Long-term persistence of Chikungunya virus neutralizing antibodies in human populations of North Eastern Thailand.

BACKGROUND: Chikungunya virus (CHIKV) outbreak recurrences in Thailand are unpredictable and separated by unexplained and often long silent epidemiological periods that can last for several years. These silent periods could be explained in part by the fact that infection with one CHIKV strain confers lasting natural immunity, even against other CHIKV strains. In this study we evaluated the persistence of CHIKV-specific neutralizing antibodies in the population of Chumpae District, Khon Kaen Province, nineteen years after a CHIKV outbreak occurred in the same area in 1991. FINDINGS: Overall 39% (44/111) of 111 former patients had neutralizing antibodies reacting against CHIKV ECSA strain. Consistently high titers of neutralizing antibodies were found in 75% (33/44) of all positively-reacting sera, 70% of which (23/33) were collected from individuals amongst the >60 years old age group. Although the prevalence found in Pong Haeng village (70%) was significantly higher than the prevalence detected in the Nong Thum village (14%), control study villages without known previous Chikungunya epidemics had a high Chikungunya neutralizing antibody prevalence (65%). CONCLUSIONS: More than one-third of the pre-exposed population had persisting natural immunity that was more likely boosted by recent and repetitive exposure to the emerging ECSA CHIKV in Thailand. Also, Chikungunya virus appears to largely circulate in the country with a great variability appears between villages or area probably associated with the vector abundance and efficiency. Altogether these results show a potential for a lifelong immunity against CHIKV. Given the rapid spread of the highly pathogenic ECSA strain in Southern Thailand, the development of CHIK vaccine is strongly recommended.

Sergentomyia (Neophlebotomus) gemmea, a potential vector of Leishmania siamensis in southern Thailand.

BACKGROUND: Leishmaniasis, caused by Leishmania siamensis, is an emerging disease in Thailand. Although reported cases have been increasing, epidemiological information of the disease including host and vector aspects is not clearly known. This study was a preliminary survey of the potential vector of L. siamensis in an affected area of leishmaniasis, Trang Province, southern Thailand. METHODS: The collection of sandflies was performed around the area where a case of leishmaniasis was reported using CDC light traps. Species of sandfly were identified based on morphological characteristics according to Lewis's key. PCR amplification and sequencing of the heat shock protein 70 gene (hsp70) was used to identify L. siamensis DNA in sandflies. RESULTS: A total of 146 male and female sandflies were collected in the affected areas. Of 71 female sandflies, four species were identified, i.e., Sergentomyia (Neophlebotomus) gemmea, S. (Neophlebotomus) iyengari, S. (Parrotomyia) barraudi and Phlebotomus (Anaphlebotomus) stantoni. Among these species, S. (Neophlebotomus) gemmea was the most predominant species in all areas. DNA of L. siamensis was identified in S. (Neophlebotomus) gemmea. Nucleotide sequences of PCR products using DNA extracted from S. (Neophlebotomus) gemmea showed 99.8% identity to L. siamensis. CONCLUSION: S. (Neophlebotomus) gemmea might be a potential vector of L. siamensis in an affected area, Trang Province, southern Thailand. However further studies are needed to prove whether these sandflies can be natural vectors of leishmaniasis.

The relationship between the abundance of Mansonia mosquitoes inhabiting a peat swamp forest and remotely sensed data.

The present study aimed to demonstrate the relationship of some environmental factors, vegetation greenness index (NDVI) and land surface temperature (LST), with the seasonal variations of Mansonia bonneae and Ma. uniformis in Khosit Subdistrict, Narathiwat Province. It was found that the Mansonia population lagged one month behind but correlated positively to NDVI, LST and rainfall. A rise in the number of mosquitoes was directly related to a rise in vegetation, temperature and rainfall.

Application of GIS to the characterization of filariasis transmission in Narathiwat Province.

The geographical information system (GIS) and available survey data (both from the Filariasis Annual Reports, 1985-1999 and from the published literature) for the microfilarial infection rates are used to develop the first subdistrict-level endemicity maps of lymphatic filariasis in Narathiwat Province. The maps demonstrated the subdistrict-level geographical distribution of filariasis and the subdistricts at varied degrees of infection rate. The maps also indicate that, since 1985, there was a marked decrease in endemicity at the subdistrict level and in some areas, the infection rates were zero. However, transmission remained in the subdistricts surrounding peat swamp forest (Su-ngai Padi, Paluru, Puyo, Pasemat, Bang Khunthong, and Phron subdistricts). The house locations of infected cases, as well as the vector breeding places were geo-registered and placed as symbolic dots on the base maps obtained from Landsat's Thematic Mapper (TM) 5 and the land use map of Narathiwat to display the distribution of filariasis foci. Of 102 houses mapped, there were 40 houses in primary peat swamp forest (39.22%), 26 in rice fields (25.49%), 15 in fruit orchards (14.70%), 10 in coconut fields (9.80%) and others (10.78%). All the houses were close to the larval habitats presented in the survey. A 2-km buffer zone around the conservation boundary of primary peat swamp forest was created to locate risk areas of filariasis transmission. The buffer zone covered an area of 544.11 km2 and included 88.89% of the houses of infected cases found in 2002. It was able to identify 54 villages located in the buffer area, which might help in the determination of resource needs and resource allocation for filariasis control in Narathiwat Province.