Vector competence of Aedes aegypti and Ae. albopictus from Singapore for dengue and other arboviruses

Globally, vector-borne diseases are the leading cause of morbidity and mortality, with dengue considered to be the most important mosquito-borne diseases affecting man. Singapore is hyperendemic to dengue with all serotypes co-circulating in different proportions. Despite having a comprehensive vector surveillance and control programme, the country is not spared from the resurgence of dengue. During the past 10 years, the country has experienced three major dengue epidemics, with the latest epidemic in 2013-14 considered to be the worst. Interestingly, these outbreaks occurred while the Aedes premise index was consistently kept at a low level of 1% or less. It was hypothesized that the aggressive vector control measures in Singapore has led to a very low mosquito population that could have selected for the circulation of a virus lineage with better fitness or population of vectors that are highly competent to transmit DENV. To elucidate this, vector competence studies were carried out to determine the susceptibility of local Aedes aegypti and Ae. albopictus to different dengue serotype 2 viral lineages. Results from these experiments seemed to be in agreement to both hypotheses. Although, both mosquitoes were highly susceptible to the virus, Ae. aegypti was found to have higher competence to transmit dengue 2. This further reinforces the role of Ae. aegypti as the key driver of dengue epidemics in Singapore. At present, there is neither a licensed vaccine nor a specific treatment for dengue, thus, control and prevention of the disease is still reliant in controlling vector population. The increase in human population, continued urbanization, coupled with human migration from dengue endemic countries and the worsening global dengue situation will make the future control of dengue in Singapore more challenging. In this study, Ae. aegypti and to a lesser extent, Ae. albopictus were also found to be highly susceptible to Zika and Ross River viruses. Introduction and establishment of exotic arboviruses in Singapore is highly plausible, especially with a population that is immunologically naïve to these pathogens. The first chikungunya outbreak in 2008 attests to these possibilities. Thus, there is an urgent need to enhance current vector surveillance and control programmes with alternative entomological approaches. The use of Wolbachia for the control and prevention of dengue and other vector-borne diseases can be a promising alternative control tool that can be implemented in Singapore. Although, the use of Wolbachia induced cytoplasmic incompatibility for suppression of Ae. aegypti population seemed to be the logical approach as it aligned with current vector control method being implemented. Tapping on the pathogen blocking ability induced by Wolbachia in Ae. aegypti should also be explored. Results from the current study showed that Wolbachia is able to reduce the ability of local Ae. aegypti in transmitting dengue to a level where it may have a significant impact on dengue transmission, as deduced from a recent Wolbachia transmission model study. In addition, this blocking ability is extended to chikungunya and zika viruses. This study also shows, for the first time, the ability of wMel to block Zika virus