Characterization of amplified esterase Estβ12 associated with organophosphate resistance in a multi-resistant population of the mosquitoCulex quinquefasciatus from Cuba

Esterase amplification is the major organophosphorus (OP) insecticide resistance mechanism in Culex mosquitoes. The amplified Estα2 ^1\ Estβ2 ¹ esterases are found in > 90% of resistant populations worldwide, whereas amplified DNAs (amplicons) containing Estβ1s are much rarer. Individuals with the Estβ1 amplicons appear to be at a selective disadvantage in competition with those carrying the Estα2 ¹\ Estβ2 ¹ amplicons. To test the hypothesis that this is because Estβ1 is less able to bind insecticide than the common amplified esterases, Estβ1² was purified from the multi-resistant Habana strain of Culex quinquefasciatus , from Cuba. In its native form Estβ1 is a monomeric enzyme of 66 kDa, with a pI of 4.8. The bimolecular rate constants for interaction of Estβ1² with several OP insecticides were similar to those for the commonly elevated esterases Estα2¹ and Estβ2¹, and much higher than for the electrophoretically identical non-elevated Estβ1³ and Estα3. Hence the apparent selective advantage of the Estα2 ¹\ Estβ2 ¹ amplicon is not due to its greater efficiency of insecticide binding, as OP insecticides are significantly better inhibitors of all the amplified esterases than of their non-amplified counterparts and therefore should be equally effective at conferring resistance.     

Resistance management strategies in malaria vector mosquito control. Baseline data for a large-scale field trial against Anopheles albimanus in Mexico

Abstract. A high level of DDT resistance and low levels of resistance to organophosphorus, carbamate and pyrethroid insecticides were detected by discriminating dose assays in field populations of Anopheles albimanus in Chiapas, southern Mexico, prior to a large-scale resistance management project described by Hemingway et al. (1997) . Biochemical assays showed that the DDT resistance was caused by elevated levels of glutathione S-transferase (GST) activity leading to increased rates of metabolism of DDT to DDE. The numbers of individuals with elevated GST and DDT resistance were well correlated, suggesting that this is the only major DDT resistance mechanism in this population. The carbamate resistance in this population is conferred by an altered acetylcholinesterase (AChE) -based resistance mechanism. The level of resistance observed in the bioassays correlates with the frequency of individuals homozygous for the altered AChE allele. This suggests that the level of resistance conferred by this mechanism in its heterozygous state is below the level of detection by the WHO carbamate discriminating dosage bioassay. The low levels of organophosphate (OP) and pyrethroid resistance could be conferred by either the elevated esterase or monooxygenase enzymes. The esterases were elevated only with the substrate pNPA, and are unlikely to be causing broad spectrum OP resistance. The altered AChE mechanism may also be contributing to the OP but not the pyrethroid resistance. Significant differences in resistance gene frequencies were obtained from the F₁ mosquitoes resulting from adults obtained by different collection methods. This may be caused by different insecticide selection pressures on the insects immediately prior to collection, or may be an indication that the indoor- and outdoor-resting A. albimanus collections are not from a randomly mating single population. The underlying genetic variability of the populations is currently being investigated by molecular methods.     

Permethrin resistance in the head louse Pediculus capitis from Israel

Head lice, Pediculus capitis , were collected from children aged 3–12 years in Maale Adumin, a town near Jerusalem, after reports of control failure with the pyrethroid insecticide permethrin. A total of 1516 children were examined: living lice and eggs were found on 12.1% of the children; or another 22.8% of the children only nits were found. Twice as many girls as boys (8.1% v 4%) were infested with lice and or nits. Head lice collected from infested children were exposed to permethrin impregnated filter-papers. Log time probit mortality (ltp) regression lines were calculated for mortality data and compared to ltp lines for a similar collection of head lice made in 1989. The regression lines for the two years were significantly different, with a 4-fold decrease in susceptibility at the LT50 level between 1989 and 1994. The slopes of the lines also suggested that the 1994 population was more heterogenous in its response to permethrin than the 1989 population. In contrast, a laboratory population of body lice (Pediculus humanus) tested with the same batch of permethrin-impregnated papers showed a slight but non-significant increase in susceptibility between 1989 and 1994. The results suggest that resistance to pyrethroids has developed rapidly among head lice since permethrin was introduced in 1991 as a pediculicide in Israel.