Molecular Analysis of the Bloodmeals of Culex spp. Mosquitoes at Natural Habitats in Singapore to Investigate the Potential Risk of Japanese Encephalitis Virus and West Nile Virus Transmission

Japanese encephalitis virus: epidemiology and risk-based surveillance approaches for New Zealand

The introduction and subsequent rapid spread of Japanese encephalitis virus genotype IV across all Australian mainland states and the Northern Territory since late 2021 has increased the risk of an incursion of this mosquito-transmitted zoonotic virus disease into New Zealand, with serious implications for both animal and human health. The potential modes of entry are through introduction of infected mosquitoes as hitchhikers on ships or aircraft, windborne transfer of mosquitoes, or arrival of infected reservoir bird species. A competent vector mosquito, Culex quinquefasciatus, is endemic in New Zealand and other mosquito species may also become involved. If infection becomes established in New Zealand, the scale of transmission may be considerably less than has occurred in Australia because climatic and epidemiological factors are not so favourable. Early evidence of an incursion could come from detection of clinical disease in horses or pigs, or from human cases. Targeted surveillance to confirm or refute indications of an incursion could be undertaken by antibody detection in a number of species. Dogs have been shown to be a particularly valuable sentinel species due to their cohabitation with people and high seroconversion rate. Other novel methods of surveillance could include reverse transcriptase PCR (RT-PCR) on oronasal secretions of pigs. Should evidence of the disease be detected, prompt action would be required to vaccinate at-risk human populations and clarify the epidemiological situation with respect to mammalian hosts and mosquito vector species, including whether a new mosquito species had arrived in the country.Abbreviations: AHL: Animal Health Laboratory; JE: Japanese encephalitis disease; JEV: Japanese encephalitis virus; RT-PCR: Reverse transcriptase PCR.

Blood meal source identification and RNA virome determination in Japanese encephalitis virus vectors collected in Ishikawa Prefecture, Japan, show distinct avian/mammalian host preference

In Asia, Culex mosquitoes are of particular interest because of their role in maintaining endemic mosquito-borne viral diseases, including the Japanese encephalitis virus (JEV). Nonetheless, host-feeding preferences, along with naturally infecting RNA viruses in certain Culex species, remain understudied. In this study, selected blood-fed mosquitoes were processed for avian and mammalian blood meal source identification. Concurrently, cell culture propagation and high-throughput sequencing (HTS) approaches were used to determine the RNA virome of Culex mosquitoes collected in Ishikawa Prefecture, Japan. The identification of blood meal sources from wild-caught Culex spp. revealed that Culex (Culex) tritaeniorhynchus Giles, 1901, has a robust preference toward wild boar (62%, 26/42), followed by heron (21%, 9/42). The other two species, Culex (Oculeomyia) bitaeniorhynchus Giles, 1901, and Culex (Culex) orientalis Edwards, 1921, showed a distinct preference for avian species, including migratory birds. From the HTS results, 34 virus sequences were detected, four of which were newly identified virus sequences of unclassified Aspiviridae, Qinviridae, Iflaviridae, and Picornaviridae. The absence of observable cytopathic effects in mammalian cells and phylogenetic analysis suggested that all identified virus sequences were insect-specific. Further investigations involving other mosquito populations collected in different areas are warranted to explore previously unknown vertebrate hosts that may be linked to JEV dispersal in nature.

Isolation and Genetic Characterization of Japanese Encephalitis Virus Two Decades after Its Elimination in Singapore

Japanese encephalitis virus (JEV) is an important arbovirus in Asia that can cause serious neurological disease. JEV is transmitted by mosquitoes in an enzootic cycle involving porcine and avian reservoirs, in which humans are accidental, dead-end hosts. JEV is currently not endemic in Singapore, after pig farming was abolished in 1992; the last known human case was reported in 2005. However, due to its location along the East-Asian Australasian Flyway (EAAF), Singapore is vulnerable to JEV re-introduction from the endemic regions. Serological and genetic evidence in the last decade suggests JEV's presence in the local fauna. In the present study, we report the genetic characterization and the first isolation of JEV from 3214 mosquito pools consisting of 41,843 Culex mosquitoes, which were trapped from April 2014 to May 2021. The findings demonstrated the presence of genotype I of JEV (n = 10), in contrast to the previous reports of the presence of genotype II of JEV in Singapore. The genetic analyses also suggested that JEV has entered Singapore on several occasions and has potentially established an enzootic cycle in the local fauna. These observations have important implications in the risk assessment and the control of Japanese encephalitis in non-endemic countries, such as Singapore, that are at risk for JEV transmission.

Modelling Japanese encephalitis virus transmission dynamics and human exposure in a Cambodian rural multi-host system

Japanese encephalitis (JE) is a vector-borne zoonosis and the leading cause of human viral encephalitis in Asia. Its transmission cycle is usually described as involving wild birds as reservoirs and pigs as amplifying hosts. JE is endemic in Cambodia, where it circulates in areas with low pig densities (<70 pigs per km²), and could be maintained in a multi-host system composed of pigs, but also poultry as competent hosts, and dogs, cattle and humans as non-competent hosts. We used a mathematical model representing Japanese encephalitis virus (JEV) transmission in a traditional Cambodian village that we calibrated with field data collected in 3 districts of Kandal province, Cambodia. First, R₀ calculations allowed us to assess the capacity of the epidemiological system to be invaded by JEV and sustain virus transmission in villages in the 3 districts, and we predicted human exposure at the epidemiological equilibrium, based on simulations. Changes in spatial density of livestock, in agricultural practices, and epizootics (e.g., African swine fever), can profoundly alter the composition of host communities, which could affect JEV transmission and its impact on human health. In a second step, we then used the model to analyse how host community composition affected R₀ and the predicted human exposure. Lastly, we evaluated the potential use of dog JE seroprevalence as an indicator of human exposure to JEV. In the modeled villages, the calculated R₀ ranged from 1.07 to 1.38. Once the equilibrium reached, predicted annual probability of human exposure ranged from 9% to 47%, and predicted average age at infection was low, between 2 and 11 years old, highlighting the risk of severe forms of JEV infection and the need to intensify child immunization. According to the model, increasing the proportion of competent hosts induced a decrease in age at infection. The simulations also showed that JEV could invade a multi-host system with no pigs, reinforcing the assumption of poultry acting as reservoirs. Finally, the annual human exposure probability appeared linearly correlated with dog seroprevalence, suggesting that in our specific study area, dog seroprevalence would be a good proxy for human exposure.

Japanese encephalitis virus (JEV) is endemic in Cambodia and remains the most common cause of acute viral encephalitis, particularly in children and adolescents. The traditionally described cycle of JEV, involving wild birds as reservoirs, pigs as amplifying hosts and Culex mosquitoes as vectors is questioned, with increasing evidence of a more complex multi-host system involved in areas where densities of pigs are low. In Cambodia, the infection could be maintained in a multi-host system consisting of pigs and poultry as competent hosts, and dogs, cattle and humans as non-competent hosts. We defined a compartmental dynamic model of JEV transmission in a multi-host system representing a rural Cambodian village, to predict human exposure to JEV in the studied area, and to analyse how host community composition may affect human exposure and R₀ value. Our theoretical approach showed that variations of the composition of the multi-host system may have an impact on human exposure to JEV, and thus on the disease burden in humans, especially in young children. Besides children vaccination in JEV endemic areas, a proper evaluation of the impact on human health is needed to target prevention actions and reduce JEV burden in Cambodia.

Japanese encephalitis in Bali, Indonesia: ecological and socio-cultural perspectives

The increasing number of cases of acute encephalitis syndrome, a key presenting clinical sign of Japanese encephalitis infection in humans, along with increasing laboratory confirmed cases in Bali over recent years have led to the Indonesian government developing a national program of vaccination against Japanese encephalitis virus. In order to inform multidisciplinary management, a review was conducted to assess Japanese encephalitis virus-related cases in humans and animals including their determinants and detection in vectors. Along with published literature, key data from local authorized officers in Bali have been used to convey the recent situation of the disease. Related surveys detected up to 92% of the local children had antibodies against the virus with the annual incidence estimated to be 7.1 per 100,000 children. Additionally, reports on young and adult cases of infection within international travellers infected in Bali were documented with both non-fatal and fatal outcomes. Further seroprevalence surveys detected up to 90% with antibodies to the virus in animal reservoirs. The detection of the virus in certain Culex mosquito species and high levels of seropositivity may be associated with greater risk of the virus transmission to the human population. It was also highlighted that local sociocultural practices for agriculture and livestock were potentially associated with the high density of the vector and the reservoirs, which then may lead to the risk of the disease transmission in the ecology of Bali.