Parasitism of Ascogregarina taiwanensis and Ascogregarina culicis (Apicomplexa: Lecudinidae) in larvae of Aedes albopictus and Aedes aegypti (Diptera: Culicidae) from Manaus, Amazon region, Brazil

The aim of the study was to determine the existence of Ascogregarina spp. in larvae of Aedes albopictus and Aedes aegypti collected in urban and suburban areas of Manaus, Amazon region, Brazil. Between May 2004 and July 2005, the mid-gut of 3rd and 4th instar larvae, collected in tire traps in six neighborhoods of Manaus, was examined for the presence of trophozoites of Ascogregarina. Coexistence of Ae. albopictus larvae infected by A. taiwanensis, and Ae. aegypti larvae by A. culicis, was detected in traps in the field. The percentage of Ae. albopictus larvae infected by A. taiwanensis ranged from 21% to 93.5% and of Ae. aegypti larvae infected by A. culicis from 22% to 95%. The mean infection intensity was similar in both species of Aedes. In traps located in Mauazinho, the replacement of Ae. aegypti by Ae. albopictus larvae was observed. In Manaus, Ae. albopictus larvae were parasitized by A. taiwanensis, and Ae. aegypti larvae by A. culicis. Infection rates were high when the species of Aedes were found separately.     

Natural vertical transmission of dengue viruses by Aedes aegypti in Bolivia

The natural transmission of dengue virus from an infected female mosquito to its progeny, namely the vertical transmission, was researched in wild caught Aedes aegypti during an important outbreak in the town of Santa Cruz de la Sierra, Bolivia. Mosquitoes were collected at the preimaginal stages (eggs, larvae and pupae) then reared up to adult stage for viral detection using molecular methods. Dengue virus serotypes 1 and 3 were found to be co-circulating with significant higher prevalence in male than in female mosquitoes. Of the 97 pools of Ae. aegypti (n = 635 male and 748 female specimens) screened, 14 pools, collected in February-May in 2007, were found positive for dengue virus infection: five DEN-1 and nine DEN-3. The average true infection rate (TIR) and minimum infection rate (MIR) were respectively 1.08% and 1.01%. These observations suggest that vertical transmission of dengue virus may be detected in vectors at the peak of an outbreak as well as several months before an epidemic occurs in human population.     

Investigations of dengue-2 susceptibility and body size among Aedes aegypti populations

The mosquito Aedes aegypti (L.) (Diptera: Culicidae) is the primary global vector for dengue virus (DENV), yet considerable genetic variation exists among populations in terms of its competence to vector DENV. Variability in adult body size has also been observed among various mosquito populations and several studies have reported a relationship between body size and arbovirus dissemination, although most of these relied on artificially derived variation in body size. Here we examine the relationship between body size and disseminated DENV infection among 10 Ae. aegypti populations reared under optimum laboratory conditions. Body size variability was inferred from wing length measurements and DENV competence was evaluated as the proportion of individuals with disseminated infections following exposure to the dengue-2 JAM1409 strain. There were significant differences in mean wing lengths among populations (anova, F(9,22)= 7.10, P < 0.0001), ranging from 2.16 mm (Bangkok population) to 2.79 mm (MOYO-S [susceptible] population). We also observed significant differences among some populations in mean DENV infection rates (Waller-Duncan K-ratio t-test), ranging from 19.54% (MOYO-R [refractory] population) to 56.60% (MOYO-S population). However, we did not observe evidence for significant interactions between body size and DENV dissemination. We suggest that either the two traits are genetically independent or that our ability to detect interactions between them was limited by their respective inheritances as quantitative traits.     

Quantitative trait loci that control dengue-2 virus dissemination in the mosquito Aedes aegypti

The mosquito Aedes aegypti is the most important vector of yellow fever and dengue fever flaviviruses. Ae. aegypti eradication campaigns have not been sustainable and there are no effective vaccines for dengue viruses. Alternative control strategies may depend upon identification of mosquito genes that condition flavivirus susceptibility and may ultimately provide clues for interrupting transmission. Quantitative trait loci affecting the ability of Ae. aegypti to develop a dengue-2 infection in the midgut have been mapped previously. Herein we report on QTL that determine whether mosquitoes with a dengue-2-infected gut can then disseminate the virus to other tissues. A strain selected for high rates of dengue-2 dissemination was crossed to a strain selected for low dissemination rates. QTL were mapped in the F(2) and again in an F(5) advanced intercross line. QTL were detected at 31 cM on chromosome I, at 32 cM on chromosome II, and between 44 and 52 cM on chromosome III. Alleles at these QTL were additive or dominant in determining rates of dengue-2 dissemination and accounted for approximately 45% of the phenotypic variance. The locations of dengue-2 midgut infection and dissemination QTL correspond to those found in earlier studies.     

The community ecology of Aedes egg hatching: implications for a mosquito invasion

Abstract. 1. A recently introduced treehole mosquito from Asia, Aedes albopictus , is spreading throughout eastern North America, especially in tyre-refuse piles. Previous studies have identified inhibitory effects of larvae on egg hatch as a potential population regulatory mechanism within Aedes. Larva-egg interactions may also occur between species. This experiment assesses the ability of larvae of A. albopictus and two possible competitors in North America, A.triseriatus and A.aegypti , to suppress hatching of conspecific and congeneric eggs.
2. We exposed eggs of each species to varying combinations of larval species and density for 24h and assessed subsequent hatch rates. Aedes albopictus eggs exhibited the lowest level of inhibition when exposed to high larval densities; moreover, at the lowest larval density they imposed the most intense interspecific hatch inhibition.
3. Discretionary hatching in response to larval density may influence community composition by promoting the spread of A.albopictus , perhaps even leading to its dominance within North American Aedes communities.     

Role of gregarine parasite Ascogregarina culicis (Apicomplexa: Lecudinidae) in the maintenance of Chikungunya virus in vector mosquito

Ascogregarina culicis and Ascogregarina taiwanensis are common gregarine parasites of Aedes aegypti and Aedes albopictus mosquitoes, respectively. These mosquito species are also known to transmit dengue and Chikungunya viruses. The sporozoites of these parasites invade the midgut epithelial cells and develop intracellularly and extracellularly in the gut to complete their life cycles. The midgut is also the primary site for virus replication in the vector mosquitoes. Therefore, studies were carried out with a view to determine the possible role of these gregarines in the vertical transmission of dengue and Chikungunya viruses from larval to adult stage. Experiments were performed by exposing first instar mosquito larvae to suspensions containing parasite oocysts and viruses. Since Ascogregarina sporozoites invade the midgut of first instar larvae, the vertical transmission was determined by feeding the uninfected first instar larvae on the freshly prepared homogenates from mosquitoes, which were dually infected with viruses and the parasite oocysts. Similarly, the role of protozoan parasites in the vertical transmission of viruses was determined by exposing fresh first instar larvae to the dried pellets of homogenates prepared from the mosquitoes dually infected with viruses and the parasite oocysts. Direct vertical transmission and the vertical transmission of CHIK virus through the oocyst of the parasites were observed in the case of Ae. aegypti mosquitoes. It is suggested that As. culicis may have an important role in the maintenance of CHIK virus during the inter-epidemic period.     

白纹伊蚊感染登革Ⅱ型病毒后的屏障

以高、低两种浓度病毒液分别经胸接种和经口感染海南株白纹伊蚊Aedes albopictus后,经不同时间内对蚊虫中肠、脑、神经节和涎腺进行IFA染色检查.比较各器官感染率后可知该蚊存在中肠感染屏障(mesenteronal infection barrier MIB)、中肠释放屏障(mesenteronal escape barrier,MEB)和涎腺感染屏障(salivary gland infection barrier,SGIB)并且各屏障率分别与时间及剂量呈负相关.     

Susceptibility of Aedes aegypti and Aedes albopictus larvae to Ascogregarina culicis and Ascogregarina taiwanensis (Apicomplexa: Lecudinidae) from Florida

The susceptibility of Aedes aegypti to Ascogregarina culicis and Aedes albopictus to Ascogregarina taiwanensis was examined with mosquito and parasite strains from Tampa, FL. When each host was bioassayed with its natural gregarine, the infection intensity indicated that Ae. aegypti was 59% more susceptible to A. culicis (87 gamonts/larva) than Ae. albopictus to A. taiwanensis (47 gamonts/larva). Infections in single and mixed host populations exposed to 100 oocysts/larva of one and both parasites demonstrated that Ae. aegypti harbors higher A. culicis gamont loads than Ae. albopictus of A. taiwanensis. In dual gregarine exposures of single host populations, the A. culicis infection intensity in Ae. aegypti was reduced by approximately 50%. A. taiwanensis exhibited the same capability of infecting Ae. albopictus in single and dual exposures. In mixed host populations there were no cross infections, but A. taiwanensis in Ae. albopictus produced an infection intensity of approximately 70% lower than that of A. culicis in Ae. aegypti.     

Competition and resistance to starvation in larvae of container-inhabiting Aedes mosquitoes

Abstract. 1. Hypotheses about declining populations of container-inhabiting Aedes mosquitoes following the invasion by additional species were tested.
2. The larval competition hypothesis was studied experimentally in pure and mixed cultures of Aedes aegypti (L.), A.albopictus (Skuse) and A.triseriatus (Say). The experiments used decomposing leaf litter in the laboratory, as opposed to most previous research which used non-natural food.
3. Resistance to starvation is introduced as a new measure of larval performance and competitiveness. The hypothesis is that more successful larvae store larger energy reserves and resist the lack of food longer.
4. Contrary to previous research showing better performance of A.aegypti in mixed cultures, A.albopictus developed faster and had greater survival when natural food was used.
5. Resistance to starvation was greater in the better performing species (i.e. A.aegypti with non-natural food and A.albopictus with leaf litter). Oxygen consumption by starved larvae was similar in the three container species, and in the ground-water mosquito, A.taeniorhynchus (Wied.), whose resistance to starvation was comparatively very low.     

登革热媒介昆虫抗药性的研究进展

近年来,登革热在全球的流行迅速增长,埃及伊蚊Aedes aegypti(L.)和白纹伊蚊Aedes albopictus(Skuse)是登革热传播的主要媒介。目前,化学防治仍然是防治蚊虫的主要手段,每年化学杀虫剂的大量使用直接或间接地导致其产生了抗药性。关于埃及伊蚊和白纹伊蚊的抗药性研究,主要集中于DDT、有机磷和拟除虫菊酯类杀虫剂导致的抗药性,并已深入到抗药性分子机理的研究。文章就埃及伊蚊和白纹伊蚊对杀虫剂诱导的抗药性现状及抗性机理的研究进展进行综述。     

Wolbachia induces density-dependent inhibition to dengue virus in mosquito cells

Wolbachia is a maternal transmitted endosymbiotic bacterium that is estimated to infect up to 65% of insect species. The ability of Wolbachia to both induce viral interference and spread into mosquito vector population makes it possible to develop Wolbachia as a biological control agent for dengue control. While Wolbachia induces resistance to dengue virus in the transinfected Aedes aegypti mosquitoes, a similar effect was not observed in Aedes albopictus, which naturally carries Wolbachia infection but still serves as a dengue vector. In order to understand the mechanism of this lack of Wolbachia-mediated viral interference, we used both Ae. albopictus cell line (Aa23) and mosquitoes to characterize the impact of Wolbachia on dengue infection. A serial of sub-lethal doses of antibiotic treatment was used to partially remove Wolbachia in Aa23 cells and generate cell cultures with Wolbachia at different densities. We show that there is a strong negative linear correlation between the genome copy of Wolbachia and dengue virus with a dengue infection completely removed when Wolbacha density reaches a certain level. We then compared Wolbachia density between transinfected Ae. aegypti and naturally infected Ae. albopictus. The results show that Wolbachia density in midgut, fatbody and salivary gland of Ae. albopictus is 80-, 18-, and 24-fold less than that of Ae. aegypti, respectively. We provide evidence that Wolbachia density in somatic tissues of Ae. albopictus is too low to induce resistance to dengue virus. Our results will aid in understanding the mechanism of Wolbachia-mediated pathogen interference and developing novel methods to block disease transmission by mosquitoes carrying native Wolbachia infections.     

Dengue virus detection by using reverse transcription-polymerase chain reaction in saliva and progeny of experimentally infected Aedes albopictus from Brazil

Oral susceptibility and vertical transmission of dengue virus type 2 (DENV-2) in an Aedes albopictus sample from Rio de Janeiro was estimated. The infection (36.7%) and transmission (83.3%) rates for Ae. albopictus were higher than those of an Ae. aegypti colony used as control, 32.8 and 60%, respectively. Fourth instar larvae and females descendants of 48.5 and 39.1% of experimentally infected Ae. albopictus showed to harbor the virus. The oral susceptibility and the high capacity to assure vertical transmission exhibited by Ae. albopictus from Brazil reinforce that this species may play a role in the maintenance of the virus in nature and be a threat for dengue control in the country.     

Are Aedes albopictus or other mosquito species from northern Italy competent to sustain new arboviral outbreaks?

The Asian tiger mosquito Aedes albopictus (Skuse) (Diptera: Culicidae), native to Southeast Asia, has extended its geographical distribution to invade new temperate and tropical regions. This species was introduced in 1990 to Italy and has since become the main pest in urban settings. It was incriminated as a principal vector in the first European outbreak of chikungunya virus (CHIKV) in the province of Ravenna (Italy) in 2007. This outbreak was associated with CHIKV E1-226V, efficiently transmitted by Ae. albopictus . The occurrence of this outbreak in a temperate country led us to estimate the potential of Ae. albopictus to transmit CHIKV and dengue virus (DENV), and to determine the susceptibility to CHIKV of other mosquito species collected in northern Italy. Experimental infections showed that Ae. albopictus exhibited high disseminated infection rates for CHIKV (75.0% in Alessandria; 90.3% in San Lazzaro) and low disseminated infection rates for DENV-2 (14.3% in San Lazzaro; 38.5% in Alessandria). Moreover, Ae. albopictus was able to attain a high level of viral replication, with CHIKV detectable in the salivary glands at day 2 after infection. In addition, the other three mosquito species, Anopheles maculipennis Meigen, Aedes vexans vexans (Meigen) and Culex pipiens L., showed variable susceptibilities to infection with CHIKV, of 0%, 7.7% and 0–33%, respectively. This information on vector competence is crucial in assessing the risk for an outbreak of CHIKV or DENV in Italy.     

Proteome of Aedes aegypti larvae in response to infection by the intracellular parasite Vavraia culicis

We report on the modification of the Aedes aegypti larval proteome following infection by the microsporidian parasite Vavraia culicis. Mosquito larvae were sampled at 5 and 15 days of age to compare the effects of infection when the parasite was in two different developmental stages. Modifications of the host proteome due to the stress of infection were distinguished from those of a more general nature by treatments involving hypoxia. We found that the major reaction to stress was the suppression of particular protein spots. Older (15 days) larvae reacted more strongly to infection by V. culicis (46% of the total number of spots affected; 17% for 5 days larvae), while the strongest reaction of younger (5 days) larvae was to hypoxia for pH range 5-8 and to combined effects of infection and hypoxia for pH range 3-6. MALDI-TOF results indicate that proteins induced or suppressed by infection are involved directly or indirectly in defense against microorganisms. Finally, our MALDI-TOF results suggest that A. aegypti larvae try to control or clear V. culicis infection and also that V. culicis probably impairs the immune defense of this host via arginases-NOS competition.     

Distributions of Competing Container Mosquitoes Depend on Detritus Types, Nutrient Ratios, and Food Availability

Coexistence of competitors may result if resources are sufficiently abundant to render competition unimportant, or if species differ in resource requirements. Detritus type has been shown to affect interspecific competitive outcomes between Aedes albopictus (Skuse) and Aedes aegypti (L.) larvae under controlled conditions. We assessed the relationships among spatial distributions of detritus types, nutrients, and aquatic larvae of these species in nature. We collected mosquitoes, water, and detritus from artificial containers across 24 Florida cemeteries that varied in relative abundances of Ae. aegypti and Ae. albopictus.We measured nutrient content of fine particulate organic matter in water samples as total N, P, and C and ratios of these nutrients. We quantified food availability via a bioassay, raising individual Aedes larvae in the laboratory in standard volumes of field-collected, particulate-containing water from each cemetery. Quantities of detritus types collected in standard containers were significant predictors of nutrients and nutrient ratios. Nutrient abundances were significant predictors of relative abundance of Ae. aegypti, and of larval survival and development by both species in the bioassay. Survival and development of larvae reared in particulate-containing water from sites decreased with decreasing relative abundance of Ae. aegypti. These data suggest that N, P, and C availabilities are determined by detritus inputs to containers and that these nutrients in turn determine the feeding environment encountered by larvae, the intensity of interspecific competition among larvae, and subsequent relative abundances of species at sites. Detritus inputs, nutrients, and food availability thus seem to contribute to distributions of Ae. aegypti and Ae. albopictus in cemetery containers throughout Florida.     

Temperature and density-dependent effects of larval environment on Aedes aegypti competence for an alphavirus

Mosquito larvae experience multiple environmental stressors that may modify how subsequent adults interact with pathogens. We evaluated the effect of larval rearing temperature and intraspecific larval competition on adult mosquito immunity and vector competence for Sindbis virus (SINV). Aedes aegypti larvae were reared at two intraspecific densities (150 and 300 larvae) at 20° C and 30° C and the adults were fed artificially on citrated bovine blood containing 10(5) plaque forming units of SINV. Expression of cecropin, defensin, and transferrin was also evaluated in one- and five-day-old female adults. There was a direct relationship between larval density and SINV infection and dissemination rates at low temperature (20° C) and an inverse relationship between larval density and SINV infection rate at high temperature (30° C). Cecropin was only expressed in five-day-old adults that were raised at high temperature as larvae and was 20-fold over-expressed at low compared to high density treatments. Defensin and transferrin were under-expressed in one-day-old adults and over-expressed in five-day-old adults in all competition-temperature combinations relative to low density treatments at 20° C. These findings suggest that interaction between biotic and abiotic conditions of the larval environment may alter adult mosquito immunity resulting in enhanced vector competence for arboviruses.     

First report of the oriental mosquito Aedes albopictus on the West African island of Bioko,Equatorial Guinea

The invasive oriental mosquito Aedes (Stegomyia) albopictus (Skuse) (Diptera: Culicidae) was detected on Bioko Island for the first time in November 2001. It was found to be well established breeding in artificial containers at Planta, near Malabo, the capital of Equatorial Guinea. Associated species of mosquito larvae were Aedes aegypti (L.), Ae. africanus (Theobald), Culex near decens Theobald, Cx. duttoni Theobald, Cx. quinquefasciatus Say, Cx. tigripes De Grandpré & De Charmoy, Eretmapodites quinquevittatus Theobald and Mansonia africana (Theobald). This is the third tropical African country to be invaded by Ae. albopictus, which has recently spread to many parts of the Americas and Europe--with vector competence for dengue, yellow fever and other arboviruses. In the Afrotropical environment, it will be interesting to monitor the ecological balance and/or displacement between introduced Ae. albopictus and indigenous Ae. aegpyti (domestic, peri-domestic and sylvatic populations).     

Dynamics of the "popcorn" Wolbachia infection in outbred Aedes aegypti informs prospects for mosquito vector control

Forty percent of the world's population is at risk of contracting dengue virus, which produces dengue fever with a potentially fatal hemorrhagic form. The wMelPop Wolbachia infection of Drosophila melanogaster reduces life span and interferes with viral transmission when introduced into the mosquito Aedes aegypti, the primary vector of dengue virus. Wolbachia has been proposed as an agent for preventing transmission of dengue virus. Population invasion by Wolbachia depends on levels of cytoplasmic incompatibility, fitness effects, and maternal transmission. Here we characterized these traits in an outbred genetic background of a potential target population of Ae. aegypti using two crossing schemes. Cytoplasmic incompatibility was strong in this background, and the maternal transmission rate of Wolbachia was high. The infection substantially reduced longevity of infected adult females, regardless of whether adults came from larvae cultured under high or low levels of nutrition or density. The infection reduced the viability of diapausing and nondiapausing eggs. Viability was particularly low when eggs were laid by older females and when diapausing eggs had been stored for a few weeks. The infection affected mosquito larval development time and adult body size under different larval nutrition levels and densities. The results were used to assess the potential for wMelPop-CLA to invade natural populations of Ae. aegypti and to develop recommendations for the maintenance of fitness in infected mosquitoes that need to compete against field insects.