Search

To verify the genetic proximity of Anopheles sinensis strains in Korea, China, Japan, and Thailand, we established 5 iso-female lines, and investigated their genetic relationships by analyzing crossing tests and comparing the DNA sequences of rDNA internal transcribed spacer 2 (ITS2) and mitochondrial cytochrome c oxidase subunit 2 (CO2). The results of reciprocal and F1-hybrid crosses between Thailand Form B (X, Y2) with Japanese Form A (X, Y1), Japanese Form B with Korean Form B, and Korean Form B with Chinese Form B indicated that these strains were genetically compatible, providing viable progenies and completely synaptic polytene chromosomes. The sequences of ITS2 and CO2 among 10 Asian An. sinensis strains were almost identical to each other, and the sequence variation was very low (<0.6% in both regions). Based on the above results, it was confidently concluded that An. sinensis is a single cosmopolitan species even though its geographical distribution ranges are comparatively wide (>2000 mi) and that the malaria-vector abilities of the various strains may vary depending on their geographical origins.

A larval survey of dengue vectors was conducted from July to November 1966 and from May to November 1997 in Chiangmai Province, Thailand. Three villages in urban, transition, and rural areas were selected for the survey to clarify the spatial distribution of Ae. aegypti and Ae. albopictus along an urban-rural ecological gradient. The average number of Ae. aegypti larvae in larvitraps was higher in the urban area than in the rural area, as we expected, whereas the opposite was found for Ae. albopictus, rural area > urban area. A house survey of larvae-inhabiting containers showed significant differences in the number and composition of these containers among the study areas. Significant differences were also found in the average distance between houses, average tree height, and average percentage of vegetation cover for each house. The seasonal pattern of rainfall recorded in each study area did not show great differences among the study areas. The response of Ae. aegypti and Ae. albopictus to the urban-rural gradient is discussed in relation to the possibility of applying geographic information system techniques to plan the control strategy and surveillance of dengue vectors.

Background: Betel-chewing, a risk factor for oral carcinoma, is a common habit of elderly Vietnamese females, but concomitant habits of tobacco and alcohol are uncommon., Methods: In the present study, 36 paraffin-embedded betel-associated oral carcinoma samples including 27 squamous cell carcinoma (SCC) and nine verrucous carcinomas (VC) were analyzed for the hypermethylation of tumor suppressor genes (TSGs) and loss of heterozygosity (LOH) of important TSG loci. Methylation-specific polymerase chain reaction (MSP) was used to identify promoter hypermethylation of p16INK4a and RASSF1A. For LOH analysis, 39 microsatellite markers at 12 chromosomal arms were examined by p olymerase chain reaction (PCR)-based microsatellite assay., Results: Hypermethylation of p16IKN4a was detected in 63% of SCC and 67% of VC. In addition, LOH at 9p21 (locus for p16INK4a) was 58% for SCC and 22% for VC, and hypermethylation of RASSF1A was 93% for SCC and 100% for VC. LOH at 3p21.3-3p22.1 (where RASSF1A is located) was detected in only 12% of SCC and 0% of VC. LOH of other chromosomal arms were infrequent., Conclusion: As LOH for chromosomes other than 9p was uncommon, epigenetic silencing of RASSF1A and p16INK4a gene expression by promoter hypermethylation may play a critical role in betel-associated oral carcinogenesis.

Mae Hong Son Province in northwestern Thailand has a long history of malaria. During the last two decades the province has had one of the highest malaria incidences of all provinces in Thailand. Data were analyzed to determine whether the vector populations were stable or increasing during the last two decades and to determine the seasonal prevalence of the main vectors, and whether or not they were related to the malaria transmission peak, in the wet season. We compiled and analyzed accumulated entomological records from 1977 to 1999. The aim was to investigate long-term changes in mean densities of malaria vectors between two periods (1977-1989 and 1990-1999), and the differences in vector densities between two seasons (wet and dry). A total of 141,144 adult anophelines of 29 species were collected on indoor and outdoor human baits and animal baits during the study period. Of the main malaria vectors, the densities of Anopheles minimus s.l. and Anopheles maculatus complex increased significantly. Anopheles dirus s.l., however, was stable between the two periods. These vector populations were associated with consistently high malaria incidence in the province during the last two decades. An. minimus s.l. density was not significantly different between seasons. However, in the second period, both An. dirus s.l. and An. maculatus complex showed a tendency for higher wet season densities. This can explain the high malaria incidence in the rainy season in Mae Hong Son. Environmental and climatic factors seem to have been favorable for supporting a consistently high vector population in the province, and consequently a high malaria transmission rate during the period of study.

A mark-release-recapture experiment was conducted at Mae Taeng, Chiangmai, Thailand, in November 1990 to examine the movement of released host-seeking mosquitoes in a heterogeneous environment. A total of 1,848 mosquitoes of nine anopheline species was field-collected, marked with fluorescent dye, and released. Adult collections were made for four nights after the release at five collection sites surrounding the release site. Three different attractants, dry ice (5 kg/site/night), human, and cow bait were used to collect a large number and a wide variety of adult mosquitoes. The recapture rate of released mosquitoes differed among species, ranging from 0 to 7.5%. The species composition was significantly different among collection sites and collection methods, and samples from dry ice collection showed intermediate species composition between those from human and cow bait collections. The spatial distribution of released mosquitoes was not significantly different from that of unmarked mosquitoes. Different behavioral responses to heterogeneous environments by different species of host-seeking mosquitoes was suggested as the underlying mechanism of species-specific spatial distribution of mosquitoes in the study area.

Experiments, using the capture-mark-release-recapture technique inside large nets, were carried out in Chiang Mai, northern Thailand, to examine heterogeneity in the host preference of Japanese encephalitis (JE) vectors. A significantly higher proportion of the vector species that were initially attracted to a cow fed when released into a net with a cow than when released into a net containing a pig. However, Culex vishnui individuals that had been attracted to a pig had a higher feeding rate in a net containing a pig rather than a cow. When mosquitoes were given a choice by being released into a net containing both animals, they exhibited a tendency to feed on the host to which they had originally been attracted. This feeding preference was, however, not shown by the offspring of pig-fed individuals. We have therefore shown evidence of physiological/behavioural conditioning in the host preference of JE vectors rather than genetic variability. Our results suggest that effective control of JE might be achieved by increasing the availability of cows (the dead-end hosts of JE virus) to deflect the vectors from pigs (the amplifying host). The behavioural imprinting which we have found would tend to re-inforce the initial tendency of the vectors to bite cows.

Adult mosquitoes were collected from 3 areas in northern Thailand with different availabilities of rice fields for larval habitats by using blacklight, truck, and pig-baited traps. Culex tritaeniorhynchus Giles and Culex gelidus Theobald were dominant in all samples. Significant study-area differences were found in light and truck trap collections for Cx. tritaeniorhynchus and Cx. gelidus, but not for Culex vishnui Theobald + Culex pseudovishnui Colless. In pig-baited sample area differences were significant only for Cx. gelidus. Adults of Culex fuscocephala Theobald were relatively rare, comprising only 0.6-7.9% of the total mosquitoes collected. When compared with the results from previous studies in the Chiangmai area, we conclude that Cx. fuscocephala has become a minor species and that changes in the mosquito fauna have occurred in northern Thailand.

In 1991-1993, the density (numbers per 10 dips) and abundance (density x flooded area) of mosquitoes were studied in 3 areas of northern Thailand with different environmental conditions. Culex tritaeniorhynchus Giles, Cx. vishnui Theobald, and Cx. gelidus Theobald were predominant among the 8 Culex species collected. Abundance of these 3 species varied significantly among the 3 areas. Type of habitat classified according to agricultural practices in rainy and dry season significantly influenced larval abundance of Cx. tritaeniorhynchus and Cx. vishnui. Seasonal variation in larval abundance was significant only in Cx. vishnui. The response of the 3 vector mosquitoes to environmental heterogeneity is discussed.

Two strains of Japanese encephalitis (JE) virus were isolated from a pool of Culex tritaeniorhynchus captured in 1992 and another pool of Cx. vishnui captured in 1993, in Chiang Mai Area, Northern Thailand. These two strains, ThCMAr44/92 and ThCMAr67/93, could not be identified either as Nakayama or JaGAr01 subtype by the hemagglutination-inhibition (HI) and the neutralization (N) tests using immune sera raised against these standard JE virus strains. Reverse transcription-polymerase chain reaction showed the presence of JE-specific conserved sequences in these strains. Sequencing of 240 nucleotides in their PrM gene region identified that these two strains belong to the genotype 1 of JE virus. Nucleotide and encoded amino acid sequences of their envelope glycoprotein gene revealed 98.8 and 99.8% identity, respectively. These two strains shared 77.8 to 87.7% homology in the nucleotide sequence and 90.0 to 98.8% homology in the amino acid sequence with other reported JE strains. Five strain-specific amino acid changes were noted in ThCMAr44/92 strain, while one in ThCMAr67/93. In addition, four common amino acid changes were found in both strains. Thus, the findings indicated that these two strains were structurally different from each other as well as different from all the reported strains which was in agreement with the serological tests by hemagglutination-inhibition and neutralization.