No abstract

The mosquito Anopheles stephensi Liston (Diptera: Culicidae) is the urban vector of malaria in several countries of the Middle East and Indian subcontinent. Extensive use of residual insecticide spraying for malaria vector control has selected An. stephensi resistance to DDT, dieldrin, malathion and other organophosphates throughout much of its range and to pyrethroids in the Middle East. Metabolic resistance mechanisms and insensitivity to pyrethroids, so-called knockdown resistance (kdr), have previously been reported in An. stephensi. Here we provide molecular data supporting the hypothesis that a kdr-like pyrethroid-resistance mechanism is present in An. stephensi. We found that larvae of a pyrethroid-selected strain from Dubai (DUB-R) were 182-fold resistant to permethin, compared with a standard susceptible strain of An. stephensi. Activities of some enzymes likely to confer pyrethroid-resistance (i.e. esterases, monooxygenases and glutathione S-transferases) were significantly higher in the permethrin-resistant than in the susceptible strain, but the use of synergists--piperonyl butoxide (PBO) to inhibit monooxygenases and/or tribufos (DEF) to inhibit esterases--did not fully prevent resistance in larvae (permethrin LC50 reduced by only 51-68%), indicating the involvement of another mechanism. From both strains of An. stephensi, we obtained a 237-bp fragment of genomic DNA encoding segment 6 of domain II of the para type voltage-gated sodium channel, i.e. the putative kdr locus. By sequencing this 237 bp fragment, we identified one point mutation difference involving a single A-T base change encoding a leucine to phenylalanine amino acid substitution in the pyrethroid-resistant strain. This mutation appears to be homologous with those detected in An. gambiae and other insects with kdr-like resistance. A diagnostic polymerase chain reaction assay using nested primers was therefore designed to detect this mechanism in An. stephensi.

The population structure of the malaria vector Anopheles arabiensis was investigated using data from six microsatellite loci in samples from localities in Mozambique and Tanzania. Genotype frequencies were neither signi®cantly di erent between houses in a village in Tanzania nor between villages within a 20-km radius in Mozambique. Thus a deme has an area greater than 20 km in radius. At ®ve of the six loci the heterozygosity of the population from Mozambique was lower than that from Tanzania, implying a lower e ective population size (N e ) at this southern edge of the species range. There were signi®cant di erences in genotype frequencies between the Tanzanian and Mozambique populations at ®ve of the six loci (P<0.05). Values for both F ST (mean 0.069) and R ST (mean 0.025) were signi®cantly di erent from zero (P<0.05) at four and three out of ®ve loci, respectively, but there was no signi®cant correlation between the two statistics. The wide variation in values of F ST and R ST across loci suggests that care should be taken in interpreting values derived from averaging across loci. Whether the variation results from sampling e ects or selectional constraints on some loci is unclear. Although there is evidence for signi®cant di erentiation between these populations, estimates of gene¯ow (Nm) calculated from mean F ST and R ST statistics were relatively high, 3.4 and 4.9, respectively. We argue that this is more likely to re¯ect recent separation of these populations and/or large e ective population size rather than large-scale present day migrations.

We describe the geographical population structure of the malaria vector Anopheles arabiensis in Eastern Africa. Allelic variation at eight microsatellite loci was scored in samples from nine localities along a 4500 km transect from Sudan to Mozambique. Highly significant differences in genotype frequencies were found between all populations separated by more than 200 km. Populations within Malawi separated by 191 km were indistinguishable, as were those within Sudan separated by 134 km. FST and rhoST gave significant estimates of isolation by distance. These data, lead us to conclude that there are extensive barriers to gene flow in this region. The high estimates of Nm (9.4 from FST and 5.2 from rhoST) indicate recent range expansion in this species rather than extensive contemporary gene flow.

There are already 40 cities in Africa with over 1 million inhabitants and the United Nations Environmental Programme estimates that by 2025 over 800 million people will live in urban areas. Recognizing that malaria control can improve the health of the vulnerable and remove a major obstacle to their economic development, the Malaria Knowledge Programme of the Liverpool School of Tropical Medicine and the Systemwide Initiative on Malaria and Agriculture convened a multi-sectoral technical consultation on urban malaria in Pretoria, South Africa from 2nd to 4th December, 2004. The aim of the meeting was to identify strategies for the assessment and control of urban malaria. This commentary reflects the discussions held during the meeting and aims to inform researchers and policy makers of the potential for containing and reversing the emerging problem of urban malaria.