Vu Sinh Nam

New strategy against Aedes aegypti in Vietnam

The container-breeding mosquito, Aedes aegypti, is the major global vector of dengue viruses, causing around 50 million infections annually. We have developed a mosquito control strategy, incorporating four elements: (1) a combined vertical and horizontal approach that depends on community understanding; (2) prioritised control according to the larval productivity of major habitat types; (3) use of predacious copepods of the genus Mesocyclops as a biological control agent; delivered by (4) community activities of health volunteers, schools, and the public. We have previously reported that, from 1998 to 2003, community-based vector control had resulted in A aegypti elimination in six of nine communes, with only small numbers of larvae detected in the others. Here, we report eradication in two further communes and, as a result of local expansion after the project in three northern provinces, elimination from 32 of 37 communes (309730 people). As a result, no dengue cases have been detected in any commune since 2002. These findings suggest that this strategy is sustainable in Vietnam and applicable where the major sources of A aegypti are large water storage containers.

Characterizing the Aedes aegypti Population in a Vietnamese Village in Preparation for a Wolbachia-Based Mosquito Control Strategy to Eliminate Dengue

Background A life-shortening strain of the obligate intracellular bacteria Wolbachia, called wMelPop, is seen as a promising new tool for the control of Aedes aegypti. However, developing a vector control strategy based on the release of mosquitoes transinfected with wMelPop requires detailed knowledge of the demographics of the target population. Methodology/Principal Findings In Tri Nguyen village (611 households) on Hon Mieu Island in central Vietnam, we conducted nine quantitative entomologic surveys over 14 months to determine if Ae. aegypti populations were spatially and temporally homogenous, and to estimate population size. There was no obvious relationship between mosquito (larval, pupal or adult) abundance and temperature and rainfall, and no area of the village supported consistently high numbers of mosquitoes. In almost all surveys, key premises produced high numbers of Ae. aegypti. However, these premises were not consistent between surveys. For an intervention based on a single release of wMelPop-infected Ae. aegypti, release ratios of infected to uninfected adult mosquitoes of all age classes are estimated to be 1.8–6.7∶1 for gravid females (and similarly aged males) or teneral adults, respectively. We calculated that adult female mosquito abundance in Tri Nguyen village could range from 1.1 to 43.3 individuals of all age classes per house. Thus, an intervention could require the release of 2–78 wMelPop-infected gravid females and similarly aged males per house, or 7–290 infected teneral female and male mosquitoes per house. Conclusions/Significance Given the variability we encountered, this study highlights the importance of multiple entomologic surveys when evaluating the spatial structure of a vector population or estimating population size. If a single release of wMelPop-infected Ae. aegypti were to occur when wild Ae. aegypti abundance was at its maximum, a preintervention control program would be necessary to ensure that there was no net increase in mosquito numbers. However, because of the short-term temporal heterogeneity, the inconsistent spatial structure and the impact of transient key premises that we observed, the feasibility of multiple releases of smaller numbers of mosquitoes also needs to be considered. In either case, fewer wMelPop-infected mosquitoes would then need to be released, which will likely be more acceptable to householders.

Critical Evaluation of Quantitative Sampling Methods for Aedes aegypti (Diptera: Culicidae) Immatures in Water Storage Containers in Vietnam

Abstract In response to an identified paucity of information on the size and composition of immature Aedes aegypti (L.) (Diptera: Culicidae) populations in large field containers, we assessed net sampling and pumping/sieving methods for estimating and enumerating third (III)/fourth (IV) instar and pupal populations. Sweep net detection thresholds (number above which ≥90% chance of a positive sample) were ≤28 immatures for seven different container types (115–3000 liter jars and tanks) in the laboratory, and mean recovery percentages varied by container type (6.15–41.29 and 7.39–33.10% for III/IV instars and pupae, respectively). A pumping method or hand bailing was applied in the field for the collection of III/IV instars and pupae from 406 receptacles, of which 343 had been previously sampled via a five-sweep netting technique. Larvae were 9.30 times more prevalent than pupae, and abundance varied by container type with means of 36–537 III/IV instars and 6–53 pupae per receptacle. Sweep netting for III/IV instars effectively identified 86.2% of Ae. aegypti-positive containers, whereas sampling for pupae detected only 43.1% of positive containers. When conversion factors (inverse of laboratory recovery percentages) were applied to field net sampling data, estimates of container populations were more accurate for III/IV instars than pupae (maximum R2 = 0.610 and 0.328, respectively); however, the relationship between immature abundance and emergent adult populations remains to be defined.

Quantifying the Emergence of Dengue in Hanoi, Vietnam: 1998-2009

Background An estimated 2.4 billion people live in areas at risk of dengue transmission, therefore the factors determining the establishment of endemic dengue in areas where transmission suitability is marginal is of considerable importance. Hanoi, Vietnam is such an area, and following a large dengue outbreak in 2009, we set out to determine if dengue is emerging in Hanoi. Methods and Principal Findings We undertook a temporal and spatial analysis of 25,983 dengue cases notified in Hanoi between 1998 and 2009. Age standardized incidence rates, standardized age of infection, and Standardized Morbidity Ratios (SMR) were calculated. A quasi-Poisson regression model was used to determine if dengue incidence was increasing over time. Wavelet analysis was used to explore the periodicity of dengue transmission and the association with climate variables. After excluding the two major outbreak years of 1998 and 2009 and correcting for changes in population age structure, we identified a significant annual increase in the incidence of dengue cases over the period 1999–2008 (incidence rate ratio  = 1.38, 95% confidence interval  = 1.20–1.58, p value  = 0.002). The age of notified dengue cases in Hanoi is high, with a median age of 23 years (mean 26.3 years). After adjusting for changes in population age structure, there was no statistically significant change in the median or mean age of dengue cases over the period studied. Districts in the central, highly urban, area of Hanoi have the highest incidence of dengue (SMR>3). Conclusions Hanoi is a low dengue transmission setting where dengue incidence has been increasing year on year since 1999. This trend needs to be confirmed with serological surveys, followed by studies to determine the underlying drivers of this emergence. Such studies can provide insights into the biological, demographic, and environmental changes associated with vulnerability to the establishment of endemic dengue.

Genetic Structure of Aedes aegypti in Australia and Vietnam Revealed by Microsatellite and Exon Primed Intron Crossing Markers Suggests Feasibility of Local Control Options

ABSTRACT The distribution of Aedes aegypti (L.) in Australia is currently restricted to northern Queensland, but it has been more extensive in the past. In this study, we evaluate the genetic structure of Ae. aegypti populations in Australia and Vietnam and consider genetic differentiation between mosquitoes from these areas and those from a population in Thailand. Six microsatellites and two exon primed intron crossing markers were used to assess isolation by distance across all populations and also within the Australian sample. Investigations of founder effects, amount of molecular variation between and within regions and comparison of FST values among Australian and Vietnamese populations were made to assess the scale of movement of Ae. aegypti. Genetic control methods are under development for mosquito vector populations including the dengue vector Ae. aegypti. The success of these control methods will depend on the population structure of the target species including population size and rates of movement among populations. Releases of modified mosquitoes could target local populations that show a high degree of isolation from surrounding populations, potentially allowing new variants to become established in one region with eventual dispersal to other regions.

Sustainability and cost of a community-based strategy against Aedes aegypti in northern and central Vietnam.

We previously reported a new community-based mosquito control that resulted in the elimination of Aedes aegypti in 40 of 46 communes in northern and central Vietnam. During 2007 and 2008, we revisited Nam Dinh and Khanh Hoa provinces in northern and central Vietnam, respectively, to evaluate whether or not these programs were still being maintained 7 years and 4.5 years after formal project activities had ceased, respectively. Using a previously published sustainability framework, we compared 13 criteria from Tho Nghiep commune in Nam Dinh where the local community had adopted our community-based project model using Mesocyclops from 2001. These data were compared against a formal project commune, Xuan Phong, where our successful intervention activities had ceased in 2000 and four communes operating under the National Dengue Control Program with data available. In Khanh Hoa province, we compared 2008 data at Ninh Xuan commune with data at project completion in 2003 and benchmarked these, where possible, against an untreated control commune, Ninh Binh, where few control activities had been undertaken. The three communes where the above community-based strategy had been adopted were rated as well-sustained with annual recurrent total costs (direct and indirect) of

Community-Based Control of Aedes aegypti By Using Mesocyclops in Southern Vietnam

0.28-0.89 international dollars per person.

Isolation and Molecular Characterization of Banna Virus from Mosquitoes, Vietnam

We previously reported a new community-based mosquito control strategy that resulted in elimination of Aedes aegypti (Linn.) in 40 of 46 communes in northern and central Vietnam, and with annual recurrent total costs (direct and indirect) of only

Climate Variability and Dengue Hemorrhagic Fever in Ba Tri District, Ben Tre Province, Vietnam during 2004–2014

0.28–

Control of Aedes vectors of dengue in three provinces of Vietnam by use of Mesocyclops (Copepoda) and community-based methods validated by entomologic, clinical, and serological surveillance

0.89 international dollars per person. This control strategy was extended to four provinces in southern Vietnam in Long An and Hau Giang (2004–2007) and to Long An, Ben Tre, and Vinh Long (2005–2010). In a total of 14 communes with 124,743 residents, the mean ± SD of adult female Ae. aegypti was reduced from 0.93 ± 0.62 to 0.06 ± 0.09, and the reduction of immature Ae. aegypti averaged 98.8%. By the final survey, no adults could be collected in 6 of 14 communes, and one commune, Binh Thanh, also had no immature forms. Although the community-based programs also involved community education and clean-up campaigns, the prevalence of Mesocyclops in large water storage containers > 50 liters increased from 12.77 ± 8.39 to 75.69 ± 9.17% over periods of 15–45 months. At the conclusion of the study, no confirmed dengue cases were detected in four of the five communes for which diagnostic serologic analysis was performed. The rate of progress was faster in communes that were added in stages to the program but the reason for this finding was unclear. At the completion of the formal project, sustainability funds were set up to provide each commune with the financial means to ensure that community-based dengue control activities continued.