Illustrated keys to the mosquitoes of Thailand. II. Genera Culex and Lutzia.

Illustrated keys for the identification of the fourth-instar larvae and adult females of the mosquito genera Culex and Lutzia is presented, along with information on the geographic distribution of each species, and bionomics. Eighty-two species belonging to subgenera Culex, Culiciomyia, Eumelanomyia, Lophoceraomyia, and Oculeomyia of genus Culex, and three species belonging to subgenus Metalutzia of genus Lutzia are recognized in Thailand. Subgenus Oculeomyia includes a probable new species near Cx. infula and Lt. vorax is recognized as a new country record.

Morphological variations of Anopheles minimus A in Tak Province, Thailand.

Anopheles minimus Theobald is one of the major vectors of malaria throughout the Oriental Region, and it's complex consists of at least 2 sibling species (A and C) in Thailand. This study aimed to determine the morphological variations of wings of An. minimus A and to clarify the specific status of An. minimus in Ban Khun Huay, Ban Pa Dae, and Ban Tham Seau, Mae Sot district, Tak Province, Thailand. Anopheline larvae were collected from the fields between October 2002 and September 2003, allowed to emerge into adults in the laboratory and identified by morphological and molecular characterization. About 1,715 of female An. minimus A were separated into 8 groups based on their wing scale patterns. Polymerase Chain Reaction Restriction Fragment Length Polymorphism (PCR-RFLP) assay (ribosomal DNA ITS2) confirmed the identification of An. minimus A in all 8 groups.

Bottle and biochemical assays on temephos resistance in Aedes aegypti in Thailand.

The bottle bioassay measuring the time-mortality rate is a simplified procedure for detecting insecticide resistance. It can be used with a biochemical microplate assay to identify the mechanism involved. This integrated approach was used to detect temephos resistance in Aedes aegypti from Nonthaburi (lowest use) and Roi Et (highest use). Ae. aegypti BKK1 laboratory strain was used as the susceptible reference strain. The appropriate concentration of insecticide for bottle bioassay was determined empirically for Ae. aegypti BKK1 strain and found to be in the range of 800-1,050 microg/bottle. The time-mortality rate at 800 microg/bottle was 170 +/- 8.66 minutes, significantly different from the time-mortality rates in the 850, 900, 950, and 1,050 microg/bottle (p = 0.008) concentrations, which were 135 +/- 15.00, 140 +/- 8.66, 135 +/- 15.00, and 125 +/- 8.66 minutes, respectively. The cut-off concentration selected for resistance detection was 850 microg/bottle. The time-mortality rate for the Roi Et strain was 382 +/- 26.41 minutes, significantly higher than the Nonthaburi (150 +/- 25.10 minutes) and BKK1 strains (145 +/- 20.49 minutes) (p < 0.001). The temephos resistance ratio (RR100) for the Ae. aegypti Roi Et strain was 2.64-fold higher at lethal time (LT100) than for the reference Ae. aegypti BKK1 strain. The mean optical density (OD) value from the biochemical microplate assay for the non-specific esterase of the Roi Et strain was higher than the mean OD for the non-specific esterase of both the Nonthaburi and BKK1 strains. Insensitive acetylcholinesterase was not found to be responsible for the resistance in the field-collected mosquitos. This study suggests that esterase detoxification is the primary cause of resistance in the Ae. aegypti population from Roi Et. Both the bottle bioassay and the biochemical microplate assay were proven to be promising tools for initial detection and field surveillance for temephos resistance.

Water quality and breeding habitats of anopheline mosquito in northwestern Thailand.

Malaria transmission is dependent upon many hydrology-driven ecological factors that directly affect the vectorial competence, including the presence of suitable habitats for the development of anopheline larvae. Larval habitats were identified and characterized at three malaria endemic villages (Ban Khun Huay, Ban Pa Dae, and Ban Tham Seau) in Mae Sot district, Tak Province, in northwestern Thailand between July 2002 and June 2003. The Global Positioning System (GPS) was used to provide precise locational data for the spatial distribution of anopheline mosquito larvae and their habitats. Ten habitat categories were identified. Eighteen adult Anopheles species were identified from larvae in all the surveyed habitats. An. minimus was the most common species throughout the year. The relationship between eight abiotic variables (temperature, hardness, carbon dioxide, dissolved oxygen, nitrate, phosphate, silica and pH) and the abundance of four major species of malaria vectors (An. (Cel.) dirus, An. (Cel.) minimus, An. (Cel.) maculatus, and An. (Cel.) sawadwongporni), and six species of non-vectors (An. (Cel.) kochi, An. (Cel.) jamesii, An. (Ano.) peditaeniatus, An. (Ano.) barbirostris, An. (Ano.) campestris, and An (Cel.) vagus) larvae was investigated. The results from the multiple regression models suggest that hardness, water temperature and carbon dioxide are the best predictor variables associated with the abundance of An. minimus larvae (p < 0.001); water pH for An. dirus larvae (p < 0.001); temperature and pH for An. kochi larvae (p < 0.01); temperature and silica concentration for An. jamesii larvae (p < 0.001); dissolved oxygen and silica concentration for An. campestris larvae (p < 0.001); and pH and silica concentration for An. vagus larvae (p < 0.001). We could not identify key environmental variables for An. maculatus, An. sawadwongporni, An. peditaeniatus, and An. barbirostris.

Ikonos-derived malaria transmission risk in northwestern Thailand.

We mapped overall malaria cases and located each field observed major malaria vector breeding habitat using Global Positioning System (GPS) instruments from September 2000 to October 2003 around the three malaria-endemic villages of Ban Khun Huay, Ban Pa Dae, and Ban Tham Seau, Mae Sod district, Tak Province, Thailand. The land-use/land-cover classifications of the three villages and surrounding areas were performed on IKONOS satellite images acquired on 12 November 2001 with a spatial resolution of 1 x 1 m. Stream network was delineated and displayed. Proximity analysis was performed on the locations of the houses with and without malaria cases within a 1.5 km buffer from An. minimus immature mosquito breeding habitats, mainly stream margins. The 1.5 km used in our proximity analysis was arbitrarily estimated based on the An. minimus flight range. A statistical t-test at 5% significance level was performed to evaluate whether houses with malaria cases have higher proximities to streams than houses without malaria cases. The result shows no significant difference between proximity to streams between houses with malaria cases and houses without malaria cases. We suspect that the actual flight range of An. minimus may be greater than 1.5 km. The An. minimus larval habitat deserves more detailed investigation. Further studies on human behavior contrary to that required for adequate malaria control among these three villages are also recommended.

Some entomological observations on temporal and spatial distribution of malaria vectors in three villages in northwestern Thailand using a geographic information system.

This spatial and temporal heterogeneity in the distribution of Anopheles mosquitos were studied during August 2001 to December 2002 in three villages Ban Khun Huay, Ban Pa Dae, and Ban Tham Seau, in northwestern Thailand in Mae Sot district, Tak Province. The three Karen villages are located about 20 km east of the city of Mae Sot near the Myanmar border. Twenty-one species were collected on human collections during 68 nights of 17 months. Anopheles minimus comprised of 86% of the specimens biting man. An. minimus was implicated as a vector based on the detection of sporozoite infections using enzyme-linked immunosorbent assays for Plasmodium falciparum and P. vivax. Seasonal comparison of vectorial capacity and entomological inoculation rate was calculated. An. dirus was rarely encountered and probably played little part in transmission in these three villages during the period of study. Information is provided on nightly biting activity, parity rate, infectivity, and adult and larval bionomics. Spatial and temporal comparisons among the collections were displayed on different satellite images including the Normalized Difference Vegetation Index data from on the National Oceanographic and Atmospheric Administration satellites (NOAA/NDVI), the LANDSAT satellite Thematic Mapper (spatial resolution 30x30 m) and the IKONOS satellite (spatial resolution 1x1 m) in a Geographical Information System (GIS).

Use of GIS-based spatial modeling approach to characterize the spatial patterns of malaria mosquito vector breeding habitats in northwestern Thailand.

We sampled 291 bodies of water for Anopheles larvae around three malaria-endemic villages of Ban Khun Huay, Ban Pa Dae, and Ban Tham Seau, Mae Sot district, Tak Province, Thailand during August 2001-December 2002 and collected 4,387 larvae from 12 categories of breeding habitat types. We modeled surface slope and wetness indices to identify the extent and spatial pattern of potential mosquito breeding habitats by digitizing base topographical maps of the study site and overlaying them with coordinates for each larval habitat. Topographical contours and streamlines were incorporated into the Geographical Information System (GIS). We used Global Positioning System (GPS) instruments to locate accurately each field observed breeding habitat, and produced a 30-m spatial resolution Digital Elevation Model (DEM). The slope (of less than 12 degrees) and wetness (more than 8 units) derived from spatial modeling were positively associated with the abundance of major malaria vectors An. dirus, An. maculatus, An. minimus, and An. sawadwongporni. These associations permit real-time monitoring and possibly forecasting of the distributions of these four species, enabling public health agencies to institute control measures before the mosquitos emerge as adults and transmit disease.