House mouse abundance and Ross River virus notifications in Victoria, Australia

Mosquito-Borne Viruses and Non-Human Vertebrates in Australia: A Review

Mosquito-borne viruses are well recognized as a global public health burden amongst humans, but the effects on non-human vertebrates is rarely reported. Australia, houses a number of endemic mosquito-borne viruses, such as Ross River virus, Barmah Forest virus, and Murray Valley encephalitis virus. In this review, we synthesize the current state of mosquito-borne viruses impacting non-human vertebrates in Australia, including diseases that could be introduced due to local mosquito distribution. Given the unique island biogeography of Australia and the endemism of vertebrate species (including macropods and monotremes), Australia is highly susceptible to foreign mosquito species becoming established, and mosquito-borne viruses becoming endemic alongside novel reservoirs. For each virus, we summarize the known geographic distribution, mosquito vectors, vertebrate hosts, clinical signs and treatments, and highlight the importance of including non-human vertebrates in the assessment of future disease outbreaks. The mosquito-borne viruses discussed can impact wildlife, livestock, and companion animals, causing significant changes to Australian ecology and economy. The complex nature of mosquito-borne disease, and challenges in assessing the impacts to non-human vertebrate species, makes this an important topic to periodically review.

Scoping review on vector-borne diseases in urban areas: transmission dynamics, vectorial capacity and co-infection

BACKGROUND

Transmission dynamics, vectorial capacity, and co-infections have substantial impacts on vector-borne diseases (VBDs) affecting urban and suburban populations. Reviewing key factors can provide insight into priority research areas and offer suggestions for potential interventions.

MAIN BODY

Through a scoping review, we identify knowledge gaps on transmission dynamics, vectorial capacity, and co-infections regarding VBDs in urban areas. Peer-reviewed and grey literature published between 2000 and 2016 was searched. We screened abstracts and full texts to select studies. Using an extraction grid, we retrieved general data, results, lessons learned and recommendations, future research avenues, and practice implications. We classified studies by VBD and country/continent and identified relevant knowledge gaps. Of 773 articles selected for full-text screening, 50 were included in the review: 23 based on research in the Americas, 15 in Asia, 10 in Africa, and one each in Europe and Australia. The largest body of evidence concerning VBD epidemiology in urban areas concerned dengue and malaria. Other arboviruses covered included chikungunya and West Nile virus, other parasitic diseases such as leishmaniasis and trypanosomiasis, and bacterial rickettsiosis and plague. Most articles retrieved in our review combined transmission dynamics and vectorial capacity; only two combined transmission dynamics and co-infection. The review identified significant knowledge gaps on the role of asymptomatic individuals, the effects of co-infection and other host factors, and the impacts of climatic, environmental, and socioeconomic factors on VBD transmission in urban areas. Limitations included the trade-off from narrowing the search strategy (missing out on classical modelling studies), a lack of studies on co-infections, most studies being only descriptive, and few offering concrete public health recommendations. More research is needed on transmission risk in homes and workplaces, given increasingly dynamic and mobile populations. The lack of studies on co-infection hampers monitoring of infections transmitted by the same vector.

CONCLUSIONS

Strengthening VBD surveillance and control, particularly in asymptomatic cases and mobile populations, as well as using early warning tools to predict increasing transmission, were key strategies identified for public health policy and practice.

Epidemic host community contribution to mosquito-borne disease transmission: Ross River virus

Most vector-borne diseases infect multiple host species, but disentangling the relative importance of different host species to transmission can be complex. Here we study how host species' abundance and competence (duration and titre of parasitaemia) influence host importance during epidemic scenarios. We evaluate this theory using Ross River virus (RRV, family Togaviridae, genus Alphavirus), a multi-host mosquito-borne disease with significant human health impacts across Australia and Papua New Guinea. We used host contribution models to find the importance of key hosts (possums, wallabies, kangaroos, horses, humans) in typical mammal communities around five Australian epidemic centres. We found humans and possums contributed most to epidemic RRV transmission, owing to their high abundances, generally followed by macropods. This supports humans as spillover hosts, and that human-mosquito and possum-mosquito transmission is predominant during epidemics. Sensitivity analyses indicate these findings to be robust across epidemic centres. We emphasize the importance of considering abundance and competence in identifying key hosts (during epidemics in this case), and that competence alone is inadequate. Knowledge of host importance in disease transmission may help to equip health agencies to bring about greater effectiveness of disease mitigation strategies.

Improving public health intervention for mosquito-borne disease: the value of geovisualization using source of infection and LandScan data

Epidemiological studies use georeferenced health data to identify disease clusters but the accuracy of this georeferencing is obfuscated by incorrectly assigning the source of infection and by aggregating case data to larger geographical areas. Often, place of residence (residence) is used as a proxy for the source of infection (source) which may not be accurate. Using a 21-year dataset from South Australia of human infections with the mosquito-borne Ross River virus, we found that 37% of cases were believed to have been acquired away from home. We constructed two risk maps using age-standardized morbidity ratios (SMRs) calculated using residence and patient-reported source. Both maps confirm significant inter-suburb variation in SMRs. Areas frequently named as the source (but not residence) and the highest-risk suburbs both tend to be tourist locations with vector mosquito habitat, and camping or outdoor recreational opportunities. We suggest the highest-risk suburbs as places to focus on for disease control measures. We also use a novel application of ambient population data (LandScan) to improve the interpretation of these risk maps and propose how this approach can aid in implementing disease abatement measures on a smaller scale than for which disease data are available.

Ross River Virus (RRV) infection in horses and humans: a review

A fascinating and important arbovirus is Ross River Virus (RRV) which is endemic and epizootic in nature in certain parts of the world. RRV is a member of the genus Alphavirus within the Semliki Forest complex of the family Togaviridae, which also includes the Getah virus. The virus is responsible for causing disease both in humans as well as horses. Mosquito species (Aedes camptorhynchus and Aedes vigilax; Culex annulirostris) are the most important vector for this virus. In places of low temperature as well as low rainfall or where there is lack of habitat of mosquito there is also limitation in the transmission of the virus. Such probability is higher especially in temperate regions bordering endemic regions having sub-tropical climate. There is involvement of articular as well as non-articular cells in the replication of RRV. Levels of pro-inflammatory factors viz., tumor necrosis factor-alpha (TNF-α); interferon-gamma (IFN-γ); and macrophage chemo-attractant protein-1 (MAC-1) during disease pathogenesis have been found to be reduced. Reverse transcription-polymerase chain reaction (RT-PCR) is the most advanced molecular diagnostic tool along with epitope-blocking enzyme-linked immunosorbent assay (ELISA) for detecting RRV infection. Treatment for RRV infection is only supportive. Vaccination is not a fruitful approach. Precise data collection will help the researchers to understand the RRV disease dynamics and thereby designing effective prevention and control strategy. Advances in diagnosis, vaccine development and emerging/novel therapeutic regimens need to be explored to their full potential to tackle RRV infection and the disease it causes.

Sampling frequency differentially influences interpretation of zoonotic pathogen and host dynamics: Sin Nombre virus and deer mice

Reports of novel emerging and resurging wildlife and zoonotic diseases have increased. Consequently, integration of pathogen sampling into wildlife monitoring programs has grown. Sampling frequency influences interpretations of coupled host-pathogen dynamics, with direct implication to human exposure risk, but has received little empirical attention. To address this, a 15-year study, based on monthly sampling, of deer mouse (Peromyscus maniculatus) populations and Sin Nombre virus (SNV; a virulent disease in humans) dynamics was evaluated. Estimates of deer mouse abundance, number infected with SNV, and SNV prevalence from sampling less frequently than each month (achieved by deletion of months and recalculation of these parameters) were compared to monthly sampling frequencies. Deer mouse abundance was underestimated (10%-20%), SNV prevalence was overestimated when prevalence was high (>15%), and fewer annual extremes of abundance and infection were detected when sampling frequency was less than monthly. Effort necessary to detect temporal dynamics of SNV differed from effort to detect demographic patterns in deer mouse abundance. Findings here are applicable to sampling strategies for other host-pathogen dynamics and have direct implications for allocation of public health resources and intervention programs.

A temporal dilution effect: hantavirus infection in deer mice and the intermittent presence of voles in Montana

The effect of intermittently occurring, non-reservoir host species on pathogen transmission and prevalence in a reservoir population is poorly understood. We investigated whether voles, Microtus spp., which occur intermittently, influenced estimated standing antibody prevalence (ESAP) to Sin Nombre hantavirus (SNV, Bunyaviridae: Hantavirus) among deer mice, Peromyscus maniculatus, whose populations are persistent. We used 14 years of data from central Montana to investigate whether ESAP among deer mice was related to vole presence or abundance while controlling for the relationship between deer mouse abundance and ESAP. We found a reduction in deer mouse ESAP associated with the presence of voles, independent of vole abundance. A number of studies have documented that geographic locations which support a higher host diversity can be associated with reductions in pathogen prevalence by a hypothesized dilution effect. We suggest a dilution effect may also occur in a temporal dimension at sites where host richness fluctuates. Preservation of host diversity and optimization of environmental conditions which promote occurrence of ephemeral species, such as voles, may result in a decreased ESAP to hantaviruses among reservoir hosts. Our results may extend to other zoonotic infectious diseases.