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.     

Aedes aegypti in Senegal: genetic diversity and genetic structure of domestic and sylvatic populations

Aedes aegypti is the main vector of dengue viruses. The epidemiology of dengue fever remains poorly understood in Senegal. A sylvatic transmission seems to predominate. However, despite the sylvatic circulation of the dengue virus and the presence of vectors in urban areas, only sporadic cases have been reported. Ae. aegypti is a polytypic species. In Senegal, a purely sylvatic form is found in the forest gallery areas and a domestic form is found in the villages in savannah and sahelian areas and in urban areas. Using allozymes, we analyzed the genetic diversity and the genetic structure of Ae. aegypti populations differing in their ecological characteristics. Populations from Senegal were significantly structured but with a low level of genetic differentiation. Ae. aegypti from the "domestic" populations show a decreased genetic diversity and a lower genetic differentiation compared with "sylvatic" populations. These findings suggest that environmental conditions, ecological factors, and human activities may impact the genetic structure of Ae. aegypti populations in Senegal.     

Population genetic structure and competence as a vector for dengue type 2 virus of Aedes aegypti and Aedes albopictus from Madagascar.

Starch gel electrophoresis was used to assess the polymorphism of 7 isoenzymes in single mosquitoes (field-collected F0 or F1 generation) for Aedes albopictus (8 strains) from northern Madagascar. Mosquitoes of the F2 generation (3 strains of Aedes aegypti and 10 strains of Ae. albopictus) were tested for oral susceptibility to dengue type 2 virus. Aedes aegypti was less susceptible to viral infection than Ae. albopictus. The genetic differentiation was less high between Ae. albopictus populations collected in agglomerations connected by highly frequented roads, indicating that human ground transportation favors mosquito dispersal. These results have implications for the ecology, pattern of migration, and relative importance in epidemic transmission of dengue viruses between the 2 Aedes species.     

Using adult mosquitoes to transfer insecticides to Aedes aegypti larval habitats

Vector control is a key means of combating mosquito-borne diseases and the only tool available for tackling the transmission of dengue, a disease for which no vaccine, prophylaxis, or therapeutant currently exists. The most effective mosquito control methods include a variety of insecticidal tools that target adults or juveniles. Their successful implementation depends on impacting the largest proportion of the vector population possible. We demonstrate a control strategy that dramatically improves the efficiency with which high coverage of aquatic mosquito habitats can be achieved. The method exploits adult mosquitoes as vehicles of insecticide transfer by harnessing their fundamental behaviors to disseminate a juvenile hormone analogue (JHA) between resting and oviposition sites. A series of field trials undertaken in an Amazon city (Iquitos, Peru) showed that the placement of JHA dissemination stations in just 3–5% of the available resting area resulted in almost complete coverage of sentinel aquatic habitats. More than control mortality occurred in 95–100% of the larval cohorts of Aedes aegypti developing at those sites. Overall reductions in adult emergence of 42–98% were achieved during the trials. A deterministic simulation model predicts amplifications in coverage consistent with our observations and highlights the importance of the residual activity of the insecticide for this technique.     

Importance of ecology in Aedes aegypti control.

Aedes aegypti is one of the world's most widely distributed mosquitos and is of considerable medical importance as a vector of dengue and yellow fever. Not surprisingly therefore there has been more written on its biology than any other mosquito. The present paper summarizes ecological studies undertaken on this vector, including those on pre-adult mortalities, life-tables, adult dispersal and survival rates. In discussing surveillance techniques it is pointed out there are still no reliable methods for monitoring adult populations. The importance of the resting habits of adults and house construction in insecticidal control of Ae. aegypti is discussed. The question is, have ecological studies and population modeling resulted in any more understanding of the epidemiology of dengue, or helped formulate better control strategies? The answer seems to be not usually, possible because there have actually been relatively few good ecological studies on Ae. aegypti. Although mathematical modeling indicates that better control might be achieved if it were directed at the larvae, not adults, this has not usually been taken into consideration by those engaged in control. There is clearly need for greater collaboration between those practicing control, and ecologists and modelers.     

Role of Aedes aegypti and Aedes albopictus during the 2011 dengue fever epidemics in Hanoi,Vietnam

Objective: To record the human cases of dengue fever(DF) and investigate the Aedes mosquito species circulating during the Hanoi 2011 DF epidemics. Methods: 24 different outbreak points were recorded in 8 districts between August and December 2011. Results: 140 patients were hospitalized following dengue diagnostic with a predominance of males(59.3%) and the 15-34 age class. Only DENV-1(11.27%) and DENV-2(88.73%) serotypes were detected in human samples. Mosquito sampling performed in and around patients households revealed the predominance of Aedes aegypti(95.15%) versus Aedes albopictus(4.85%). There is a positive correlation between the population density of Aedes aegypti and the number of human cases and duration of outbreaks. Conclusions: This was not observed for Aedes albopictus. 3 pools of Aedes aegypti were positive with dengue virus, two with DENV-1 and one with DENV-2.     

Community-based control of Aedes aegypti by using Mesocyclops in southern 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 $0.28-$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.     

Phylogeography and population structure of Aedes aegypti in Arizona

Aedes aegypti, the mosquito responsible for transmitting dengue, has colonized many cities and towns throughout Arizona. Determining both the migration between, and the origin of, local Ae. aegypti populations is important for vector control and disease prevention purposes. Amplified fragment length polymorphism was used to infer geographic structure and local substructure, and effective migration rates (M, migrants per generation) between populations, and to determine genetic differentiation between populations (PhiPT). Three geographically and genetically differentiated groups of populations were identified. Population substructure was only detected in the border town of Nogales. Reliable estimates of M between regions ranged from 1.02 to 3.41 and between cities within regions from 1.66 to 4.44. In general, pairwise PhiPT were lowest between cities within regions. The observed patterns of genetic differentiation suggest infrequent migration between populations and are compatible with the idea of human transport facilitating dispersal between regions.     

Larval environmental temperature and insecticide exposure alter Aedes aegypti competence for arboviruses

Temperature is a key factor influencing mosquito growth and development and is also known to affect insecticide efficacy. We evaluated the effects of larval rearing temperature and exposure to insecticides on adult mosquito fitness and competence for arboviral infection using Sindbis virus (SINV). We exposed newly hatched larvae of Aedes aegypti to an environmentally realistic level of insecticide malathion at 20°C and 30°C and allowed the resulting adults to feed on SINV-infected blood meal. Exposure to malathion significantly reduced survival to adulthood. Statistically significant interactions between temperature and malathion were observed for body size, estimated population growth, and SINV infection and dissemination. Malathion-exposed Ae. aegypti cohorts had significantly higher population growth at 20°C than at 30°C. Body size decreased with higher temperature and malathion-exposed females were larger than unexposed females at 20°C but not at 30°C. Viral infection and dissemination increased with larval rearing temperature and were higher in malathion-exposed than unexposed females at 30°C but not at 20°C. These results show that environmental factors, including those factors used in controlling mosquitoes, experienced by immature stages have latent effects that continue to adulthood and alter vector competence to arboviruses.     

Aedes aegypti (L) in Port Blair, Andaman and Nicobar islands-distribution and larval ecology

A comprehensive survey was carried out in the Port Blair town in Andaman and Nicobar islands, in order to study the distribution and relative prevalence of Aedes aegypti during the monsoon (July'1997-October'1997) season using larval survey and adult collection methods. Ae. aegypti was found in all 21 localities surveyed. Spatial variations in distribution was evident which was closely related to population density. The nature of the larval habitats was observed to be similar in all the localities. For both outdoor and in-door containers, breeding preference ratio was highest for mud/brick/cement containers, followed by metal and plastic containers. These findings are correlated with water storage habits of the residents in the localities surveyed.     

Study on susceptibility status in aquatic and adult stages of Aedes aegypti and Ae. albopictus against insecticides at international airports of south India

A study on susceptibility status in aquatic and adult stages of Aedes aegypti and Ae. albopictus was undertaken at International Airports of Thiruvananthapuram and Cochin located in southern India. WHO standard test kits were used to conduct insecticide susceptibility tests against various organophosphates, organochlorines, carbamates and synthetic pyrethroids. The results indicate that adult Aedes aegypti and Ae. albopictus were resistant to DDT and dieldrin, but susceptible to propoxur, fenitrothion, malathion, deltamethrin, permethrin and lambdacyhalothrin. The susceptibility test conducted on immature stages of Ae. aegypti and Ae. albopictus revealed that they are susceptible to the larvicides commonly used under the National Vector Borne Diseases Control Programme viz. Temephos (0.02 ppm), Fenthion (0.05 ppm) Malathion (1.0 ppm) and Fenitrothion (0.06 ppm).     

A machine-learning approach to map landscape connectivity in Aedes aegypti with genetic and environmental data

Mapping landscape connectivity is important for controlling invasive species and disease vectors. Current landscape genetics methods are often constrained by the subjectivity of creating resistance surfaces and the difficulty of working with interacting and correlated environmental variables. To overcome these constraints, we combine the advantages of a machine-learning framework and an iterative optimization process to develop a method for integrating genetic and environmental (e.g., climate, land cover, human infrastructure) data. We validate and demonstrate this method for the Aedes aegypti mosquito, an invasive species and the primary vector of dengue, yellow fever, chikungunya, and Zika. We test two contrasting metrics to approximate genetic distance and find Cavalli-Sforza–Edwards distance (CSE) performs better than linearized F_ST. The correlation (R) between the model’s predicted genetic distance and actual distance is 0.83. We produce a map of genetic connectivity for Ae. aegypti’s range in North America and discuss which environmental and anthropogenic variables are most important for predicting gene flow, especially in the context of vector control.

Landscape genetics—explicitly quantifying the effects of a heterogenous landscape on gene flow—is an important tool for both conservation biology and the control of invasive species and disease vectors including the “yellow fever mosquito” (Aedes aegypti) (1, 2). We demonstrate that current limitations in landscape genetics can be addressed with a machine-learning approach integrated into an iterative optimization process. Isolation by distance (IBD) is a classical model in population genetics that assumes dispersal is limited in proportion to geographic distance, resulting in increasing genetic differentiation with increasing geographic distance between populations (3–5). Although this pattern is commonly seen in nature, factors such as history and dispersal limitations caused by the environment (i.e., “isolation by resistance”) (6) can produce deviations from IBD. Landscape resistance (alias friction) and its inverse, connectivity, determine how organisms move through a landscape (7). Modeling landscape connectivity can be used to identify the environmental variables that affect the organisms’ gene flow and genetic structure; predict how climate and land use change will affect their gene flow and distribution in the future; and inform conservation, vector control, and other management decisions (1, 8–13). Our goals are to use environmental data (the predictors) to build a model of genetic connectivity (the observed data) that improves on IBD and to identify environmental drivers of gene flow patterns.We implement a machine-learning approach that offers a number of advantages over classical methods in landscape genetics: The machine-learning approach is more objective, it allows the inclusion of correlated variables, and it is able to account for different shapes and magnitudes of correlations between predictor and response variables at different locations in the landscape (14–17). In comparison, a common approach in landscape genetics called resistance surface mapping involves the subjective process of creating resistance surfaces for environmental variables, in which each pixel represents a hypothesized resistance to the organism’s movement often based on expert opinion (6, 18). Effective landscape distances through the resistance surfaces can be found with least cost path or circuit theory analysis (19) and then analyzed for associations with genetic distance (20).One option to circumvent the subjectivity of creating resistance surfaces is to model genetic connectivity directly from environmental data. Bouyer et al. (7) took this approach and used a maximum-likelihood method to integrate genetic data and environmental data to map landscape resistance in tsetse flies. Additionally, they introduced an iterative optimization approach in which each subsequent iteration used least cost path lines through the previously predicted resistance surface—an improvement over modeling organism movement as straight lines (16, 17). While this presented a major advance, the maximum-likelihood methodology requires exclusion of correlated data, establishing the relationship between environmental variables and genetic distance before building the model, and transforming or discretizing nonlinear relationships. Additionally, this approach assumes one relationship between each environmental variable and the genetic data across the whole landscape. To build on previous advances while overcoming some of their limitations, we combine iterative optimization with a machine-learning method called random forest (RF).RF is a nonlinear classification and regression tree analysis that can handle many inputs, including redundant or irrelevant variables, as well as continuous and categorical data types (14, 15). RF creates many internal training/testing subdatasets and aggregates the predictors, resulting in stable and consistent results that generally do not overfit the data and can be evaluated through validation processes (14). It is easier to tune and less likely to overfit noisy data than another machine-learning method we considered, gradient boosting (21). Additionally, RF has been successfully incorporated into ecological studies (22) and a small number of landscape genetics studies (16, 17, 23). These studies considered only the environmental predictor values at the genetic collection sites (23) or along straight lines between each pair of sites (16, 17), in contrast to the least cost path analysis we implement here (7).We demonstrate the efficacy of our method to map landscape connectivity for an important disease vector. Ae. aegypti is highly invasive and the primary vector of yellow fever, Zika, dengue, and chikungunya. Except for yellow fever, there are no reliable, widely used vaccines for these diseases, so vector control is essential. Ae. aegypti originated in Africa and is now found throughout the tropics and increasingly in temperate regions (24–26). The species is temperature constrained, preferring warm, humid areas close to humans (the females’ preferred source for bloodmeals outside their native African range) (27). In the United States, it has a patchy distribution throughout southern states, especially Texas, Florida, and California (28). Although Ae. aegypti can disperse >1 km, its usual lifetime dispersal is only around 200 m (29–32). Passive “hitchhiking” via human transportation networks is responsible for long-distance invasions and worldwide spread of Ae. aegypti and its close relative (33–35). Climate change is also expanding the range of Aedes species, which could expose nearly 1 billion additional people to diseases carried by these mosquitoes for the first time (26).Although IBD is common in nature and a helpful null model in landscape genetics (20), geographic distance is often an inadequate sole predictor of genetic distance (as in the case of our dataset; SI Appendix, Fig. S1). Therefore, a more complex model is needed to explain and predict genetic distance and corresponding landscape connectivity. In this paper we introduce an iterative machine-learning approach to integrate environmental predictors and genetic observation data and apply it to map landscape connectivity for the Ae. aegypti mosquito in North America. We also find and examine the most important variables for building the connectivity model and provide validation of our proposed method.     

Antituberculosis drug resistance in the south of Vietnam: prevalence and trends

BACKGROUND: There is limited evidence that the DOTS (directly observed therapy, short course) strategy for tuberculosis (TB) control can contain the emergence and spread of drug resistance in the absence of second-line treatment. We compared drug-resistance levels between 1996 and 2001 in the south of Vietnam, an area with a well-functioning DOTS program. METHODS: Sputum specimens were collected from consecutively diagnosed patients with smear-positive TB at 40 randomly selected public TB clinics. Mycobacterium tuberculosis isolates were tested for susceptibility to first-line drugs. RESULTS: Among 888 new patients in 2001, resistance to any drug was observed in 238 (26.3%), resistance to isoniazid was observed in 154 (16.6%), resistance to rifampin was observed in 22 (2.0%), resistance to ethambutol was observed in 12 (1.1%), resistance to streptomycin was observed in 173 (19.4%), and resistance to both isoniazid and rifampicin (multidrug resistance [MDR]) was observed in 20 (1.8% [95% confidence interval, 1.0%-3.3%]). Among 136 previously treated patients in 2001, any resistance was observed in 89 (62.9%), and MDR was observed in 35 (23.2%). The prevalence of any drug resistance and of streptomycin resistance among new patients had decreased significantly (P<.01) since 1996; there was no increase in the prevalence of MDR. CONCLUSION: The prevalence of drug resistance decreased despite high initial levels of resistance to isoniazid and streptomycin and despite the absence of second-line treatment. Therefore, a DOTS program can contain drug-resistant TB in this setting.     

Geographic and temporal genetic patterns of Aedes aegypti populations in Rio de Janeiro, Brazil

Rio de Janeiro is considered as the most important entry point for dengue viruses in Brazil. Using isoenzyme markers, we investigated the genetic structure of the mosquito vector Aedes aegypti sampled at three-month intervals in 14 districts in Rio de Janeiro from December 2002 to December 2003. We detected high levels of genetic differentiation (i.e. high F(ST) values and significant P values), which tended to persist throughout the year. The species does not take advantage of routes and railways to disperse. Genetic structuring was higher in the rainy season, suggesting low dispersion of Ae. aegypti at this time of year when all dengue epidemics have been reported in the city.     

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.     

Insecticide resistance mechanisms of Brazilian Aedes aegypti populations from 2001 to 2004

In Brazil, Aedes aegypti resistance to temephos, used since 1967, was detected in several municipalities in 2000. Organophosphates were substituted by pyrethroids against adults and, in some localities, by Bti against larvae. However, high temephos resistance ratios were still detected between 2001 and 2004. Field-simulated assays confirmed a low temephos residual effect. Acethylcholinesterase and Mixed Function Oxidase profiles were not altered. In contrast, higher Esterase activity, studied with three substrates, was found in all examined populations collected in 2001. From 2001 to 2004, a slight reduction in alpha-Esterase (EST) and beta-EST activity together with a gradual increase of p-nitrophenyl acetate (PNPA)-EST was noted. Gluthathione-S-transferase alteration was encountered only in the northeast region in 2001, spreading the entire country thereafter. In general, except for alpha-EST and beta-EST, only one enzyme class was altered in each mosquito specimen. Data are discussed in the context of historic application of insecticides in Brazil.