Australian vertebrate hosts of Japanese encephalitis virus; a review of the evidence

Japanese Encephalitis Virus (JEV) transmission in temperate Australia has underscored a critical need to characterise transmission pathways and identify probable hosts of infection within the country. This systematic review consolidates existing research on the vertebrate hosts of JEV that are known to exist in Australia. Specifically, we aim to identify probable species for JEV transmission, their potential role as either a spillover or maintenance host and identify critical knowledge gaps. Data were extracted from studies involving experimental infection, seroprevalence, and virus isolation and were available for 22 vertebrate species known to reside in Australia. A host competence score was calculated to assess the potential for a given species to infect JEV vectors and to quantity their possible role in JEV transmission. Based on the host competence score and ecology of each species, we find ardeid birds, feral pigs, and flying foxes have potential as maintenance hosts for JEV in the Australian context. We also note that brushtail possums and domestic pigs have potential as spillover hosts under certain outbreak conditions. However, evidence to confirm these roles in localized transmission or outbreaks is sparse, emphasizing the need for further targeted research. This review provides a foundation for future investigations into JEV transmission in Australia, advocating for enhanced surveillance and standardized research methodologies to better understand and mitigate the virus's impact.

Extensive public health initiatives drive the elimination of Aedes aegypti (Diptera, Culicidae) from a town in regional Queensland: A case study from Gin Gin, Australia

Aedes aegypti is the primary vector of exotic arboviruses (dengue, chikungunya and Zika) in Australia. Once established across much of Australia, this mosquito species remains prevalent in central and northern Queensland. In 2011, Ae. aegypti was re-discovered in the town of Gin Gin, Queensland, by health authorities during routine larval surveillance. This town is situated on a major highway that provides a distribution pathway into the highly vulnerable and populous region of the state where the species was once common. Following the detection, larval habitat and adult control activities were conducted as a public health intervention to eliminate the Ae. aegypti population and reduce the risk of exotic disease transmission. Importantly, genetic analysis revealed a homogenous cluster and small effective population vulnerable to an elimination strategy. By 2015, adult surveillance revealed the population had expanded throughout the centre of the town. In response, a collaboration between research agencies and local stakeholders activated a second control program in 2016 that included extensive community engagement, enhanced entomologic surveillance and vector control activities including the targeting of key containers, such as unsealed rainwater tanks. Here we describe a model of the public health intervention which successfully reduced the Ae. aegypti population below detection thresholds, using source reduction, insecticides and novel, intensive genetic surveillance methods. This outcome has important implications for future elimination work in small towns in regions sub-optimal for Ae. aegypti presence and reinforces the longstanding benefits of a partnership model for public health-based interventions for invasive urban mosquito species.

Simulating an invasion: unsealed water storage (rainwater tanks) and urban block design facilitate the spread of the dengue fever mosquito, Aedes aegypti, in Brisbane, Australia

Aedes aegypti (Linnaeus) was once highly prevalent across eastern Australia, resulting in epidemics of dengue fever. Drought conditions have led to a rapid rise in semi-permanent, urban water storage containers called rainwater tanks known to be critical larval habitat for the species. The presence of these larval habitats has increased the risk of establishment of highly urbanised, invasive mosquito vectors such as Ae. aegypti. Here we use a spatially explicit network model to examine the role that unsealed rainwater tanks may play in population connectivity of an Ae. aegypti invasion in suburbs of Brisbane, a major Australian city. We characterise movement between rainwater tanks as a diffusion-like process, limited by a maximum distance of movement, average life expectancy, and a probability that Ae. aegypti will cross wide open spaces such as roads. The simulation model was run against a number of scenarios that examined population spread through the rainwater tank network based on non-compliance rates of tanks (unsealed or sealed) and road grids. We show that Ae. aegypti tank infestation and population spread was greatest in areas of high tank density and road lengths were shortest e.g. cul-de-sacs. Rainwater tank non-compliance rates of over 30% show increased connectivity when compared to less than 10%, suggesting rainwater tanks non-compliance should be maintained under this level to minimize the spread of an invading Ae. aegypti population. These results presented as risk maps of Ae. aegypti spread across Brisbane, can assist health and government authorities on where to optimally target rainwater tank surveillance and educational activities.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10530-021-02619-z.

Mark-release-recapture of male Aedes aegypti (Diptera: Culicidae): Use of rhodamine B to estimate movement, mating and population parameters in preparation for an incompatible male program

Rapid advances in biological and digital support systems are revolutionizing the population control of invasive disease vectors such as Aedes aegypti. Methods such as the sterile and incompatible insect techniques (SIT/IIT) rely on modified males to seek out and successfully mate with females, and in doing so outcompete the wild male population for mates. Currently, these interventions most frequently infer mating success through area-wide population surveillance and estimates of mating competitiveness are rare. Furthermore, little is known about male Ae. aegypti behaviour and biology in field settings. In preparation for a large, community scale IIT program, we undertook a series of mark- release-recapture experiments using rhodamine B to mark male Ae. aegypti sperm and measure mating interactions with females. We also developed a Spatial and Temporally Evolving Isotropic Kernel (STEIK) framework to assist researchers to estimate the movement of individuals through space and time. Results showed that ~40% of wild females captured daily were unmated, suggesting interventions will need to release males multiple times per week to be effective at suppressing Ae. aegypti populations. Males moved rapidly through the landscape, particularly when released during the night. Although males moved further than what is typically observed in females of the species, survival was considerably lower. These unique insights improve our understanding of mating interactions in wild Ae. aegypti populations and lay the foundation for robust suppression strategies in the future.

Incompatible insect techniques for controlling populations of the dengue vector, Aedes aegypti, utilize the mating biology of adult male mosquitoes to achieve suppression through a sterilization process. As the study of Ae. aegypti control has typically focused on adult female mosquitoes, knowledge on male movement, survival and mating interactions in the field is lacking. Here we undertook several mark-release-recapture experiments on adult male Ae. aegypti in Innisfail, Australia, and measured important biological parameters. For the first time in large field experiments, we employed rhodamine B as a marker that when fed to adult males, identified both marked males and the wild females they mated with. We observed males moving further through the landscape, but surviving for a shorter period, than previous measurements undertaken on females in a field setting. A high proportion (~40%) of unmated females suggests individuals are constantly available for mating. As such, sterile male strategies may need to release at regular intervals to achieve effective population suppression. The unique insights provided by this study will assist in designing future sterile male field interventions.

Modelling the Wolbachia incompatible insect technique: strategies for effective mosquito population elimination

BACKGROUND

The Wolbachia incompatible insect technique (IIT) shows promise as a method for eliminating populations of invasive mosquitoes such as Aedes aegypti (Linnaeus) (Diptera: Culicidae) and reducing the incidence of vector-borne diseases such as dengue, chikungunya and Zika. Successful implementation of this biological control strategy relies on high-fidelity separation of male from female insects in mass production systems for inundative release into landscapes. Processes for sex-separating mosquitoes are typically error-prone and laborious, and IIT programmes run the risk of releasing Wolbachia-infected females and replacing wild mosquito populations.

RESULTS

We introduce a simple Markov population process model for studying mosquito populations subjected to a Wolbachia-IIT programme which exhibit an unstable equilibrium threshold. The model is used to study, in silico, scenarios that are likely to yield a successful elimination result. Our results suggest that elimination is best achieved by releasing males at rates that adapt to the ever-decreasing wild population, thus reducing the risk of releasing Wolbachia-infected females while reducing costs.

CONCLUSIONS

While very high-fidelity sex separation is required to avoid establishment, release programmes tend to be robust to the release of a small number of Wolbachia-infected females. These findings will inform and enhance the next generation of Wolbachia-IIT population control strategies that are already showing great promise in field trials.

Urban Landscape Features Influence the Movement and Distribution of the Australian Container-Inhabiting Mosquito Vectors Aedes aegypti (Diptera: Culicidae) and Aedes notoscriptus (Diptera: Culicidae)

Urban landscape features play an important role in the distribution and population spread of mosquito vectors. Furthermore, current insecticide and novel rear-and-release strategies for urban mosquito management rarely consider the spatial structure of the landscape when applying control practices. Here, we undertake a mark-recapture experiment to examine how urban features influence the movement and distribution of Australian container-inhabiting Aedes vectors. We pay attention to the role of semipermanent water storage containers, called rainwater tanks, and the influence of movement barriers, such as roads, on the spread and distribution of vector populations. Results suggest that Aedes aegypti (Linnaeus) (Diptera: Culicidae) were more likely to be captured around rainwater tanks, and that released males travel throughout residential blocks but do not cross roads. Conversely, female Aedes notoscriptus (Skuse) (Diptera: Culicidae) movement was uninhibited by roads and rainwater tanks did not influence female distribution or oviposition behavior. Using an isotropic Gaussian kernel framework, we show that vector movement is likely to be greater when applying a temporal effect, than when estimated by traditional methods. We conclude that a greater understanding on the role of urban features on vector movement will be important in the new age of rear-and-release mosquito control strategies, particularly those where estimations of movement are important for ensuring efficacy of application.

Life on the margin: Rainwater tanks facilitate overwintering of the dengue vector, Aedes aegypti, in a sub-tropical climate

A key determinant of insect persistence in marginal habitats is the ability to tolerate environmental extremes such as temperature. Aedes aegypti is highly invasive and little is known about the physiological sensitivity of the species to fluctuating temperature regimes at the lower critical threshold for development. A temperature that may limit the establishment and persistence of the species in sub-optimal regions. Daily winter temperatures were measured in common Australian larval habitats, replicated in environmental chambers and used to investigate the effect of fluctuating temperatures on the development and survival of tropical and subtropical strains of Australian Ae. aegypti. Development was slow for all treatments but both strains were able to complete development to the adult stage, suggesting previous models underestimate the potential for the species to persist in eastern Australia. Results suggested that thermal buffering in large volume habitats, and water that persists for greater than 32 days, will facilitate completion of the life cycle during sub-tropical winters. Furthermore, we provide a non-linear estimate of the lower critical temperature for Ae. aegypti development that suggests the current threshold may be incorrect. Our study demonstrates that the current re-introduction of water storage containers such as rainwater tanks, into major Australian population centres will increase the risk of Ae. aegypti establishment by permitting year-round development in locations south of its current distribution.

The elimination of the dengue vector, Aedes aegypti, from Brisbane, Australia: The role of surveillance, larval habitat removal and policy

Aedes aegypti (L.) (Diptera: Culicidae) is a highly invasive mosquito whose global distribution has fluctuated dramatically over the last 100 years. In Australia the distribution of Ae. aegypti once spanned the eastern seaboard, for 3,000 km north to south. However, during the 1900s this distribution markedly reduced and the mosquito disappeared from its southern range. Numerous hypotheses have been proffered for this retraction, however quantitative evidence of the mechanisms driving the disappearance are lacking. We examine historical records during the period when Ae. aegypti disappeared from Brisbane, the largest population centre in Queensland, Australia. In particular, we focus on the targeted management of Ae. aegypti by government authorities, that led to local elimination, something rarely observed in large cities. Numerous factors are likely to be responsible including the removal of larval habitat, especially domestic rainwater tanks, in combination with increased mosquito surveillance and regulatory enforcement. This account of historical events as they pertain to the elimination of Ae. aegypti from Brisbane, will inform assessments of the risks posed by recent human responses to climate change and the reintroduction of 300,000 rainwater tanks into the State over the past decade.

We examined the historical role that water storage practices and the enforcement of anti-mosquito regulations played in the elimination of Aedes aegypti from Brisbane, a major urban centre in Australia. We examined changes in regulations pertaining to mosquitoes, collected government records documenting surveillance, and the response by the community to the actions of local authorities. Our findings indicate that anti-mosquito regulations, underpinned by effective implementation, were successful in gaining community support and removing the risk of mosquito presence at non-compliant properties. In particular, we argue that the removal of rainwater tanks which provided a permanent larval habitat in otherwise suboptimal environments, played a major role in the elimination of the species from Brisbane. Public Health regulations were supported by a large surveillance effort by local government health officers that were empowered to enforce legislation where necessary. Our findings are of importance to health authorities managing the ongoing expansion of Aedes populations, particularly in regions of sub-optimal climate and where water storage has become a major concern.

Adult survivorship of the dengue mosquito Aedes aegypti varies seasonally in central Vietnam

The survival characteristics of the mosquito Aedes aegypti affect transmission rates of dengue because transmission requires infected mosquitoes to survive long enough for the virus to infect the salivary glands. Mosquito survival is assumed to be high in tropical, dengue endemic, countries like Vietnam. However, the survival rates of wild populations of mosquitoes are seldom measured due the difficulty of predicting mosquito age. Hon Mieu Island in central Vietnam is the site of a pilot release of Ae. aegypti infected with a strain of Wolbachia pipientis bacteria (wMelPop) that induces virus interference and mosquito life-shortening. We used the most accurate mosquito age grading approach, transcriptional profiling, to establish the survival patterns of the mosquito population from the population age structure. Furthermore, estimations were validated on mosquitoes released into a large semi-field environment consisting of an enclosed house, garden and yard to incorporate natural environmental variability. Mosquito survival was highest during the dry/cool (January-April) and dry/hot (May-August) seasons, when 92 and 64% of Hon Mieu mosquitoes had survived to an age that they were able to transmit dengue (12 d), respectively. This was reduced to 29% during the wet/cool season from September to December. The presence of Ae. aegypti older than 12 d during each season is likely to facilitate the observed continuity of dengue transmission in the region. We provide season specific Ae. aegypti survival models for improved dengue epidemiology and evaluation of mosquito control strategies that aim to reduce mosquito survival to break the dengue transmission cycle.

Increased container-breeding mosquito risk owing to drought-induced changes in water harvesting and storage in Brisbane, Australia

BACKGROUND

Extended drought conditions in south-east Queensland during the early 2000s have resulted in a culture of water harvesting and legislated water restrictions. Aedes notoscriptus is a container-breeding mosquito vector of Ross River and Barmah Forest viruses.

METHODS

From 2008-2009, the larval habitats and seasonal abundance of domestic container-breeding mosquitoes were recorded from three suburbs of Brisbane. A knowledge, attitudes and practice questionnaire was administered to householders. A low-cost, desktop methodology was used to predict the proportion of shaded premises compared with front-of-property estimates.

RESULTS

We highlight changes in the frequency of container categories for A. notoscriptus as a response to human behavioural changes to drought. Garden accoutrements, discarded household items and water storage containers accounted for 66.2% (525/793) of positive containers and 77.5% (73 441/94 731) of all immature mosquitoes. Of all household premises surveyed, 52.6% (550/1046) contained rainwater tanks and 29.4% (308/1046) harvested water in other containers, contrasting with a previous 1995 survey where neither category was observed. Both Premise Condition Index and shade directly correlated with positive premises.

CONCLUSIONS

Human response to drought has resulted in new habitats for domestic container-breeding mosquitoes. This recent trend of prolific water storage is similar to earlier years (1904-1943) in Brisbane when Aedes aegypti was present and dengue epidemics occurred.