Aedes aegypti larval indices and risk for dengue epidemics

We assessed in a case-control study the test-validity of Aedes larval indices for the 2000 Havana outbreak. "Cases" were blocks where a dengue fever patient lived during the outbreak. "Controls" were randomly sampled blocks. Before, during, and after the epidemic, we calculated Breteau index (BI) and house index at the area, neighborhood, and block level. We constructed receiver operating characteristic (ROC) curves to determine their performance as predictors of dengue transmission. We observed a pronounced effect of the level of measurement. The BI(max) (maximum block BI in a radius of 100 m) at 2-month intervals had an area under the ROC curve of 71%. At a cutoff of 4.0, it significantly (odds ratio 6.00, p<0.05) predicted transmission with 78% sensitivity and 63% specificity. Analysis of BI at the local level, with human-defined boundaries, could be introduced in control programs to identify neighborhoods at high risk for dengue transmission.     

Experimental infection of Aedes aegypti and Aedes albopictus with dengue viruses

The susceptibility of two Aedes (Stegomyia) species, Ae. aegypti and Ae. albopictus, to infection with dengue virus was assessed using both natural feeding on a viraemic gibbon and a membrane feeding technique. Both species were highly susceptible to dengue virus infection.     

Distribution of Aedes aegypti and dengue in the State of Maranh?o, Brazil]

Dengue and Aedes aegypti are widespread in the State of Maranh?o. During 1995, 87 of the 136 of the State's counties, including 176 towns and 480,687 households, were studied, of which 30 counties (34. 4%), 118 towns (67.0%) and 10,357 households (2.1%) were positive for Aedes aegypti. The positive counties are distributed as follow: 3 on S?o Luís Island, 7 in the Amazonia of Maranh?o, 12 in the southern zone of the cerrados or savannas, and 5 in the mixed forest/savanna/palm grove zone. One positive county for Ae. aegypti was located in each of the following: alluvial fields, forest/palm grove, and sand dune/shoals areas. The Ae. aegypti-positive household rates were higher in the Amazonia of Maranh?o (3.5%) and on S?o Luís Island (2.5%), because they are the most heavily traveled migratory and trade routes between Maranh?o and neighboring states and are also the State's main economic centers. Household infestation rates for Ae. aegypti and reported dengue case rates were higher during the rainy season, showing the importance of rain in forming prime breeding sites for Aedes and spreading dengue.     

Surveillance for the dengue vector Aedes aegypti in Tobago, West Indies

An island-wide house survey was conducted in January 2002 to determine the geographic distribution, container profile, and population density of the Aedes aegypti in Tobago, West Indies. The results showed the Ae. aegypti infestation levels were significantly different (P > 0.01) among the 4 districts, with greater infestation levels (P > 0.01) observed in the Northern and Windward districts than in the Central and Leeward districts. From the 50 towns in Tobago, houses were found positive in Delaford (21), Argyle (18), and Goodwood (14). representing 42.1% of the total number of positive houses in the Windward district (3,971 houses); Parlatuvier (15), Whim (14), Castara (12), and Bloody Bay (12), representing 62.3% of the total number of positive houses in the Northern district (3,087 houses); Calderhall (12), Mason Hall (11), and Government House (10), representing 46.5% of the total number of positive houses in the Central district (4,706 houses); and Lambeau (10), Bucco (6), and Bethel (6), representing 53.7% of the total number of positive houses in the Leeward district (3,175 houses). The majority (66 or 63.5%) of dengue cases occurred in the Central district where the Breteau indices ranged from 7.1 to 44.0 (mean = 16.6). These results suggest that a more systematic and sustained vector control program that uses both biological and chemical control methods should be adopted to reduce Ae. aegypti populations to below dengue transmission thresholds.     

Aedes aegypti (l.) population in an endemic area of dengue in Southeast Brazil

OBJECTIVE: A Aedes aegypti population from an area of dengue transmission was studied to determine mosquito's frequency, spatial distribution, parity, its ovarian development and midgut contents. METHODS: The study carried out in S?o José do Rio Preto country, S?o Paulo, SP, Brazil. Low and intermediate socioeconomic level regions were chosen to the study, which was carried out 1996 and 1997. Using a manual electric aspirator, captures were performed inside and outside the houses. Dissections were conducted to allow classify female mosquitoes according to their physiologic state. RESULTS: One hundred and eighty-eight males and 189 females were captured in both regions. A general index of 0.46 Ae. aegypti females per house was determined. Of the total sample, 82.4% of males and 87.3% of females were collected indoors. Greater proportions of females were found in the region of low socioeconomic level, and higher population density. One hundred and forty-eight female mosquitoes were analyzed: 27.0% were nulliparous and 10.1% parous. The rest were classified as Christophers and Mer's (C & M) phases III to V, 28.0% of which had red blood in their midgut. Of the total female population, 87.9% had bloodmeals. CONCLUSIONS: The mosquito species revealed a high degree of endophily. There was a higher proportion of nulliparous mosquitoes, although the majority of the females were classified as C & M's phases III to V. These results highlight both the great number of females who had had bloodmeals, and the lack of gonotrophic concordance.     

我国登革热重要媒介埃及伊蚊的研究进展

登革热是经蚊虫叮咬人体传播的重要传染病,广泛流行于热带和亚热带地区。埃及伊蚊是登革热病毒的主要媒介,随着全球气候变暖,旅游业及交通业的迅速发展,该蚊在我国的分布发生了区域上的变化;长期的蚊虫控制措施,引起了一些生态习性的改变;通过对埃及伊蚊及登革热病毒传播关系的研究表明,垂直传播和水平传播方式是该蚊保存登革热病毒的重要方式,该蚊对登革热病毒高度易感和带病毒率较高。现就埃及伊蚊在我国的地理分布、生态习性及与登革热传播关系的研究做一综述。     

Evaluation of the efficacy of lambda-cyhalothrin applied as ultra-low volume and thermal fog for emergency control of Aedes aegypti in Honduras.

An extended-duration formulation of lambda-cyhalothrin (Demand CS) applied as either an ultra-low volume (ULV) or thermal fog spray from a new handheld sprayer (Terrier) against Aedes aegypti was evaluated in Honduras. Spray applications were made at the front door for 1 min or to each room for 15 sec, both for the ULV and thermal fog applications to houses in separate blocks for each treatment. The efficacy and duration of effectiveness of the spray was determined from sentinel caged mosquito mortality and collection of mosquitoes within houses with a backpack power aspirator. Sentinel caged mosquito mortality in both open and sequestered locations was 97-100% for all spray treatments, with control mortality less than 2%. Both ULV applications (front door and each room) provided 4 wk of significant control (P < 0.01) based on adult Ae. aegypti house collections.     

Simulation modelling of Aedes aegypti prevalence,an environmental hazard surveillance tool for the control of dengue epidemics

We performed a five-year simulation of Aedes aegypti population dynamics in Townsville, Queensland to better understand local Ae. aegypti ecology and thus improve the potential effectiveness of interventions aimed at dengue control. Using the Container Inhabiting Mosquito Simulation Model (CIMSiM), public health interventions can use a wealth of environmental knowledge about Ae. aegypti, a disease vector hazard, towards its surveillance. We conducted larval surveys to initialise the simulation of Ae. aegypti prevalence and parameterise Townsville's artificial container habitats. Our simulations indicate (1) a seasonal, but stable Ae. aegypti prevalence, (2) the importance of bromeliads, striking vases, pot plant bases and car tire containers as breeding sources and (3) the potential effectiveness of breeding container source reduction and pesticide space spraying. Although it is unlikely that simulation modelling will ever be used to forecast impending dengue epidemics, the limitations of modelling do not preclude their role in environmental health programs aimed at improving the effectiveness of available interventions.     

Evaluation of surveillance devices for monitoring Aedes aegypti in an urban area of northeastern Peru

In this study, we assessed the efficacy of the American Biophysics Corporation Standard Professional (ABC-PRO) light trap, the Omni-Directional Fay-Prince trap (with and without CO2), and the Centers for Disease Control and Prevention Wilton trap as a means of evaluating populations of adult Aedes aegypti in an urban area of northeastern Peru. Efficacies of collections from each of the trap types were compared to backpack-aspirator collections and human-landing collections. Collections were conducted twice daily, 3 days per week, for 27 wk from July 2001 to July 2002. Backpack-aspirator collections yielded significantly more mosquitoes (1,764) than any of the other collecting methods with a mean of 21.80 mosquitoes collected per sampling period. This method was less specific for Ae. aegypti than were human-landing collections because only 28.3% of mosquitoes collected with backpack aspirators were Ae. aegypti. Human-landing collections yielded only 23% (554/2,411) of the total mosquitoes collected. However, more than 80% (445/554) of the mosquitoes collected by this method were Ae. aegypti. None of the trapping devices evaluated collected mosquitoes, specifically Ae. aegypti, as effectively as backpack-aspirator or human-landing collections. The ABC-PRO trap, which was the most effective device in collecting mosquitoes, particularly Ae. aegypti, collected less than 2% of the total mosquitoes (mean of 0.12 mosquitoes/sampling period), and less than 3% of total Ae. aegypti (mean of 0.11 Ae. aegypti/sampling period). We conclude that none of the trap devices evaluated in this study is an acceptable alternative to backpack-aspirator or human-landing collections for monitoring populations of adult Ae. aegypti in Peru.     

Aedes aegypti infestation and occurrence of dengue in the city of Belo Horizonte, Brazil

OBJECTIVE: To assess the association between the proportion of buildings positive for Aedes aegypti larvae measured by means of building infestation rate and dengue incidence rate. METHODS: Autochthonous dengue cases were selected and building infestation rates assessed in the coverage areas of health districts in the city of Belo Horizonte, Brazil, from October 1997 to May 2001. After grouping building infestation rates according to their distribution in quartiles, mean dengue incidence rates (for the month following the building infestation assessment) were compared using ANOVA. RESULTS: A weak though statistically significant correlation was observed between monthly dengue incidence and building infestation rates for the health districts (r=0.21; p=0.02) and coverage areas (r=0.14; p=0.00) in the study period. After grouping building infestation rates in quartiles, coverage areas of building infestation between 0.46% and 1.32% (second quartile) showed a mean monthly dengue incidence rate twice as high as areas of building infestation lower or equal to 0.45% (first quartile). Areas of building infestation between 1.33% and 2.76% (third quartile) and equal to or higher than 2.77% had a mean monthly dengue incidence rate five and seven times, respectively, higher than those areas showing 0.45% or less. CONCLUSIONS: Despite the well-known limitations of building infestation rates for estimating vector infestation and prediction of dengue epidemics, the study results indicate that higher building infestation rates were associated to a higher risk of disease transmission in the health districts and coverage areas of Belo Horizonte.     

Infection and dissemination of dengue virus type 2 in Aedes aegypti, Aedes albopictus, and Aedes scutellaris from the Torres Strait, Australia

To determine their relative roles in transmission of dengue virus (DENV) in the Torres Strait region of northern Australia, we examined infection and dissemination of a sympatric strain of dengue virus type 2 (DENV-2) in Aedes scutellaris, Ae. albopictus, and Ae. aegypti. In experiments using membrane feeders for virus exposure, infection rates were 83% and 43% for Ae. scutellaris and Ae. aegypti, respectively. Salivary gland infection rates for both species were 43%. In experiments using pledgets for virus exposure, infection rates for Ae. aegypti, Ae. scutellaris, and Ae. albopictus were 68%, 55%, and 37%, respectively. Aedes albopictus exhibited the greatest barriers to infection with only 7% tested developing a salivary gland infection, compared to 42% and 24% of Ae. aegypti and Ae. scutellaris, respectively. These results suggest that Ae. scutellaris may have been responsible for DENV transmission on Torres Strait islands, where Ae. aegypti does not occur. In contrast, Ae. albopictus may not be an important vector of DENV-2 from the Torres Strait.     

Genetic structure and phylogeography of Aedes aegypti, the dengue and yellow-fever mosquito vector in Bolivia

Between the 16th and 18th centuries, Aedes aegypti (Diptera: Culicidae), a mosquito native to Africa, invaded the Americas, where it was successively responsible for the emergence of yellow fever (YF) and dengue (DEN). The species was eradicated from numerous American countries in the mid-20th century, but re-invaded them in the 1970s and 1980s. Little is known about the precise identities of Ae. aegypti populations which successively thrived in South America, or their relation with the epidemiological changes in patterns of YF and DEN. We examined these questions in Bolivia, where Ae. aegypti, eradicated in 1943, re-appeared in the 1980s. We assessed the genetic variability and population genetics of Ae. aegypti samples in order to deduce their genetic structure and likely geographic origin. Using a 21-population set covering Bolivia, we analyzed the polymorphism at nine microsatellite loci and in two mitochondrial DNA regions (COI and ND4). Microsatellite markers revealed a significant genetic structure among geographic populations (F(ST)=0.0627, P<0.0001) in relation with the recent re-expansion of Ae. aegypti in Bolivia. Analysis of mtDNA sequences revealed the existence of two genetic lineages, one dominant lineage recovered throughout Bolivia, and the second restricted to rural localities in South Bolivia. Phylogenic analysis indicated that this minority lineage was related to West African Ae. aegypti specimens. In conclusion, our results suggested a temporal succession of Ae. aegypti populations in Bolivia, that potentially impacted the epidemiology of dengue and yellow fever.     

Genetic diversity of the dengue vector Aedes aegypti in Australia and implications for future surveillance and mainland incursion monitoring

In February 2004, the discovery of an incursion of the dengue vector Aedes aegypti into the town of Tennant Creek in the Northern Territory caused concern for the Northern Territory health authorities who proceeded to implement a Commonwealth-funded eradication program. To determine the origin of the incursion, we performed a genetic analysis on Ae. aegypti from several Queensland and overseas localities. A comparison of DNA sequences from the mitochondrial cytochrome oxidase 1 gene indicated that the incursion was probably from Cairns or Camooweal. This genetic marker was also useful in identifying a separate Townsville haplotype population and another population on Thursday Island in the Torres Strait that was genetically divergent to the mainland populations. The possible use of this marker as a surveillance tool for identifying the origins of local and overseas incursions is discussed.     

Vector competence of Aedes aegypti populations from Senegal for sylvatic and epidemic dengue 2 virus isolated in West Africa

Vector competence of Aedes aegypti populations from six locations in different bioclimatic zones and habitats of Senegal was assessed for sylvatic (ArD 140875) and epidemic (ArA 6894) dengue 2 virus (DENV-2) isolates. F1 generation mosquitoes were fed with a blood meal containing 10(6.5) and 1.6 x 10(7) tissue culture infectious dose (TCID50) of the sylvatic DENV-2 isolate or 10(6.5) TCID50 of the epidemic isolate and incubated for 14 days. After this period, legs and wings of each individual were pooled and the corresponding body was collected separately. An immunofluorescence assay was used to test bodies and wings/legs for DENV-2 infection separately. The results showed that: (i) Senegalese Ae. aegypti populations have lower vector competence (infection rate 0-26%, dissemination rate 10-100%) for sylvatic and epidemic dengue viruses than populations from Asia and America; (ii) there was no obvious geographic variation in susceptibility among mosquito populations; (iii) the only apparent factor affecting the susceptibility was the virus titre; and (iv) except for that of Kédougou, all populations were able to disseminate the virus. Overall our findings, while extended to four other populations, confirm previous studies showing low susceptibility of Senegalese Ae. aegypti populations.     

Integrated control of the dengue vector Aedes aegypti in Liu-Chiu village, Ping-Tung County, Taiwan

Because of an inadequate supply of potable water, villagers of Small Liu-Chiu Isle, Ping-Tung County, Taiwan, store water in containers supporting a large population of Aedes aegypti. In 1989-96, integrated control measures against Ae. aegypti were implemented on the basis of community participation. These measures included release of mosquito larvivorous fish in the drinking water storage facilities, application of larvicides to the water storage facilities in vegetable gardens, removal of discarded and unused containers and tires, improvement of household water storage facilities, and increase of potable water supply. Before implementation of the integrated control measures in 1988, 74% of the water-containing vessels were water storage facilities, and 24% of those were infested by Ae. aegypti. In 1989, the Breteau index for the entire island, indicating the average distribution density for larval Ae. aegypti, was 53.9, as compared to an index of 1.2 in 1996. In 4 villages located at the southwest and middle of the island, Ae. aegypti nearly became extinct because of the enthusiastic participation of the community. Before the implementation of integrated control, Ae. aegypti was the dominant species in containers both inside and outside the household, but after the integrated control, Aedes albopictus became predominant outside.     

Malathion resistance in Aedes aegypti and Culex quinquefasciatus after its use in Aedes aegypti control programs

The continued widespread use of malathion in Aedes aegypti control programs in Latin America has generated insecticide resistance to this chemical in Culex quinquefasciatus but not in Ae. aegypti. To determine the extent of this resistance, the susceptibility of Cx. quinquefasciatus and Ae. aegypti from several countries to malathion was evaluated. Bioassay results indicated that all Ae. aegypti strains evaluated from Cuba, Venezuela, Costa Rica, and Jamaica were susceptible to malathion in spite of the historical use of this insecticide in Ae. aegypti control programs in these countries. In contrast, a high level of resistance to this insecticide was found in Cx. quinquefasciatus from Venezuela, Colombia, Brazil, and Cuba. Synergist assays indicated that neither esterases nor mixed-function oxidases (MFOs) were involved as the resistance mechanism to malathion in any of the Ae. aegypti strains tested. In Cx. quinquefasciatus, synergist assays confirmed that esterases played an important role in malathion resistance but MFOs were not involved in causing malathion resistance in this species. Biochemical assays showed that both resistance mechanisms were present in the Ae. aegypti and Cx. quinquefasciatus populations. Acrylamide electrophoresis gels revealed that all Ae. aegypti strains had a strongly staining, clear band, named A4, and had a relative mobility (Rm) value of 0.7. Analysis if the results of this study suggested that malathion could continue to be used for the emergency control of Ae. aegypti, the mosquito vector for dengue and dengue hemorrhagic fever in the Americas, but that malathion is probably not effective for the control of adult Cx. quinquefasciatus in urban areas. Therefore, control operations should integrate nonorganophosphate insecticides such as pyrethroids for control of these 2 species found in the urban environment.