BackgroundZika virus (ZIKV) is a little known arbovirus until it caused a major outbreak in the Pacific Island of Yap in 2007. Although the virus has a wide geographic distribution, most of the known vectors are sylvatic Aedes mosquitoes from Africa where the virus was first isolated. Presently, Ae. aegypti is the only known vector to transmit the virus outside the African continent, though Ae. albopictus has long been a suspected vector. Currently, Ae. albopictus has been shown capable of transmitting more than 20 arboviruses and its notoriety as an important vector came to light during the recent chikungunya pandemic. The vulnerability of Singapore to emerging infectious arboviruses has stimulated our interest to determine the competence of local Ae. albopictus to transmit ZIKV.Methodology/Principal FindingsTo determine the competence of Ae. albopictus to ZIKV, we orally infected local mosquito strains to a Ugandan strain virus. Fully engorged mosquitoes were maintained in an environmental chamber set at 29°C and 80–85%RH. Twelve mosquitoes were then sampled daily from day one to seven and on day 10 and 14 post infection (pi). Zika virus titre in the midgut and salivary glands of each mosquito were determined using tissue culture infectious dose50 assay, while transmissibility of the virus was determined by detecting viral antigen in the mosquito saliva by qRT-PCR. High dissemination and transmission rate of ZIKV were observed. By day 7-pi, all mosquitoes have disseminated infection and 73% of these mosquitoes have ZIKV in their saliva. By day 10-pi, all mosquitoes were potentially infectious.Conclusions/SignificanceThe study highlighted the potential of Ae. albopictus to transmit ZIKV and the possibility that the virus could be established locally. Nonetheless, the threat of ZIKV can be mitigated by existing dengue and chikungunya control program being implemented in Singapore.

BackgroundZika virus (ZIKV) is a little known flavivirus that caused a major outbreak in 2007, in the South-western Pacific Island of Yap. It causes dengue-like syndromes but with milder symptoms. In Africa, where it was first isolated, ZIKV is mainly transmitted by sylvatic Aedes mosquitoes. The virus has also been isolated from Ae. aegypti and it is considered to be the vector involved in the urban transmission of the virus. Transmission of the virus by an African strain of Ae. aegypti has also been demonstrated under laboratory conditions. The aim of the present study is to describe the oral susceptibility of a Singapore strain of Ae. aegypti to ZIKV, under conditions that simulate local climate.Methodology/Principal FindingsTo assess the receptivity of Singapore's Ae. aegypti to the virus, we orally exposed a local mosquito strain to a Ugandan strain of ZIKV. Upon exposure, fully engorged mosquitoes were maintained in an environmental chamber set at 29°C and 70–75% RH. Eight mosquitoes were then sampled daily from day 1 to day 7, and subsequently on days 10 and 14 post exposure (pe). The virus titer of the midgut and salivary glands of each mosquito were determined using a tissue culture infectious dose50 (TCID50) assay. High midgut infection and salivary gland dissemination rates were observed. By day 5 after the infectious blood meal, ZIKV was found in the salivary glands of more than half of the mosquitoes tested (62%); and by day 10, all mosquitoes were potentially infective.Conclusions/SignificanceThis study showed that Singapore's urban Ae. aegypti are susceptible and are potentially capable of transmitting ZIKV. The virus could be established in Singapore should it be introduced. Nevertheless, Singapore's current dengue control strategy is applicable to control ZIKV.

In August 2008, a team from the National Environmental Agency conducted an entomological investigation of a chikungunya cluster in Singapore, with the primary aim of identifying the vector responsible for the outbreak and to assess the vector control operation. A total of 173 adult mosquitoes were caught using both the sweep-net method and the BG Sentinel Traps in and around the affected workers' quarters. Of these, 120 (69.4%) were Aedes albopictus and the rest were Culex quinquefasciatus. More than 2700 Ae. albopictus larvae were also collected from 33 breeding habitats detected. No Aedes aegypti was found. During the preintervention period, 6 (8.4%) out of 71 adult female Ae. albopictus were found positive for the chikungunya virus (CHIKV). Vector control measures resulted in a 90% reduction of adult Ae. albopictus caught by BG Sentinel Traps. Postintervention surveillance revealed the presence of CHIKV-positive mosquitoes. These findings led to continued intensive vector control operation in the affected area that further reduced vector population and interrupted the transmission of the disease. The E1 gene sequence of the CHIKV was identical to those of CHIKV isolated from human chikungunya cases working in the affected area, and contained the A226V mutation. The incrimination of Ae. albopictus as a major vector involved in the transmission of A226V CHIKV had led to the revision of chikungunya control strategy in Singapore. This study suggests the benefit of a vector control program that includes the evaluation of control measures in conjunction to virological surveillance in vector population.

We report a case of a Singaporean who acquired Zika virus (ZIKV) during a visit to Cuba. The infection was confirmed using molecular and serological methods. This report highlights potential drawbacks of using IgG serology for diagnosis of flavivirus infections in endemic regions. The low viremia detected during the early phase of this case resulted in low mosquito infectivity rates, suggesting the possibility of ZIKV transmission prior to clinical onset. The report also emphasizes the challenges of public health interventions for Zika fever and the importance of sustaining a low vector population to reduce the risk of arbovirus transmission in vulnerable regions.

Shifting of virus serotypes and clade replacement events are known to drive dengue epidemics. However, only a few studies have attempted to elucidate the virus attributes that contribute to such epidemics. In 2007, Singapore experienced a dengue outbreak affecting more than 8000 individuals. The outbreak ensued with the shuffling of dominant clades (from clade I to clade II) of Dengue virus 2 (DENV-2) cosmopolitan genotype, at a time when the Aedes premise index was significantly low. Therefore, we hypothesized that clade II had higher epidemic potential and fitness than clade I. To test this hypothesis, we tested the replication and apoptotic qualities of clade I and II isolates in mammalian cells and their ability to infect and disseminate in a field strain of Ae. Aegypti. Our findings indicated that clade II replicated more efficiently in mammalian cells than clade I and possessed higher transmission potential in local vectors. This could collectively improve the epidemic potential of clade II, which dominated during the outbreak in 2007. The findings exemplify complex interactions between the emergence, adaptation and transmission potential of DENV, and testify the epidemiological importance of a deeper understanding of virus and vector dynamics in endemic regions.