the successful malaria control programmes in the Mekong region, a large investigation was previously conducted in the Mekong countries to assess the susceptibility of the main malaria vectors against DDT and pyrethroid insecticides. It showed that the main vector, Anopheles epiroticus, was highly pyrethroid-resistant in the Mekong delta, whereas Anopheles minimus sensu lato was pyrethroid-resistant in northern Vietnam. Anopheles dirus sensu stricto showed possible resistance to type II pyrethroids in central Vietnam. Anopheles subpictus was DDT- and pyrethroid-resistant in the Mekong Delta. The present study intends to explore the resistance mechanisms involved.


METHODS
By use of molecular assays and biochemical assays the presence of the two major insecticide resistance mechanisms, knockdown and metabolic resistance, were assessed in the main malaria vectors of the Mekong region.


RESULTS
Two FRET/MCA assays and one PCR-RFLP were developed to screen a large number of Anopheles populations from the Mekong region for the presence of knockdown resistance (kdr), but no kdr mutation was observed in any of the study species. Biochemical assays suggest an esterase mediated pyrethroid detoxification in An. epiroticus and An. subpictus of the Mekong delta. The DDT resistance in An. subpictus might be conferred to a high GST activity. The pyrethroid resistance in An. minimus s.l. is possibly associated with increased detoxification by esterases and P450 monooxygenases.


CONCLUSIONS
As different metabolic enzyme systems might be responsible for the pyrethroid and DDT resistance in the main vectors, each species may have a different response to alternative insecticides, which might complicate the malaria vector control in the Mekong region.
Verhaeghen
Katrijn
K
Department of Parasitology, Institute of Tropical Medicine, Antwerp, Belgium. kverhaeghen@itg.be
Van Bortel
Wim
W
Trung
Ho Dinh
HD
Sochantha
Tho
T
Coosemans
Marc
M
D000818:Animals; D000852:Anopheles / Q000145:classification / Q000187:drug effects* / Q000235:genetics*; D001210:Asia, Southeastern; D003634:DDT / Q000494:pharmacology; D004950:Esterases / Q000235:genetics / Q000378:metabolism; D031541:Fluorescence Resonance Energy Transfer; D055785:Gene Knockdown Techniques; D007303:Insect Vectors / Q000145:classification / Q000187:drug effects* / Q000235:genetics*; D007305:Insecticide Resistance; D007306:Insecticides / Q000494:pharmacology*; D008288:Malaria / Q000517:prevention & control*; D008541:Mekong Valley; D006899:Mixed Function Oxygenases / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D009032:Mosquito Control; D016133:Polymerase Chain Reaction / Q000662:veterinary; D012150:Polymorphism, Restriction Fragment Length; D011722:Pyrethrins / Q000494:pharmacology; D017422:Sequence Analysis, DNA
D007306:Insecticides; D011722:Pyrethrins; D003634:DDT; D006899:Mixed Function Oxygenases; D004950:Esterases