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.     

High susceptibility to Chikungunya virus of Aedes aegypti from the French West Indies and French Guiana

Objectives To estimate the vector competence of Aedes aegypti populations sampled from distinct anthropogenic environments in French Guiana, Guadeloupe and Martinique for the strain CHIKV 06.21. Methods F₁/F₂ females were orally infected at titres of 10⁶ and 10^7.5 pfu/ml in blood‐meals. Disseminated infection rates (DIR) of mosquitoes were estimated using indirect fluorescent antibody assay on heads’ squashes, 7 or 14 days post‐infection (pi). Results At a titre of 10^7.5 pfu/ml, DIR ranged from 88.9% to 100.0% and were not significantly different whether assessed at day 7 or 14 pi. At a titre of 10⁶ pfu/ml, DIR observed 7 days pi ranged from 37.6 to 62.0%. Conclusions Ae. aegypti from French Guiana and French West Indies are highly competent to transmit CHIKV. An evaluation of DIR 7 days rather than 14 days pi is adequate to estimate vector competence. The titre of 10⁶ pfu/ml allows us to distinguish Ae. aegypti populations originating from distinct environments (dense or diffuse housing) by their vector competence. This assessment is a prerequisite to better evaluate the potential risk of Chikungunya outbreaks once the virus is introduced from endemic regions.     

Detection of Persistent Chikungunya Virus RNA but not Infectious Virus in Experimental Vertical Transmission in Aedes aegypti from Malaysia

Vertical transmission may contribute to the maintenance of arthropod-borne viruses, but its existence in chikungunya virus (CHIKV) is unclear. Experimental vertical transmission of infectious clones of CHIKV in Aedes aegypti mosquitoes from Malaysia was investigated. Eggs and adult progeny from the second gonotrophic cycles of infected parental mosquitoes were tested. Using polymerase chain reaction (PCR), 56.3% of pooled eggs and 10% of adult progeny had detectable CHIKV RNA, but no samples had detectable infectious virus by plaque assay. Transfected CHIKV RNA from PCR-positive eggs did not yield infectious virus in BHK-21 cells. Thus, vertical transmission of viable CHIKV was not demonstrated. Noninfectious CHIKV RNA persists in eggs and progeny of infected Ae. aegypti, but the mechanism and significance are unknown. There is insufficient evidence to conclude that vertical transmission exists in CHIKV, as positive results reported in previous studies were almost exclusively based only on viral RNA detection.     

Abundant Aedes (Stegomyia) aegypti aegypti mosquitoes in the 2014 dengue outbreak area of Mozambique

In early 2014, dengue cases were reported from northern Mozambique, 30 years after the last outbreak. We identified potential dengue vector species in three northern towns, Pemba, Nampula and Nacala, and one southern town, Maputo, during the outbreak in April 2014. A major dengue vector species, Aedes (Stegomyia) aegypti, was found in all these towns. The dominant vector subspecies in the northern towns was Aedes aegypti aegypti, while Ae. aegypti formosus was dominant in Maputo. Considering the high proportion of Ae. aegypti aegypti and its high vector competence, the findings from this study suggest that Ae. aegypti aegypti was responsible for the outbreak in northern Mozambique.     

Experimental transmission of Mayaro virus by Aedes aegypti

Outbreaks of Mayaro fever have been associated with a sylvatic cycle of Mayaro virus (MAYV) transmission in South America. To evaluate the potential for a common urban mosquito to transmit MAYV, laboratory vector competence studies were performed with Aedes aegypti from Iquitos, Peru. Oral infection in Ae. aegypti ranged from 0% (0/31) to 84% (31/37), with blood meal virus titers between 3.4 log(10) and 7.3 log(10) plaque-forming units (PFU)/mL. Transmission of MAYV by 70% (21/30) of infected mosquitoes was shown by saliva collection and exposure to suckling mice. Amount of viral RNA in febrile humans, determined by real-time polymerase chain reaction, ranged from 2.7 to 5.3 log(10) PFU equivalents/mL. Oral susceptibility of Ae. aegypti to MAYV at titers encountered in viremic humans may limit opportunities to initiate an urban cycle; however, transmission of MAYV by Ae. aegypti shows the vector competence of this species and suggests potential for urban transmission.     

Evaluation of Aedes aegypti,Aedes albopictus,and Culex quinquefasciatus Mosquitoes Competence to Oropouche virus Infection

The emergence of new human viral pathogens and re-emergence of several diseases are of particular concern in the last decades. Oropouche orthobunyavirus (OROV) is an arbovirus endemic to South and Central America tropical regions, responsible to several epidemic events in the last decades. There is little information regarding the ability of OROV to be transmitted by urban/peri-urban mosquitoes, which has limited the predictability of the emergence of permanent urban transmission cycles. Here, we evaluated the ability of OROV to infect, replicate, and be transmitted by three anthropophilic and urban species of mosquitoes, Aedes aegypti, Aedes albopictus, and Culex quinquefasciatus. We show that OROV is able to infect and efficiently replicate when systemically injected in all three species tested, but not when orally ingested. Moreover, we find that, once OROV replication has occurred in the mosquito body, all three species were able to transmit the virus to immunocompromised mice during blood feeding. These data provide evidence that OROV is restricted by the midgut barrier of three major urban mosquito species, but, if this restriction is overcome, could be efficiently transmitted to vertebrate hosts. This poses a great risk for the emergence of permanent urban cycles and geographic expansion of OROV to other continents.     

Growth characteristics of ChimeriVax-DEN2 vaccine virus in Aedes aegypti and Aedes albopictus mosquitoes

The chimeric yellow fever (YF) 17D-dengue type 2 (ChimeriVax-DEN2) vaccine virus developed by Acambis, Inc. (Cambridge, MA) contains the prM and E genes of wild-type (wt) dengue 2 (DEN-2) (strain PUO-218) virus in the YF vaccine virus (strain 17D) backbone. The potential of ChimeriVax-DEN2 virus to infect and be transmitted by Aedes aegypti, the principal DEN and YF virus mosquito vector, and Aedes albopictus, a species that occurs in areas of active transmission of YF and DEN viruses, was evaluated. Mosquitoes were intrathoracically (IT) inoculated with virus or were fed a virus-laden blood meal, and the replication kinetics of ChimeriVax-DEN2 were compared with the wt DEN-2 and YF 17D vaccine viruses. Replication of YF 17D virus is attenuated in cultured Ae. albopictus C6/36 mosquito cells and in Ae. aegypti and Ae. albopictus mosquitoes. Growth of ChimeriVax-DEN2 virus similarly was restricted in C6/36 cells and in mosquitoes. ChimeriVax-DEN2 replicated in 56% of IT inoculated Ae. aegypti, and virus disseminated to head tissue in 36%, with a mean viral titer of 1.8 log10 PFU/mosquito. Of mosquitoes, 16% of Ae. aegypti and 24% of Ae. albopictus were infected 14 days after a blood meal containing ChimeriVax-DEN2, but virus did not disseminate to head tissue. In contrast, DEN-2 replicated in all IT inoculated and orally infected Ae. aegypti (mean titer 5.5 log10 PFU/mosquito), and virus disseminated to head tissue in 95%. Of Ae. albopictus, 84% were infected after a blood meal containing DEN-2 virus; dissemination occurred in 36%. Replication of ChimeriVax-DEN2 virus in mosquitoes corresponded to that of YF 17D vaccine virus, which is restricted in its ability to infect and replicate in mosquitoes. Therefore, transmission of ChimeriVax-DEN2 virus by vector mosquitoes is unlikely.     

Transmission potential of two chimeric Chikungunya vaccine candidates in the urban mosquito vectors, Aedes aegypti and Ae. albopictus

Chikungunya virus (CHIKV) is an emerging, mosquito-borne alphavirus that has caused major epidemics in Africa and Asia. We developed chimeric vaccine candidates using the non-structural protein genes of either Venezuelan equine encephalitis virus (VEEV) attenuated vaccine strain TC-83 or a naturally attenuated strain of eastern equine encephalitis virus (EEEV) and the structural genes of CHIKV. Because the transmission of genetically modified live vaccine strains is undesirable because of the potentially unpredictable evolution of these viruses as well as the potential for reversion, we evaluated the ability of these vaccines to infect the urban CHIKV vectors, Aedes aegypti and Ae. albopictus. Both vaccine candidates exhibited significantly lower infection and dissemination rates compared with the parent alphaviruses. Intrathoracic inoculations indicated that reduced infectivity was mediated by midgut infection barriers in both species. These results indicate a low potential for transmission of these vaccine strains in the event that a vaccinee became viremic.     

Aedes aegypti in French Guiana: susceptibility to a dengue virus

Twenty-seven samples of Aedes aegypti (F1 generation) from French Guiana were tested for their susceptibility to dengue serotype 2 virus. Very high infection rates were observed by indirect fluorescent antibody (IFA) test. Ae. aegypti samples were pooled according to two groups: the first group (N=10) represented mosquitoes from the urbanized area of Cayenne and surroundings, and the second group (N=17) corresponded to mosquitoes collected in the countryside. Infection rates were found to be similar in these two cases. These findings are discussed in relation with the history of Ae. aegypti in this part of the world.     

Larval environmental stress alters Aedes aegypti competence for Sindbis virus

Objective To evaluate how stress at the larval stage alters adult mosquito performance and susceptibility to viral infection. Methods We used a model system consisting of Sindbis virus (SINV) and the yellow fever mosquito Aedes aegypti. Larvae were either reared under optimal conditions (control) or exposed to one of four types of stressors; suboptimal nutrients, starvation, elevated temperature, and a low dose of the insecticide malathion and adult females were fed SINV infectious blood meal. Differential expressions of stress, immune‐specific and detoxification genes was measured in fourth instar larvae (HSP70, HSP83, cecropin, defensin, transferrin and CYP6Z6) and 3‐day‐old females (cecropin, defensin, transferrin) to identify plausible molecular mechanisms associated with mosquito response to stress. Results There were stress‐specific variations in mosquito performance (survival, development time, female size), but all stressors had a consistent effect of significantly increasing susceptibility to viral infection and dissemination relative to the controls. Three genes were up‐regulated in fourth instar larvae exposed to temperature stress (cecropin, defensin and CYP6Z6) compared to single genes in suboptimal nutrient (cecropin) and malathion (transferrin) stress treatments and down‐regulation of all the six genes in starvation treatments. In adult samples, transferrin was up‐regulated in all but starvation treatments while defensin was up‐regulated in starvation and temperature stress treatments. Conclusions Stress during larval development may cause alterations in adult mosquito phenotype and immunity that can increase their susceptibility to pathogens.