Dengue haemorrhagic fever emerged in the 1950s and has become a major public health concern in most Asian countries. In Vietnam, little is known about the intraspecific variation of the vector and its consequences on vectorial capacity. Here we report the use of microsatellite markers to differentiate Aedes aegypti populations in Ho Chi Minh City, a typical, overcrowded Asian city. Six microsatellite loci, with 5-14 alleles per locus, were scored in 20 mosquito samples collected in 1998 in Ho Chi Minh City. We found substantial differentiation among Ae. aegypti populations from the outskirts, whereas populations from the centre of the city showed less differentiation. These results are consistent with the hypothesis that populations of Ae. aegypti in central Ho Chi Minh City are panmictic because there are abundant larval breeding sites and an abundance of humans for adults to feed upon. In contrast, populations on the outskirts become differentiated largely through the processes of genetic drift because larval breeding sites are not as abundant. These findings implicate human activities associated with urbanization, as factors shaping the genetic structure of Ae. aegypti populations.
Aedes aegypti, the main vector of dengue viruses in Asia, displays variation in population density over time. The larval habitats of this species being unevenly distributed and transient (depending on cycles of drought and flood), the forces generating temporal variation in gene frequecies in populations are studied. We sampled seven mosquito populations from Ho Chi Minh City (Vietnam) and its suburbs on five occasions between April 1999 and August 2000. We investigated genetic variation by studying isoenzyme and microsatellite polymorphism and susceptibility to a dengue 2 virus strain. Ae. aegypti populations collected during the dry sea-
Aedes aegypti is the principal vector of dengue viruses, responsible for a viral infection that has become a major public health concern in Asia. In Viet Nam, dengue haemorrhagic fever was first detected in the 1960s and is now a leading cause of death in childhood. We studied the variability in competence of Ae. aegypti as a vector for dengue 2 virus and genetic differentiation in this mosquito species. Twenty mosquito samples collected in 1998 in Ho Chi Minh City were subjected to oral infection and isoenzyme polymorphism analysis by starch gel electrophoresis. Ae. aegypti populations from the centre of Ho Chi Minh City were genetically differentiated and their infection rates differed from those of populations from the commuter belt. These results have implications for insecticidal control during dengue outbreaks.
The electrophoretic polymorphism of loci encoding for 10 enzymes was studied in Culex p. quinquefasciatus Say from six localities of Vietnam. The analysis of 11 "neutral genes" showed that differentiation among samples was low, but significant (Fst = 0.06), and significantly related to geographic distance between sample sites. These results are similar to those observed in other countries (Europe and west Africa). A single type of overproduced esterases (A2-B2) was observed, and its frequency was high (60-100%) in all samples. This situation is in sharp contrast with that observed in other countries of South East Asia (China, South Korea and Japan), where two or more types of overproduced esterases have been reported. A map summarizing the geographic distribution of Asian Cr. p. quinquefasciatus with overproduced esterases is provided.
A Vietnamese domestic plant namely Solanum nigrum (S. nigrum) was subjected to test for larvicidal activity on two majors Dengue hemorrhagic fever (DHF) vectors Aedes aegypti (Ae. aegypti) and Aedes albopictus (Ae. albopictus). The plant was processed to get infusions in hot water or extracted in ethanol. Laboratory and field larval strains of two Aedes species were exposed to the infusions and extract at increasing concentrations for one hour and followed-up intensively for up to 72 hours. The obtained results of bioassay showed larvicidal effects of extract on all mosquito strains. The effects on laboratory strain of Ae. aegypti larvae were correlated with infusions and extract concentrations. Chopped plant infusions in hot water indicated mortality up to 77.3% of larvae. Ground plant infusions killed all of exposed larvae at day 3 postexposure. Median lethal concentrations (LC50,s) of chopped and ground plant infusions were 10.25 and 7.54%, respectively. Ethanolic extract had very strong effect on experimental subjects. Within 72 hours, 100% of laboratory strain of Ae. aegypti larvae died after exposure to extract at 100 parts per million (ppm) or higher concentrations. Ethanolic plant extract showed similar larvicidal effect on field strains of Ae. aegypti and Ae. albopictus. The percentage mortality of field strains larvae reached 100% after exposure to 100 ppm of plant extract. At concentrations of 1000 ppm, 100% of exposed larvae died with 8 hours. LC50 on tested larvae was 25.07-33.60 ppm. Strong larvicidal activity of S. nigrum suggests the possible application in DHF vector control effort.
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