Population Genetic Structure of Aedes (Stegomyia) aegypti (L.) at a Micro-Spatial Scale in Thailand: Implications for a Dengue Suppression Strategy

Background

The genetic population structure of Aedes (Stegomyia) aegypti (L.), the main vector of dengue virus, is being investigated in areas where a novel dengue suppression program is to be implemented. The aim of the program is to release and establish mosquito populations with impaired virus transmission capabilities. To model effects of the release and devise protocols for its implementation, information about the genetic structure of populations at a range of spatial scales is required.

Methodology/Principal Findings

This study investigates a potential release site in the Hua Sam Rong Subdistrict of Plaeng Yao District, Chachoengsao Province, in eastern Thailand which comprises a complex of five villages within a 10 km radius. Aedes aegypti resting indoors was sampled at four different times of year from houses within the five villages. Genetic markers were used to screen the mosquitoes: two Exon Primed Intron Crossing (EPIC) markers and five microsatellite markers. The raw allele size was determined using several statistical software packages to analyze the population structure of the mosquito. Estimates of effective population size for each village were low, but there was no evidence of genetic isolation by geographic distance.

Conclusions

The presence of temporary genetic structure is possibly caused by genetic drift due to large contributions of adults from a few breeding containers. This suggests that the introduction of mosquitoes into an area needs to proceed through multiple releases and targeting of sites where mosquitoes are emerging in large numbers.     

THE GLUCOSE TRANSPORT SYSTEM OF AMPHORA COFFEAEFORMIS (BACILLARIOPHYCEAE)1

Amphora coffeaeformis (Ag.) Kütz. var. perpusilla (Grun.) Cleve took up glucose by an inducible transport system. The system was induced by d -fructose, d -mannose, as well as glucose. Some d -pentoses also induced a glucose uptake system but it may not be the same one as that induced by hexose. d -fructose, d -mannose and 2-deoxy-d -glucose inhibited 2 mM glucose uptake at equimolar concentration, but d -pentoses did not. The uptake system decayed in ca. 5 h in the absence of glucose. The half-saturation constant for uptake, K₈ was ca. 0.1 mM glucose with a maximum uptake rate, V_max= 0.4 nmol/10⁶ cells-min^?1.     

Mosquito larvicidal activity of aqueous extracts of long pepper (Piper retrofractum vahl) from Thailand.

Aqueous extracts of nine medicinal plants were bioassayed against larvae of Culex quinquefasciatus Say and Aedes aegypt (L.). Among these plants, the long pepper, Piper retrofractum Vahl (Piperaceae), showed the highest level of activity against mosquito larvae. To gain more information on larvicidal activity of P. retrofractum, fresh fruits of this plant were extracted in water and the extracts made into powder and bioassayed against 3rd and 4th instar larvae of Cx. quinquefasciatus and Ae. aegypti in the laboratory. Extracts of unripe (001/3) and ripe (002/3 and 001/4) fruits showed different levels of activity against Cx. quinquefasciatus larvae. Extracts 001/3 and 002/3 were equi-toxic to a Bacillus sphaericus resistant and susceptible strains, both from Thailand. The ripe fruit extract 002/3 was somewhat more active against Ae. aegypti than Cx. quinquefasciatus. Another ripe fruit extract (001/4) was much more toxic to both mosquito species. Diluted solutions of the solid extract (002/3) in distilled water lost their larvicidal activity upon aging. Loss of activity at 25 degrees C was greater than that stored at 4 degrees C, and greater in water than in acetone solution.     

Combination of Mesocyclops thermocyclopoides and Bacillus thuringiensis var. israelensis: a better approach for the control of Aedes aegypti larvae in water containers.

The efficacy of a local Thai-strain of the copepod, Mesocyclops thermocyclopoides and the larvicide, Bacillus thuringiensis var. israelensis (Bti), used jointly and singly, was studied against Aedes aegypti in water containers. In a laboratory test, copepods alone produced mortality of 98-100% in 1st instar larvae of Ae. aegypti at copepod:larvae ratios ranging from 1:1 to 1:4. In an outdoor field simulated experiment that ran for 16 wk, after a single inoculation, the treatment of copepods and Bti combined yielded the better, more sustainable results than the agents used individually. Numbers of mosquito larvae per sample in the combined treatment were zero during the first 8 wk; larval numbers then increased but were maintained at a very low level for the next 4 wk after which the larval numbers increased moderately but still remained below numbers in the control. Bti alone kept the larvae at the zero level for the first 4 wk after which their numbers increased slightly and were at low levels up to 12 wk. Copepods alone maintained larval numbers at a low level as compared with those of the control. During the course of the experiment the larval numbers in the control were greater than 20 per sample. Statistically significant differences were noted among treatment means (F = 23.083, df = 3/60, P<0.01) over the total period of the study. The number of copepods in the joint treatment was significantly higher than in the copepod alone treatment for the first 8 wk (t = -4.97, df = 14, P<0.01). The density of copepods, however, for the whole 16-wk period was not significantly different in these two treatments (t = -1.51, df = 30, P>0.1).     

Pestiferous nature, resting sites, aggregation, and host-seeking behavior of the eye fly Siphunculina funicola (Diptera: Chloropidae) in Thailand.

Species of eye flies and eye gnats (Diptera: Chloropidae) are severe and persistent pests of man, domestic and wild animals, and potential vectors of pathogens. The species prevailing in the Oriental region belong to the genus Siphunculina, while those in the Neotropic and Nearctic regions belong to Liohippelates and Hippelates. These are small insects of 1-2 mm that feed on wounds, lacerations, scratches, and mucous membranes of man and higher animals. One species, Siphunculina funicola, commonly known as the Oriental eye fly, is considered the most anthropophilic in the genus, with potential involvement in the spread and mechanical transmission of infectious agents to humans and animals. Very little is known about the biology, prevalence, host-seeking, and aggregation behavior of this species in South and Southeast Asia. We initiated studies on biological aspects of this potential vector and human pest in central Thailand. The most significant findings of our study were the aggregation behaviors of S. funicola, that both sexes attack hosts, and that males outnumbered females attacking humans, dogs, and other domestic animals. They feed on wounds, scabs, lacerations, eyes, and mucous membranes. They hover around and feed on hosts during the daylight hours when host-seeking activities are more pronounced at temperatures above 25-27 degrees C under calm conditions. We noted that large masses of males and females aggregated on a variety of hanging objects such as strings, trailings, electrical lines, decorations, ropes, twines, abandoned cob webs, clothes hangers, and other hanging substrates in open shade of structures and dwellings. This behavior of eye flies brings them closer to human and animal hosts. In these aggregations, both males and females were present, with mating pairs frequently noted. In the aggregations, about 37% of the females had fully developed eggs in the rainy season, but only <1-3.6% were gravid in the hot and dry season. The average number of eggs per female was generally 42 to 44.     

Patterns and possible environmental controls of skeletogenesis of Porites lutea,South Thailand

Eleven fringing reef sites were investigated over a distance of about 50 km in the Phuket Region. There is a wide range in exposure to wave energy, and also water turbidity across the area. Annual increments of growth of shallow-water reef-front colonies of Porites lutea were calculated for the period November 1984 November 1986 using seasonal fluorescent banding (revealed with ultraviolet light) and Alizarin staining. Measurement of linear extension rate, skeletal bulk density, calcification rate, polyp numbers per unit area and colony surface morphology were made and compared. Linear extension rate and skeletal bulk density are inversely related within and between reef sites. Linear extension rate decreases and bulk density increases along a gradient of increasing hydraulic energy of the setting. Calcification (the product of linear extension rate and bulk density), although varying slightly from site to site, does not appear to relate to any obvious environmental inshore-offshore gradient. Skeletal bulk density is the most sensitive discriminator between reef sites, and we suggest that hydraulic energy of the setting is the main control on these spatial variations in skeletogenesis.     

Mannose transport in Amphora coffeaeformis var. perpusilla (Bacillariophyceae).

Amphora coffeaeformis var. perpusilla is able to concentrate but not metabolize mannose. The sugar, which can be accumulated against a concentration gradient, is taken up only after a short lag. Preincubation with glucose allows immediate mannose uptake. Substances that reduce the ATP content of the diatom cells also inhibit mannose uptake as do sugars of similar stereochemical configuration. Mannose appears to act as a non-metabolizable analogue of glucose in these respects.     

ISOLATION AND PHYSIOLOGICAL STUDIES ON THREE ISOLATES OF AMPHORA (BACILLARIOPHYCEAE)1

Three clones of the diatom Amphora were euryhaline, able to grow autotrophically at 160 lx (0.001 ly/min) and heterotrophically on glucose and fructose. Furthermore 2 clones grew on glutamate and feast extract. Light-limited growth of individual clones was stimulated by glycerol, galactose, lactate, acetate, aspartate and asparagine, although mannose torn inhibitory at low and high light levels. The half-saturation constant for growth of A. coffeaefomis var. perpusilla Grunow (Cleve) with glucose was 25 μM. Heterotrophic growth rate of this organism became saturated with respect to glucose at 0.5 mM.