Physiology and ecology combine to determine host and vector importance for Ross River virus

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 the virus. 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 involved in JEV transmission, their potential role as hosts 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 ability of each species to generate and sustain a viraemia. Based on the host competence score and ecology of each species, we find that ardeid birds, feral pigs and flying foxes have potential as maintenance hosts for JEV in the Australian context. We also note that domestic pigs are frequently infected during outbreaks, but their role as amplification hosts in Australia is unclear. Evidence to confirm these roles is sparse, emphasising the need for further targeted research. This review provides a foundation for future investigations into JEV transmission in Australia, advocating for enhanced surveillance and standardised research methodologies to better understand and mitigate the virus's impact.

Mosquito bloodmeals can be used to determine vertebrate diversity, host preference, and pathogen exposure in humans and wildlife

The surveillance and detection of zoonotic pathogens in animals is essential for predicting disease transmission pathways and the risks of spillover, but challenges include the costs, ethics and technical expertise required for vertebrate trapping, serum sampling and antibody or virus screening. Surveillance using haematophagous arthropods as a sampling tool offers a unique opportunity to obtain blood samples from a wide range of vertebrate species, allowing the study of host-mosquito associations, and host exposure to pathogens. We explored vertebrate diversity and potential Ross River virus (RRV) transmission pathways by analysing blood-fed mosquitoes collected in Brisbane, Australia. Host origins were identified using barcode sequencing, and host exposure to RRV was assessed using a modified plaque reduction neutralisation test. In total, 480 blood-fed mosquitoes were collected between February 2021 and May 2022. The host origins of 346 (72%) bloodmeals were identified, with humans (73%) and cattle (9%) comprising the dominant hosts. RRV seroprevalence was high in both vertebrate species with evidence of RRV exposure in 70% (21/30) of cattle and 52% (132/253) of humans. This is a novel, non-invasive method of estimating seroprevalence in vertebrate host populations. Our results highlight the potential of blood-fed mosquitoes to provide species-specific insights into pathogen transmission dynamics.

Independent repeated mutations within the alphaviruses Ross River virus and Barmah Forest virus indicates convergent evolution and past positive selection in ancestral populations despite ongoing purifying selection

Ross River virus (RRV) and Barmah Forest virus (BFV) are arthritogenic arthropod-borne viruses (arboviruses) that exhibit generalist host associations and share distributions in Australia and Papua New Guinea (PNG). Using stochastic mapping and discrete-trait phylogenetic analyses, we profiled the independent evolution of RRV and BFV signature mutations. Analysis of 186 RRV and 88 BFV genomes demonstrated their viral evolution trajectories have involved repeated selection of mutations, particularly in the nonstructural protein 1 (nsP1) and envelope 3 (E3) genes suggesting convergent evolution. Convergent mutations in the nsP1 genes of RRV (residues 248 and 441) and BFV (residues 297 and 447) may be involved with catalytic enzyme mechanisms and host membrane interactions during viral RNA replication and capping. Convergent E3 mutations (RRV site 59 and BFV site 57) may be associated with enzymatic furin activity and cleavage of E3 from protein precursors assisting viral maturation and infectivity. Given their requirement to replicate in disparate insect and vertebrate hosts, convergent evolution in RRV and BFV may represent a dynamic link between their requirement to selectively 'fine-tune' intracellular host interactions and viral replicative enzymatic processes. Despite evidence of evolutionary convergence, selection pressure analyses did not reveal any RRV or BFV amino acid sites under strong positive selection and only weak positive selection for nonstructural protein sites. These findings may indicate that their alphavirus ancestors were subject to positive selection events which predisposed ongoing pervasive convergent evolution, and this largely supports continued purifying selection in RRV and BFV populations during their replication in mosquito and vertebrate hosts.

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.

Does arbovirus emergence in humans require adaptation to domestic mosquitoes?

In the last few decades, several mosquito-borne arboviruses of zoonotic origin have established large-scale epidemic transmission cycles in the human population. It is often considered that arbovirus emergence is driven by adaptive evolution, such as virus adaptation for transmission by 'domestic' mosquito vector species that live in close association with humans. Here, I argue that although arbovirus adaptation to domestic mosquito vectors has been observed for several emerging arboviruses, it was generally not directly responsible for their initial emergence. Secondary adaptation to domestic mosquitoes often amplified epidemic transmission, however, this was more likely a consequence than a cause of arbovirus emergence. Considering that emerging arboviruses are generally 'preadapted' for transmission by domestic mosquito vectors may help to enhance preparedness toward future arbovirus emergence events.

Species-specific MARCO-alphavirus interactions dictate chikungunya virus viremia

Arboviruses are public health threats that cause explosive outbreaks. Major determinants of arbovirus transmission, geographic spread, and pathogenesis are the magnitude and duration of viremia in vertebrate hosts. Previously, we determined that multiple alphaviruses are cleared efficiently from murine circulation by the scavenger receptor MARCO (Macrophage receptor with collagenous structure). Here, we define biochemical features on chikungunya (CHIKV), o'nyong 'nyong (ONNV), and Ross River (RRV) viruses required for MARCO-dependent clearance in vivo. In vitro, MARCO expression promotes binding and internalization of CHIKV, ONNV, and RRV via the scavenger receptor cysteine-rich (SRCR) domain. Furthermore, we observe species-specific effects of the MARCO SRCR domain on CHIKV internalization, where those from known amplification hosts fail to promote CHIKV internalization. Consistent with this observation, CHIKV is inefficiently cleared from the circulation of rhesus macaques in contrast with mice. These findings suggest a role for MARCO in determining whether a vertebrate serves as an amplification or dead-end host following CHIKV infection.

Ecological drivers of dog heartworm transmission in California

BACKGROUND

Effectively controlling heartworm disease-a major parasitic disease threatening animal health in the US and globally-requires understanding the local ecology of mosquito vectors involved in transmission. However, the key vector species in a given region are often unknown and challenging to identify. Here we investigate (i) the key vector species associated with transmission of the parasite, Dirofilaria immitis, in California and (ii) the climate and land cover drivers of vector presence.

METHODS

To identify key mosquito vectors involved in transmission, we incorporated long-term, finely resolved mosquito surveillance data and dog heartworm case data in a statistical modeling approach (fixed-effects regression) that rigorously controls for other unobserved drivers of heartworm cases. We then used a flexible machine learning approach (gradient boosted machines) to identify the climate and land cover variables associated with the presence of each species.

RESULTS

We found significant, regionally specific, positive associations between dog heartworm cases and the abundance of four vector species: Aedes aegypti (Central California), Ae. albopictus (Southern California), Ae. sierrensis (Central California), and Culiseta incidens (Northern and Central California). The proportion of developed land cover was one of the most important ecological variables predicting the presence or absence of the putative vector species.

CONCLUSION

Our results implicate three previously under-recognized vectors of dog heartworm transmission in California and indicate the land cover types in which each putative vector species is commonly found. Efforts to target these species could prioritize surveillance in these land cover types (e.g. near human dwellings in less urbanized settings for Ae. albopictus and Cs. incidens) but further investigation on the natural infection prevalence and host-biting rates of these species, as well as the other local vectors, is needed.

Not all mosquitoes are created equal: A synthesis of vector competence experiments reinforces virus associations of Australian mosquitoes

The globalization of mosquito-borne arboviral diseases has placed more than half of the human population at risk. Understanding arbovirus ecology, including the role individual mosquito species play in virus transmission cycles, is critical for limiting disease. Canonical virus-vector groupings, such as Aedes- or Culex-associated flaviviruses, have historically been defined using virus detection in field-collected mosquitoes, mosquito feeding patterns, and vector competence, which quantifies the intrinsic ability of a mosquito to become infected with and transmit a virus during a subsequent blood feed. Herein, we quantitatively synthesize data from 68 laboratory-based vector competence studies of 111 mosquito-virus pairings of Australian mosquito species and viruses of public health concern to further substantiate existing canonical vector-virus groupings and quantify variation within these groupings. Our synthesis reinforces current canonical vector-virus groupings but reveals substantial variation within them. While Aedes species were generally the most competent vectors of canonical “Aedes-associated flaviviruses” (such as dengue, Zika, and yellow fever viruses), there are some notable exceptions; for example, Aedes notoscriptus is an incompetent vector of dengue viruses. Culex spp. were the most competent vectors of many traditionally Culex-associated flaviviruses including West Nile, Japanese encephalitis and Murray Valley encephalitis viruses, although some Aedes spp. are also moderately competent vectors of these viruses. Conversely, many different mosquito genera were associated with the transmission of the arthritogenic alphaviruses, Ross River, Barmah Forest, and chikungunya viruses. We also confirm that vector competence is impacted by multiple barriers to infection and transmission within the mesenteron and salivary glands of the mosquito. Although these barriers represent important bottlenecks, species that were susceptible to infection with a virus were often likely to transmit it. Importantly, this synthesis provides essential information on what species need to be targeted in mosquito control programs.

There are over 3,500 species of mosquitoes in the world, but only a small proportion are considered important vectors of arboviruses. Vector competence, the physiological ability of a mosquito to become infected with and transmit arboviruses, is used in combination with virus detection in field populations and analysis of vertebrate host feeding patterns to incriminate mosquito species in virus transmission cycles. Here, we quantified the vector competence of Australian mosquitoes for endemic and exotic viruses of public health concern by analyzing 68 laboratory studies of 111 mosquito-virus pairings. We found that Australia has species that could serve as efficient vectors for each virus tested and it is these species that should be targeted in control programs. We also corroborate previously identified virus-mosquito associations at the mosquito genus level but show that there is considerable variation in vector competence between species within a genus. We also confirmed that vector competence is influenced by infection barriers within the mosquito and the experimental protocols employed. The framework we developed could be used to synthesize vector competence experiments in other regions or expanded to a world-wide overview.

Seroprevalence of dengue, Zika, chikungunya and Ross River viruses across the Solomon Islands

Across the Pacific, and including in the Solomon Islands, outbreaks of arboviruses such as dengue, chikungunya, and Zika are increasing in frequency, scale and impact. Outbreaks of mosquito-borne disease have the potential to overwhelm the health systems of small island nations. This study mapped the seroprevalence of dengue, Zika, chikungunya and Ross River viruses in 5 study sites in the Solomon Islands. Serum samples from 1,021 participants were analysed by ELISA. Overall, 56% of participants were flavivirus-seropositive for dengue (28%), Zika (1%) or both flaviviruses (27%); and 53% of participants were alphavirus-seropositive for chikungunya (3%), Ross River virus (31%) or both alphaviruses (18%). Seroprevalence for both flaviviruses and alphaviruses varied by village and age of the participant. The most prevalent arboviruses in the Solomon Islands were dengue and Ross River virus. The high seroprevalence of dengue suggests that herd immunity may be a driver of dengue outbreak dynamics in the Solomon Islands. Despite being undetected prior to this survey, serology results suggest that Ross River virus transmission is endemic. There is a real need to increase the diagnostic capacities for each of the arboviruses to support effective case management and to provide timely information to inform vector control efforts and other outbreak mitigation interventions.

The occurrence of arboviruses is increasing and causing significant impacts on human health. This is of high concern in small Pacific island nations where fragile health systems are regularly overwhelmed by disease outbreaks. To effectively prevent and control disease transmission there is a need to understand which viruses have been in circulation. Therefore, we conducted a cross-sectional survey of residents from 5 study sites distributed across the Solomon Islands. The serum samples were tested for antibodies that indicate prior infection for four arboviruses. We found evidence that the residents of the Solomon Islands have been exposed to substantial transmission of dengue and Ross River viruses, with lower levels of Zika and chikungunya transmission. Two large dengue outbreaks have been recently experienced and the outbreak pattern suggests that natural herd immunity may still be a driver of dengue outbreak dynamics in the Solomon Islands. Regarding Ross River virus, transmission is endemic despite being undetected prior to this survey. There is a real need to increase the capacity to accurately diagnose each of these arboviruses to support effective case management and to provide timely information to inform vector control efforts.

Human-mediated impacts on biodiversity and the consequences for zoonotic disease spillover

Human-mediated changes to natural ecosystems have consequences for both ecosystem and human health. Historically, efforts to preserve or restore 'biodiversity' can seem to be in opposition to human interests. However, the integration of biodiversity conservation and public health has gained significant traction in recent years, and new efforts to identify solutions that benefit both environmental and human health are ongoing. At the forefront of these efforts is an attempt to clarify ways in which biodiversity conservation can help reduce the risk of zoonotic spillover of pathogens from wild animals, sparking epidemics and pandemics in humans and livestock. However, our understanding of the mechanisms by which biodiversity change influences the spillover process is incomplete, limiting the application of integrated strategies aimed at achieving positive outcomes for both conservation and disease management. Here, we review the literature, considering a broad scope of biodiversity dimensions, to identify cases where zoonotic pathogen spillover is mechanistically linked to changes in biodiversity. By reframing the discussion around biodiversity and disease using mechanistic evidence - while encompassing multiple aspects of biodiversity including functional diversity, landscape diversity, phenological diversity, and interaction diversity - we work toward general principles that can guide future research and more effectively integrate the related goals of biodiversity conservation and spillover prevention. We conclude by summarizing how these principles could be used to integrate the goal of spillover prevention into ongoing biodiversity conservation initiatives.