Sexual transmission of dengue viruses by Aedes albopictus

Male Aedes albopictus experimentally infected with dengue virus types 1, 2, 3, or 4 transmitted their infection sexually to female Ae. albopictus. Such transmission was enhanced if the females had taken a bloodmeal 2 to 7 days prior to mating. Male Ae. albopictus also transmitted dengue virus vertically to their F1 progeny. Infected progeny were found among those derived from eggs laid greater than or equal to 73 hr after mating but not among those derived from eggs laid prior to that time. This suggests that virus probably was not transmitted directly to ova but, rather, underwent prior replication in the female genital tract. Female Ae. albopictus experimentally infected with dengue type 1 virus did not transmit their infection sexually to males. This finding supports the hypothesis that male mosquitoes naturally infected with dengue virus acquired their infection vertically.     

Transovarial transmission of dengue viruses by mosquitoes: Aedes albopictus and Aedes aegypti

Transovarial transmission of all four dengue serotypes was demonstrated in Aedes albopictus mosquitoes. The rates of such transmission varied with the serotype and strain of virus. In general, the highest rates were observed with strains of dengue type 1 and the lowest with dengue type 3. Surprisingly, despite the use of viral strains of the four dengue serotypes which gave the highest rates with Ae. albopictus, transovarial transmission was observed in Aedes aegypti only with dengue type 1, and then only at a relatively low rate. Five different strains of Ae. aegypti were employed, including one that was known to be relatively susceptible to oral infection with dengue viruses. The findings support the view that Ae. aegypti, while of major importance from the point of view of transmission of dengue to man, may be relatively unimportant in the overall natural history of dengue viruses.     

Maternal transmission efficiency of Wolbachia superinfections in Aedes albopictus populations in Thailand

We examined the transmission efficiency of 2 strains of Wolbachia bacteria that cause cytoplasmic incompatibility in field populations of Aedes albopictus by polymerase chain reaction assay. We found mainland and island populations throughout Thailand to be superinfected with group A and B bacteria. Of 320 Wolbachia-positive adult mosquitoes, 97.5% were infected with both groups. Single infected individuals of each Wolbachia group were encountered in nearly equal numbers. We screened 550 offspring from 80 field-collected mothers and found the transmission efficiency of group A Wolbachia to be 96.7% and that of group B Wolbachia to be 99.6%. Mothers that did not transmit both Wolbachia infections to all of their offspring were significantly larger in size than those with perfect transmission fidelity. We discuss our findings in relation to the prospects of the use of Wolbachia as a gene-driving mechanism.     

Limited potential for transmission of live dengue virus vaccine candidates by Aedes aegypti and Aedes albopictus

To evaluate the transmission risk of four live dengue (DEN) vaccine candidates developed by the U.S. Army (DEN-1, 45AZ5 PDK 20; DEN-2, S16803 PDK 50; DEN-3, CH53489 PDK 20; and DEN-4, 341750 PDK 20), we tested 3,010 Aedes aegypti and 1,576 Aedes albopictus mosquitoes blood-fed on 21 volunteers who had been administered one of the four vaccine candidates or the licensed yellow fever (YF) vaccine (17D). We used an indirect immunofluorescence assay (IFA) to detect DEN or YF viral antigen in the heads of mosquitoes. Corresponding to the lack of a detectable viremia among volunteers inoculated 8-13 days previously with live DEN-1 or DEN-2 vaccine candidates, only six mosquitoes developed disseminated infections after feeding on these volunteers. These six mosquitoes included 4 of 247 Ae. albopictus fed on volunteers inoculated with the DEN-1 vaccine candidate and 2 of 528 Ae. aegypti fed on volunteers inoculated with the DEN-2 vaccine candidate. Infection was confirmed in each of these IFA-positive mosquitoes by isolating infectious virus from the mosquito's body in Vero-cell culture. None of the 1,252 or the 969 mosquitoes fed on DEN-3 or DEN-4 recipients, respectively, were infected. Overall, dissemination rates in Ae. albopictus and Ae. aegypti were low. Dissemination rates were 0.5%, 0.3%, < 0.1%, and < 0.1% for the DEN-1 through DEN-4 vaccine candidates, respectively. Because of the observed low dissemination rates, it is unlikely that these vaccine viruses would be transmitted under natural conditions.     

Field prevalence of Wolbachia in the mosquito vector Aedes albopictus

The endosymbiotic bacteria in the genus Wolbachia have been proposed as a potential candidate to deliver pathogen-blocking genes into natural populations of medically important insects. The successful application of Wolbachia in insect vector control depends on the ability of the agent to successfully invade and maintain itself at high frequency under field conditions. Here, we evaluated the prevalence of Wolbachia infections in a field population of the Wolbachia-superinfected mosquito Aedes albopictus. A field prevalence of 100% (n = 1,016) was found in a single population in eastern Thailand via polymerase chain reaction (PCR) testing of Wolbachia both from individual parent females and their corresponding F1 offspring. This is the first report of accurate Wolbachia prevalence in a field population of an insect disease vector. The prevalence of superinfection was estimated to be 99.41%. All single-infected individual mosquitoes (n = 6) were found to harbor group A Wolbachia. For this particular population, none was found to be single-infected with group B Wolbachia. Our results also show that PCR testing of field materials alone without checking F1 offspring overestimated the natural prevalence of single infection. Thus, the confirmation of infection status by means of F1 offspring was critical to the accurate estimates of Wolbachia prevalence under field conditions.     

First dengue virus detection in Aedes albopictus from Delhi, India: its breeding ecology and role in dengue transmission

Objective To report dengue virus and its disease transmission in Aedes albopictus in the National Capital Territory of Delhi, India. Methods Monthly Aedes surveys were carried out in 126 urban localities of Delhi in 2008 and 2009. Pools of all three species of Aedes mosquitoes were tested for Dengue virus (DENV) using an antigen‐capture enzyme‐linked immunosorbent assay. Results Aedes aegypti was the most prevalent species, breeding throughout the year. Aedes albopictus was found in 9.52% of surveyed localities including the central urban part of Delhi, in March and from August to October. Aedes albopictus and Aedes vittatus are adapting to breed in manmade containers in the urban areas of Delhi in addition to their natural habitats of bamboo bushes and rock pits. Of the 229 pools of Ae. aegypti and 34 pools of Ae. albopictus tested, 10.5% and 11.76% were positive for dengue virus, respectively. No dengue virus infection was recorded in Ae. vittatus. Conclusion This is the first report of dengue virus in Ae. albopictus from north India. Because DENV was detected in Ae. albopictus, which adapted to manmade containers, both its spread and transmission dynamics should be checked.     

Polygamy: the possibly significant behavior of Aedes aegypti and Aedes albopictus in relation to the efficient transmission of dengue virus

The polygamous behavior of male Aedes aegypti (L.) and Ae. albopictus (Skuse) was investigated by co-habiting a newly-emerged male and females in a 30 cm3 cage (1 male: 20 females) for up to 5 consecutive days. As determined by insemination rates, the results indicated that one Ae. aegypti and Ae. albopictus male could successfully mate with 1.10 (0-4), 4.10 (1-8), 5.40 (4-8), 5.10 (2-8), 5.15 (3-9) and 0.20 (0-3), 1.70 (0-3), 2.35 (1-4), 2.30 (0-4), 2.35 (1-4) Ae. aegypti and Ae. albopictus females, respectively on day 1,2,3,4 and 5 consecutively. The possibly significant role of their polygamy in relation to dengue virus transmission is discussed.     

登革热媒介白纹伊蚊mtDNA—ND4基因特征分析

目的研究不同地理种群白纹伊蚊（Aedesalbopictus）线粒体蛋白NADH脱氢酶亚基Ⅳ（mtDNA—ND4）基因多态性,探讨其遗传特征。方法采集不同海拔高度、登革热历史流行区与非流行区的白纹的幼虫,实验室饲养至成蚊,PCR扩增mtDNA—ND4基因并遗传分析。结果获mtDNA—ND4长度为324b,碱基A＋T含量分别为73：2％,17个碱基置换位点,颠换率为62．5％;19个个体存在5个单倍型,单倍型多态性（h）为0．442。结论福建省不同地理株白纹伊蚊出现小程度的遗传分化,可能由遗传漂变引起的。     

Wolbachia Do Not Induce Reactive Oxygen Species-Dependent Immune Pathway Activation in Aedes albopictus

Aedes albopictus is a major vector of dengue (DENV) and chikungunya (CHIKV) viruses, causing millions of infections annually. It naturally carries, at high frequency, the intracellular inherited bacterial endosymbiont Wolbachia strains wAlbA and wAlbB; transinfection with the higher-density Wolbachia strain wMel from Drosophila melanogaster led to transmission blocking of both arboviruses. The hypothesis that reactive oxygen species (ROS)-induced immune activation plays a role in arbovirus inhibition in this species was examined. In contrast to previous observations in Ae. aegypti, elevation of ROS levels was not observed in either cell lines or mosquito lines carrying the wild-type Wolbachia or higher-density Drosophila Wolbachia strains. There was also no upregulation of genes controlling innate immune pathways or with antioxidant/ROS-producing functions. These data suggest that ROS-mediated immune activation is not an important component of the viral transmission-blocking phenotype in this species.     

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.     

Wolbachia transinfection in Aedes aegypti: a potential gene driver of dengue vectors

The endosymbiotic bacteria in the genus Wolbachia are capable of inducing a wide range of reproductive abnormalities in their hosts, including cytoplasmic incompatibility (CI), which could lead to the replacement of uninfected host populations with infected ones. Because of this, Wolbachia have attracted considerable interest as a potential mechanism for spreading disease-blocking transgenes through vector populations. Here we report the establishment of double Wolbachia transinfection by direct adult microinjection of Wolbachia from naturally double-infected Aedes albopictus to Aedes aegypti, the most important mosquito vector of infectious viral diseases, and a mosquito in which natural Wolbachia infections are not known to occur. We further demonstrate that incomplete CI is induced in these double-transinfected mosquitoes. Comparisons of fitness traits between naturally uninfected and transinfected Ae. aegypti lines indicated one significant difference in favor of the latter, namely, an increased number of eggs laid. Levels of CI expression corresponded to the Wolbachia density. There were large differences in relative Wolbachia density between reproductive and nonreproductive tissues in both Ae. albopictus and transinfected Ae. aegypti, except Malpighian tubule, which implied the preferred establishment of Wolbachia within reproductive tissue. Results from a simulation model confirm that population replacement by transinfected Ae. aegypti is possible over time. The establishment of Wolbachia double infections in Ae. aegypti by direct adult microinjection and the demonstration of CI expression in this new host suggest that Wolbachia could be experimentally transferred into vector species and could also be used as a gene-driving system to genetically manipulate vector populations.     

Vertical transmission of dengue viruses by Aedes mediovittatus

Aedes mediovittatus, a forest and peridomestic mosquito found in the Caribbean area, has previously been shown to be highly susceptible to oral infection with dengue viruses in the laboratory. In the present study, the species was found to transmit all four dengue serotypes vertically (i.e., from one generation to another) at rates much higher than any observed previously for flaviviruses in mosquitoes. Vertical transmission rates (the percentage of parent females transmitting to one or more progeny) ranged up to 95%. Filial infection rates (the percentage of infected progeny) varied widely by family but rates greater than or equal to 20% for individual families were not uncommon. Since Ae. mediovittatus feeds readily on humans and is relatively abundant, there is no apparent reason why it would not serve as a vector of dengue. If it does, vertical transmission of the virus in this species would contribute to the maintenance of viral endemicity.     

Natural vertical transmission of dengue virus in Aedes albopictus (Diptera: Culicidae) in Kerala, a southern Indian state

The natural occurrence of vertical transmission of dengue viruses in Aedes albopictus (Skuse) mosquitoes was examined in the state of Kerala in southern India. Adults and larvae of Ae. albopictus collected from Kerala were screened for dengue viruses by using enzyme-linked immunosorbent assay with dengue-specific monoclonal antibodies. The possibility of the vertical transmission of dengue virus in Ae. albopictus was further evidenced by the detection of the virus in field-collected adult males as well as females emerged from field-collected larvae. Two pools, one pool from the adult males and one pool from the emerged females derived from field-collected larvae, were collected in the relatively hot months of June and March, respectively, and found to be positive for dengue virus antigen. Dengue serotype 2 virus was isolated from field-collected male adults in Kerala. These findings suggest that dengue virus is maintained in Ae. albopictus mosquitoes during the dry season by vertical transmission in nature.