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).     

Aedes albopictus, vector of chikungunya and dengue viruses in Reunion Island: biology and control

Chikungunya virus (CHIKV) and dengue virus (DENV) are mosquito-borne viruses transmitted by the Aedes genus. Dengue is considered as the most important arbovirus disease throughout the World. Chikungunya, known from epidemics in continental Africa and Asia, has up to now been poorly studied. It has been recently responsible for the severe 2004-2007 epidemic reported in the Indian Ocean (IO), which has caused several serious health and economic problems. This unprecedented epidemic of the IO has shown severe health troubles with morbidity and death associated, which had never been observed before. The two major vectors of those arboviruses in the IO area are Aedes aegypti and Aedes albopictus. The latest is considered as the main vector in most of the islands of the area, especially in Reunion Island. Ae. albopictus showed strong ecological plasticity. Small disposable containers were the principal urban breeding sites, and preferred natural developmental sites were bamboo stumps and rock holes in peri-urban and gully areas. The virus has been isolated from field collected Ae. albopictus females, and in two out of 500 pools of larvae, demonstrating vertical transmission. Experimental works showed that both Ae. albopictus and Ae. aegypti from west IO islands are efficient vectors of dengue and chikungunya viruses. Since 2006 and all along the epidemic of CHIKV, measures for the control of larvae (temephos then Bacillus thuringiensis) and adults (fenitrothion, then deltamethrine) of Ae. albopictus where applied along with individual and collective actions (by the use of repellents, and removal of breeding sites around houses) in Reunion Island. In order to prevent such epidemics, a preventive plan for arboviruses upsurge is ongoing processed. This plan would allow a quicker response to the threat and adapt it according to the virus and its specific vector.     

Updating the geographical distribution and frequency of Aedes albopictus in Brazil with remarks regarding its range in the Americas

The geographical distribution of Aedes albopictus in Brazil was updated according to the data recorded across the country over the last eight years. Countrywide house indexes (HI) for Ae. albopictus in urban and suburban areas were described for the first time using a sample of Brazilian municipalities. This mosquito is currently present in at least 59% of the Brazilian municipalities and in 24 of the 27 federal units (i.e., 26 states and the Federal District). In 34 Brazilian municipalities, the HI values for Ae. albopictus were higher than those recorded for Ae. aegypti, reaching figures as high as HI = 7.72 in the Southeast Region. Remarks regarding the current range of this mosquito species in the Americas are also presented. Nineteen American countries are currently infested and few mainland American countries have not confirmed the occurrence of Ae. albopictus. The large distribution and high frequency of Ae. albopictus in the Americas may become a critical factor in the spread of arboviruses like chikungunya in the new world.     

Phylogeography of Aedes (Stegomyia) aegypti (L.) and Aedes (Stegomyia) albopictus (Skuse) (Diptera: Culicidae) based on mitochondrial DNA variations

Aedes (Stegomyia) aegypti (l.) and Aedes (Stegomyia) albopictus (Skuse) are the most important vectors of the dengue and yellow-fever viruses. Both took advantage of trade developments to spread throughout the tropics from their native area: A. aegypti originated from Africa and a. albopictus from South-East Asia. We investigated the relationships between A. aegypti and A. albopictus mosquitoes based on three mitochondrial-DNA genes (cytochrome b, cytochrome oxidase I and NADH dehydrogenase subunit 5). Little genetic variation was observed for a. albopictus, probably owing to the recent spreading of the species via human activities. For A. aegypti, most populations from South America were found to be genetically similar to populations from South-East Asia (Thailand and Vietnam), except for one sample from Boa Vista (northern Amazonia), which was more closely related to samples from Africa (Guinea and Ivory Coast). This suggests that African populations of A. aegypti introduced during the slave trade have persisted in Boa Vista, resisting eradication campaigns.     

First finding of Dirofilaria repens in a natural population of Aedes albopictus

The invasive mosquito Aedes albopictus (Skuse) (Diptera: Culicidae) has become widespread in Italy during the past decade. Also Italy has foci of canine filariasis caused by Dirofilaria (Spirurida: Onchocercidae), due to subcutaneous D. repens Railliet & Henry as well as the dog heartworm D. immitis (Leidy) transmitted by various vector mosquitoes (Diptera: Culicidae). In 2002, at Fiumicino, west of Rome (Lazio Region), 17% of dogs were found to have D. repens microfilariae in peripheral blood. To evaluate the role of Ae. albopictus as a vector of Dirofilaria in this area, female mosquitoes were collected daily, June-October 2002, landing on dog or human bait in a rural house at Focene. Mosquitoes were maintained at 27 degrees C and 70% RH for 6 days, to allow development or purging of filaria larvae, then identified and frozen for subsequent molecular assay with filaria-specific ribosomal S2-S16 primers. To distinguish specimens harbouring infective L3 Dirofilaria larvae, DNA was extracted separately from the mosquito abdomen and head-thorax. Dirofilaria species were identified by sequencing, confirmed by polymerase chain reaction of positive specimens using primers specific for D. immitis and D. repens. Dirofilaria DNA was detected in 3/154 (2%) of Ae. albopictus females examined: D. repens DNA in head-thorax and abdomen of one collected 27th July; D. immitis in the abdomen of one collected 24th September; DNA of both D. immitis and D. repens in the head-thorax of one collected 11th October 2002. Thus Ae. albopictus is a potential vector of both Dirofilarias in Italy, representing risks for veterinary and human health.     

Larval development of Aedes aegypti and Aedes albopictus in peri-urban brackish water and its implications for transmission of arboviral diseases

Aedes aegypti (Linnaeus) and Aedes albopictus Skuse mosquitoes transmit serious human arboviral diseases including yellow fever, dengue and chikungunya in many tropical and sub-tropical countries. Females of the two species have adapted to undergo preimaginal development in natural or artificial collections of freshwater near human habitations and feed on human blood. While there is an effective vaccine against yellow fever, the control of dengue and chikungunya is mainly dependent on reducing freshwater preimaginal development habitats of the two vectors. We show here that Ae. aegypti and Ae. albopictus lay eggs and their larvae survive to emerge as adults in brackish water (water with <0.5 ppt or parts per thousand, 0.5-30 ppt and >30 ppt salt are termed fresh, brackish and saline respectively). Brackish water with salinity of 2 to 15 ppt in discarded plastic and glass containers, abandoned fishing boats and unused wells in coastal peri-urban environment were found to contain Ae. aegypti and Ae. albopictus larvae. Relatively high incidence of dengue in Jaffna city, Sri Lanka was observed in the vicinity of brackish water habitats containing Ae. aegypti larvae. These observations raise the possibility that brackish water-adapted Ae. aegypti and Ae. albopictus may play a hitherto unrecognized role in transmitting dengue, chikungunya and yellow fever in coastal urban areas. National and international health authorities therefore need to take the findings into consideration and extend their vector control efforts, which are presently focused on urban freshwater habitats, to include brackish water larval development habitats.     

Multiplication of chikungunya virus in salivary glands of Aedes albopictus (Oahu strain) mosquitoes: an electron microscopic study

Aedes albopictus as well as Aedes aegypti is an important vector of chikungunya and dengue viruses. Electron microscopic observations on the salivary glands of Ae. albopictus infected with chikungunya virus were performed in comparing with those of Ae. aegypti infected with dengue virus. No virus budding from the cell surface of the chikungunya-infected mosquito's salivary glands was found as shown in dengue-infected ones, in contrast to the findings of the mammalian cells such as Vero, KB, IMR, J-111 and BHK-21 cells infected with chikungunya and/or dengue virus(es).     

Importance of Aedes albopictus in veterinary medicine

To assess the role of Aedes albopictus in transmission of filarial nematodes of veterinary importance, researches were carried out in different geographical areas. In Italy a first research was performed to study the susceptibility of Ae. albopictus to Dirofilaria repens, D. immitis and Setaria labiatopapillosa. The development of L3 larvae was longer than in other species of mosquitoes but Ae. albopictus could be a suitable vector of filariae. To understand the role of Ae. albopictus in the natural transmission of Dirofilaria and to assess the risk for animal and human health, in 2000, 2001 and 2002 another study was carried out in the town of Padua. A total of 2,534 Ae. albopictus were caught on human-attracted mosquitoes. Specific primers and sequencing identified filarial DNA as D. immitis; Ae. albopictus was proved a natural vector of D. immitis. Similar results were confirmed in Central Italy also for D. repens. The presence of Ae. albopictus increased the probability of transmission of canine and human dirofilariosis in urban environment and it could change the epidemiology of dirofilariosis, in particular for what concern the time of biting and the risk season. These aspects must be considered to outline a correct prophylaxis.     

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.     

Dispersal of Aedes aegypti and Aedes albopictus (Diptera: Culicidae) in an urban endemic dengue area in the State of Rio de Janeiro,Brazil

Experimental releases of female Aedes (Stegomyia) aegypti and Aedes (Stegomyia) albopictus were performed in August and September 1999, in an urban area of Nova Igua u, State of Rio de Janeiro, Brazil, to estimate their flight range in a circular area of 1,600 m where 1,472 ovitraps were set. Releases of 3,055 Ae. aegypti and 2,225 Ae. albopictus females, fed with rubidium (Rb)-marked blood and surgically prevented from subsequent blood-feeding, were separated by 11 days. Rb was detected in ovitrap-collected eggs by atomic emission spectrophotometry. Rb-marked eggs of both species were detected up to 800 m from the release point. Eggs of Ae. albopictus were more numerous and more heterogeneously distributed in the area than those of Ae. aegypti. Eggs positively marked for Rb were found at all borders of the study area, suggesting that egg laying also occurred beyond these limits. Results from this study suggest that females can fly at least 800 m in 6 days and, if infected, potentially spread virus rapidly.     

The invasion of urban forest by dengue vectors in Rio de Janeiro.

The invasion of a secondary forest within the city of Rio de Janeiro by Aedes aegypti and Ae. albopictus was evaluated from July 1997 to June 1998 through collections of immature stages in ovitraps set at 1 m, 10 m, 100 m, 500 m, and 1,000 m into the forest from houses on the periphery. Both mosquito species were much more abundant close to houses (1-10 m). Aedes aegypti was not collected beyond 100 m, while Ae. albopictus was the most abundant species overall and in ovitraps at all distances from houses. Abundances of Ae. albopictus were significantly correlated with time-lagged rainfall and with abundances of Ae. aegypti. Co-occurrences of Ae. albopictus in traps with Ae. aegypti and Limatus durhami, but not with Culex dolosus, were more likely close to houses. The results suggest that the urban forest is a refuge for both Aedes species, but especially for Ae. albopictus, whose abundance both near houses and in the forest raises concern that this invader may transmit arboviruses to humans that are presently restricted to the sylvan environment.     

Parity and ovarian development of Aedes aegypti and Ae. albopictus (Diptera: Culicidae) in metropolitan Rio de Janeiro.

Vector blood-feeding frequency, parity, and ovarian development are important factors that can influence pathogen transmission. Parity rates of the dengue vectors Aedes aegypti and Ae. albopictus were determined from females collected from August 2002 to July 2004 in metropolitan Rio de Janeiro. A high frequency of parous Ae. aegypti (92.9%, n = 550) and Ae. albopictus (99.1%, n = 320) females suggested high survivorship of both species. A total of 69% of wild-caught Ae. aegypti females had blood in the midgut compared to 19% of Ae. albopictus. For Ae. aegypti, red-colored midgut contents were associated with ovaries in early stages of development, and brown-colored midguts were associated with ovaries in late stages of maturation. Ovaries of Ae. aegypti females without blood in the midgut were most frequently in stages I and V of Christophers.     

Ovitrap surveys of dengue vector mosquitoes in Chiang Mai, northern Thailand: seasonal shifts in relative abundance of Aedes albopictus and Ae. aegypti

Aedes aegypti (L.) and Aedes albopictus (Skuse) were surveyed using ovitraps in residential areas in Chiang Mai, northern Thailand. Egg populations (both species inclusive) remained low in the dry season, but increased/decreased exponentially during the first/latter half of the rainy season, respectively. This seasonal pattern was similar to the seasonal distribution of dengue haemorrhagic fever cases in the area. During the dry season (November-March) Ae.aegypti was dominant in urban and indoor ovitraps. With onset of the rainy season in April, relative abundance of Ae.albopictus increased in rural and outdoor ovitraps. Ae.albopictus displaced Ae.aegypti in the latter half of the rainy season in the rural area. Possible mechanisms to account for this seasonal decline of Ae.aegypti and reciprocal fluctuations in relative abundance of Ae.albopictus are discussed in relation to food availability for larvae in container habitats.     

Distribution of dengue vectors in neighborhoods with different urbanization types of Manaus, state of Amazonas, Brazil

Aedes aegypti and Ae. albopictus are vectors of dengue viruses, which cause endemic disease in the city of Manaus, capital of the state of Amazonas, Brazil. More than 53 thousand cases have been registered in this city since the first epidemic in 1998. We evaluated the hypothesis that different ecological conditions result in different patterns of vector infestation in Manaus, by measuring the infestation level in four neighborhoods with different urbanization patterns, during the rainy (April), dry (August), and transitional (November) seasons. Ae. aegypti predominated throughout the study areas and sampling periods, representing 86% of all specimens collected in oviposition traps. High frequencies of houses positive for both species were observed in all studied sites, with Ae. aegypti present in more than 84% of the houses in all seasons. Ae. albopictus, on the other hand, showed more spatial and temporal variation in abundance. We found no association between infestation level and house traits. This study highlights the homogeneity of dengue vector distribution in Manaus.     

Susceptibility of mosquitoes in central Taiwan to natural infections of Dirofilaria immitis

From October 1997 to September 1998, 3085 Culex quinquefasciatus (Say) (Diptera: Culicidae), 584 Cx. tritaeniorhynchus (Giles) (Diptera: Culicidae), 392 Cx. annulus (Theobald) (Diptera: Culicidae), 374 Aedes albopictus (Skuse) (Diptera: Culicidae) and 102 Armigeres subalbatus (Coquillet) (Diptera: Culicidae) were collected and examined for Dirofilaria immitis (Leidy) (Spirurida: Filariidae) infection. However, only Cx. quinquefasciatus and Ae. albopictus were infected, with a prevalence of 4.28% and 3.74%, respectively. The intensity of D. immitis found in Ae. albopictus (3.43 larvae/mosquito) was higher than that found in Cx. quinquefasciatus (2.89 larvae/mosquito). After being fed with canine blood containing 7500 microfilariae (mf) per mL, Cx. quinquefasciatus ingested approximately two times as many mf as Ae. albopictus (mean of 31.73 in comparison to 16.47). However, almost three times as many third-stage infective larvae developed in Ae. albopictus as in Cx. quinquefasciatus (mean of 3.25 as compared with 1.10), with a vector efficiency index (VEI) of 19.73 and 3.47, respectively. The results showed that Cx. quinquefasciatus and Ae. albopictus served as natural vectors of D. immitis in central Taiwan. Although Ae. albopictus was more efficient for heartworm transmission, Cx. quinquefasciatus may play a more prominent role on the transmission of dirofilariasis in central Taiwan.     

Yellow fever and dengue—the interactions of virus,vector and host in the re-emergence of epidemic disease

The incidence of the mosquito-borne flavivirus diseases, yellow fever, dengue and dengue hemorrhagic fever has increased dramatically in recent years. Both diseases are characterized by the emergence of explosive epidemics. Yellow fever outbreaks appear to have a periodicity dependent upon fluctuations in sylvatic (enzootic) transmission cycles and the ecological factors that influence these cycles. Spread of the virus from the sylvatic cycle to human settlements, ultimately with interhuman transmission by domestic Aedes aegypti is a repeating event in Africa, and presents a renewed threat in the Americas, where effective Ae. aegypti control collapsed in the 1970s. The incidence of dengue has also increased dramatically in recent years, with up to 80 million persons living in tropical regions of the world now affected annually—an attack rate of 4%. The severe form, dengue hemorrhagic fever (DHF) has become a leading health problem throughout Asia in the last 20 years and is emerging as an epidemic disease in the Americas. Unlike yellow fever, sylvatic dengue transmission cycles are not responsible for disease emergence. The major factors underlying dengue epidemics are changes in human ecology, increasing contact with Ae. aegypti, the co-circulation of multiple dengue serotypes, and a rising prevalence of immunity and immunopathological events that underlie the pathogenesis of DHF. In this review, the complex interplay of virus, host, vector, environment and weather in the ecology of yellow fever and dengue are explored.     

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.     

Preliminary data on the performance of Aedes aegypti and Aedes albopictus immatures developing in water-filled tires in Rio de Janeiro

A monthly survey of Aedes aegypti and Aedes albopictus immatures in discarded tires at a site in metropolitan Rio de Janeiro showed that Ae. albopictus was much more abundant in the rainy season, but Ae. aegypti abundance showed a less clear seasonal pattern. Pupal masses for Ae. albopictus showed a seasonal trend. In contrast, Ae. aegypti pupae did not show any clear trend in weight. Large Ae. albopictus pupae were found in the warmer months, when water volume was higher, pH lower and larval abundance lower. Further studies should be carried out to assess how seasonal variations in body size may impact vector competence of these species in Brazil.     

Geographic genetic variation in populations of the dengue virus vector Aedes aegypti

Isoenzyme variation was assessed in 79 mosquito samples of Aedes aegypti, and susceptibility to a dengue 2 virus strain was evaluated in 83 samples. Analysis of FST values, differentiation indexes, and geographic distances separating populations revealed that genetic differences between populations depended on the species' history of migration and colonization. Three major clusters were identified: (1). the sylvan form, Ae. ae. formosus, from West Africa and some islands in the Indian Ocean; (2). the domestic form, Ae. ae. aegypti, from Southeast Asia and South America; and (3). Ae. ae. aegypti populations from the South Pacific islands. Two groups were identified on the basis of susceptibility to the dengue virus: (1). populations with high infection rates, mostly the Ae. ae. aegypti form, and (2). mosquitoes with lower infection rates, specifically Ae. ae. formosus. Other evolutionary and epidemiological implications of the genetic variability of Ae. aegypti are also discussed.