Correlation of glutathione S-transferase and DDT dehydrochlorinase activities with DDT susceptibility in Anopheles and Culex mosquitos from northern Thailand.

Comparative DDT-susceptibility status as well as glutathione S-transferase activity and DDTase activity of Anopheles minimus (A). An. annularis and Culex quinquefasciatus were investigated to ascertain the role of these enzymes in DDT-resistance. The standard WHO susceptibility test kits was used to discriminate between resistant and susceptible populations. GST activity was measured in microtiter plates whereas DDTase activity was determined by HPLC quantitation of DDT metabolites. This is the first report of DDT-resistance in the Thai malaria vector, An. minimus species A. A positive correlation of DDT-resistance and DDTase activity was observed in this species as well as in the suspected vector, An. annularis. However, GST activity was not correlated to DDT-resistance in either species. Statistical analysis and scatter plots demonstrated the non-correlation between DDTase and GST activity in An. annularis. Studies in Culex quinquefisciatus revealed difference in GST/ DDTase and the relationship to DDT-resistance compared to the anopheline species. The Culex GST activity is correlated to DDTase activity. These results suggested that a positive correlation of GST and DDTase activity might be species dependent.

Comparative susceptibility of two forms of Anopheles sinensis Wiedemann 1828 (Diptera : Culicidae) to infection with Plasmodium falciparum, P. vivax, P. yoelii and the determination of misleading factor for sporozoite identification.

Two karyotypic forms of laboratory-raised Anopheles sinensis, ie Form A (XY1) and Form B (XY2), were experimentally infected with various indigenous strains of Plasmodium falciparum and P. vivax using an artificial membrane feeding technique, and a rodent malaria, P. yoelii, using a direct feeding technic and dissected 7-9 days and 10-15 days after feeding for oocyst and sporozoite rates, respectively. The results revealed that two forms of An. sinensis were refractory vectors for P. falciparum and P. yoelii since 0% of oocyst and sporozoite rates were obtained, but poor vectors for P. vivax since 0.00-85.71% and 0.00-5.88% of oocyst and sporozoite rates were recovered. The sporozoite-like crystal found in the median lobe of the salivary gland of An. sinensis which could be a misleading factor in identification of true sporozoites in the salivary glands is reported for the first time.

Susceptibility of Thai zoophilic Anophelines and suspected malaria vectors to local strains of human malaria parasites.

Wild caught zoophilic Anopheles and suspected malaria vector species collected in northwest Thailand were experimentally infected with local human malaria parasites using a membrane feeding. One week post-feeding a number of mosquitos were dissected for oocyst examination. The remainder were kept for another one week or more, and then the salivary glands were examined for the presence of sporozoites. The results revealed that An. vagus, An. kochi and An. annularis were susceptible to both Plasmodium falciparum and P. vivax whereas An. barbirostris and An. sinensis were susceptible to only P. vivax. The non-susceptibility to P. falciparum of these two mosquito species may indicate their poor vector status of this malaria species in the field.