Sequencing of Historical Isolates, K-mer Mining and High Serological Cross-Reactivity with Ross River Virus Argue against the Presence of Getah Virus in Australia

Getah virus (GETV) is a mosquito-transmitted alphavirus primarily associated with disease in horses and pigs in Asia. GETV was also reported to have been isolated from mosquitoes in Australia in 1961; however, retrieval and sequencing of the original isolates (N544 and N554), illustrated that these viruses were virtually identical to the 1955 GETV_MM2021 isolate from Malaysia. K-mer mining of the >40,000 terabases of sequence data in the Sequence Read Archive followed by BLASTn confirmation identified multiple GETV sequences in biosamples from Asia (often as contaminants), but not in biosamples from Australia. In contrast, sequence reads aligning to the Australian Ross River virus (RRV) were readily identified in Australian biosamples. To explore the serological relationship between GETV and other alphaviruses, an adult wild-type mouse model of GETV was established. High levels of cross-reactivity and cross-protection were evident for convalescent sera from mice infected with GETV or RRV, highlighting the difficulties associated with the interpretation of early serosurveys reporting GETV antibodies in Australian cattle and pigs. The evidence that GETV circulates in Australia is thus not compelling.

Spatial and temporal patterns of Ross River virus in south east Queensland, Australia: identification of hot spots at the rural-urban interface

Background

Ross River virus (RRV) is responsible for the most common vector-borne disease of humans reported in Australia. The virus circulates in enzootic cycles between multiple species of mosquitoes, wildlife reservoir hosts and humans. Public health concern about RRV is increasing due to rising incidence rates in Australian urban centres, along with increased circulation in Pacific Island countries. Australia experienced its largest recorded outbreak of 9544 cases in 2015, with the majority reported from south east Queensland (SEQ). This study examined potential links between disease patterns and transmission pathways of RRV.

Methods

The spatial and temporal distribution of notified RRV cases, and associated epidemiological features in SEQ, were analysed for the period 2001–2016. This included fine-scale analysis of disease patterns across the suburbs of the capital city of Brisbane, and those of 8 adjacent Local Government Areas, and host spot analyses to identify locations with significantly high incidence.

Results

The mean annual incidence rate for the region was 41/100,000 with a consistent seasonal peak in cases between February and May. The highest RRV incidence was in adults aged from 30 to 64 years (mean incidence rate: 59/100,000), and females had higher incidence rates than males (mean incidence rates: 44/100,000 and 34/100,000, respectively). Spatial patterns of disease were heterogeneous between years, and there was a wide distribution of disease across both urban and rural areas of SEQ. Overall, the highest incidence rates were reported from predominantly rural suburbs to the north of Brisbane City, with significant hot spots located in peri-urban suburbs where residential, agricultural and conserved natural land use types intersect.

Conclusions

Although RRV is endemic across all of SEQ, transmission is most concentrated in areas where urban and peri-urban environments intersect. The drivers of RRV transmission across rural-urban landscapes should be prioritised for further investigation, including identification of specific vectors and hosts that mediate human spillover.

Flooding and Arboviral Disease: Predicting Ross River Virus Disease Outbreaks Across Inland Regions of South-Eastern Australia

Flood frequency is expected to increase across the globe with climate change. Understanding the relationship between flooding and arboviral disease can reduce disease risk and associated costs. South-eastern Australia is dominated by the flood-prone Murray-Darling River system where the incidence of Australia's most common arboviral disease, Ross River virus (RRV), is high. This study aimed to determine the relationship between riverine flooding and RRV disease outbreaks in inland south-eastern Australia, specifically New South Wales (NSW). Each study month from 1991 to 2013, for each of 37 local government areas (LGAs) was assigned 'outbreak/non-outbreak' status based on long-term trimmed-average age-standardized RRV notification rates and 'flood/non-flood' status based on riverine overflow. LGAs were grouped into eight climate zones with the relationship between flood and RRV outbreak modeled using generalized estimating equations. Modeling adjusted for rainfall in the previous 1-3 mo. Spring-summer flooding increased the odds of summer RRV outbreaks in three climate zones before and after adjusting for rainfall 1, 2, and 3 mo prior to the outbreak. Flooding at any time of the year was not predictive of RRV outbreaks in the remaining five climate zones. Predicting RRV disease outbreaks with flood events can assist with more targeted mosquito spraying programs, thereby reducing disease transmission and mosquito resistance.

Infection of Western Gray Kangaroos (Macropus fuliginosus) with Australian Arboviruses Associated with Human Infection

More than 75 arboviruses (arthropod-borne viruses) have been identified in Australia. While Alfuy virus (ALFV), Barmah Forest virus (BFV), Edge Hill virus (EHV), Kokobera virus (KOKV), Murray Valley encephalitis virus (MVEV), Sindbis virus (SINV), Ross River virus (RRV), Stratford virus (STRV), and West Nile virus strain Kunjin (KUNV) have been associated with human infection, there remains a paucity of data regarding their respective transmission cycles and any potential nonhuman vertebrate hosts. It is likely that these viruses are maintained in zoonotic cycles involving native animals rather than solely by human-to-human transmission. A serosurvey (n = 100) was undertaken to determine the prevalence of neutralizing antibodies against a panel of Australian arboviruses in western gray kangaroos (Macropus fuliginosus) obtained from 11 locations in the midwest to southwest of Western Australia. Neutralizing antibodies against RRV were detected in 25%, against BFV in 14%, and antibodies to both viruses in 34% of serum samples. The prevalence of antibodies against these two viruses was the same in males and females, but higher in adult than in subadult kangaroos (p < 0.05). Twenty-one percent of samples had neutralizing antibodies against any one or more of the flaviviruses ALFV, EHV, KOKV, MVEV, and STRV. No neutralizing antibodies against SINV and KUNV were detected. If this sample of kangaroo sera was representative of the broader Australian population of macropods, it suggests that they are common hosts for RRV and BFV. The absence or low seroprevalence of antibodies against the remaining arboviruses suggests that they are not prevalent in the region or that kangaroos are not commonly infected with them. The detection of neutralizing antibodies to MVEV requires further investigation as this virus has not been identified previously so far south in Western Australia.

Neglected Australian arboviruses: quam gravis?

At least 75 arboviruses have been identified from Australia. Most have a zoonotic transmission cycle, maintained in the environment by cycling between arthropod vectors and susceptible mammalian or avian hosts. The primary arboviruses that cause human disease in Australia are Ross River, Barmah Forest, Murray Valley encephalitis, Kunjin and dengue. Several other arboviruses are associated with human disease but little is known about their clinical course and diagnostic testing is not routinely available. Given the significant prevalence of undifferentiated febrile illness in Australia, investigation of the potential threat to public health presented by these viruses is required.

Spatio-temporal patterns of Barmah Forest virus disease in Queensland, Australia

Background

Barmah Forest virus (BFV) disease is a common and wide-spread mosquito-borne disease in Australia. This study investigated the spatio-temporal patterns of BFV disease in Queensland, Australia using geographical information system (GIS) tools and geostatistical analysis.

Methods/Principal Findings

We calculated the incidence rates and standardised incidence rates of BFV disease. Moran's I statistic was used to assess the spatial autocorrelation of BFV incidences. Spatial dynamics of BFV disease was examined using semi-variogram analysis. Interpolation techniques were applied to visualise and display the spatial distribution of BFV disease in statistical local areas (SLAs) throughout Queensland. Mapping of BFV disease by SLAs reveals the presence of substantial spatio-temporal variation over time. Statistically significant differences in BFV incidence rates were identified among age groups (χ² = 7587, df = 7327,p<0.01). There was a significant positive spatial autocorrelation of BFV incidence for all four periods, with the Moran's I statistic ranging from 0.1506 to 0.2901 (p<0.01). Semi-variogram analysis and smoothed maps created from interpolation techniques indicate that the pattern of spatial autocorrelation was not homogeneous across the state.

Conclusions/Significance

This is the first study to examine spatial and temporal variation in the incidence rates of BFV disease across Queensland using GIS and geostatistics. The BFV transmission varied with age and gender, which may be due to exposure rates or behavioural risk factors. There are differences in the spatio-temporal patterns of BFV disease which may be related to local socio-ecological and environmental factors. These research findings may have implications in the BFV disease control and prevention programs in Queensland.

Molecular evolutionary dynamics of Ross River virus and implications for vaccine efficacy

Ross River virus (RRV) is a mosquito-borne member of the genus Alphavirus that causes epidemic polyarthritis in humans, costing the Australian health system at least US$10 million annually. Recent progress in RRV vaccine development requires accurate assessment of RRV genetic diversity and evolution, particularly as they may affect the utility of future vaccination. In this study, we provide novel RRV genome sequences and investigate the evolutionary dynamics of RRV from time-structured E2 gene datasets. Our analysis indicates that, although RRV evolves at a similar rate to other alphaviruses (mean evolutionary rate of approx. 8x10(-4) nucleotide substitutions per site year(-1)), the relative genetic diversity of RRV has been continuously low through time, possibly as a result of purifying selection imposed by replication in a wide range of natural host and vector species. Together, these findings suggest that vaccination against RRV is unlikely to result in the rapid antigenic evolution that could compromise the future efficacy of current RRV vaccines.

Socio-environmental predictors of Barmah forest virus transmission in coastal areas, Queensland, Australia

OBJECTIVE

To assess the socio-environmental predictors of Barmah forest virus (BFV) transmission in coastal areas, Queensland, Australia.

METHODS

Data on BFV notified cases, climate, tidal levels and socioeconomic index for area (SEIFA) in six coastal cities, Queensland, for the period 1992-2001 were obtained from the relevant government agencies. Negative binomial regression models were used to assess the socio-environmental predictors of BFV transmission.

RESULTS

The results show that maximum and minimum temperature, rainfall, relative humidity, high and low tide were statistically significantly associated with BFV incidence at lags 0-2 months. The fitted negative binomial regression models indicate a significant independent association of each of maximum temperature (beta = 0.139, P = 0.000), high tide (beta = 0.005, P = 0.000) and SEIFA index (beta = -0.010, P = 0.000) with BFV transmission after adjustment for confounding variables.

CONCLUSIONS

The transmission of BFV disease in Queensland coastal areas seemed to be determined by a combination of local social and environmental factors. The model developed in this study may have applications in the control and prevention of BFV disease in these areas.

Sindbis virus infection in resident birds, migratory birds, and humans, Finland

Resident grouse may be involved in the epidemiology of SINV in humans.

Sindbis virus (SINV), a mosquito-borne virus that causes rash and arthritis, has been causing outbreaks in humans every seventh year in northern Europe. To gain a better understanding of SINV epidemiology in Finland, we searched for SINV antibodies in 621 resident grouse, whose population declines have coincided with human SINV outbreaks, and in 836 migratory birds. We used hemagglutination-inhibition and neutralization tests for the bird samples and enzyme immunoassays and hemagglutination-inhibition for the human samples. SINV antibodies were first found in 3 birds (red-backed shrike, robin, song thrush) during their spring migration to northern Europe. Of the grouse, 27.4% were seropositive in 2003 (1 year after a human outbreak), but only 1.4% were seropositive in 2004. Among 2,529 persons, the age-standardized seroprevalence (1999–2003) was 5.2%; seroprevalence and incidence (1995–2003) were highest in North Karelia (eastern Finland). Grouse may contribute to the epidemiology of SINV in humans.

Acute symptoms and sequelae of Ross River virus infection in South-Western Australia: a follow-up study

BACKGROUND

Unusually high tides along the south-west coast of Western Australia (WA) during the spring and summer of 1988-89 provided ideal breeding conditions for mosquito vectors of Ross River virus (RRV). This was followed by the biggest outbreak of RRV infection ever documented in WA (330 notified cases).

OBJECTIVES

To describe the nature and duration of symptoms of RRV infection in WA, and associated functional disability; to determine the perceived effectiveness of treatments; to determine the usefulness of available information on RRV infection.

STUDY DESIGN

A retrospective study of all cases of RRV infection reported from South West WA during the 1988-89 outbreak, using a self-administered patient questionnaire.

RESULTS

The most common symptoms were arthralgia, tiredness and lethargy, and joint stiffness and swelling. In patients with joint manifestations, the knees, wrists and ankles were almost always affected. Non-steroidal anti-inflammatory agents, rest, simple analgesics and hydrotherapy were subjectively the most helpful treatments. Only 27% of patients had recovered completely within 6 months of onset of symptoms. Three years after the outbreak, up to 57% still experienced at least intermittent joint symptoms. Less than one-third of people reported that the available information adequately explained the consequences of RRV infection.

CONCLUSIONS

Compared with patients elsewhere in Australia, people infected with RRV in the South West of WA experience a slightly different spectrum of clinical symptoms, with a longer period of disability. This may be related to the presence of a different topotype of the virus to that found elsewhere in Australia. Better information for doctors and patients on the likely course of the illness is needed. Future studies should examine the economic cost associated with RRV infection, and evaluate treatments to shorten the period of disability.

Prevalence of neutralising antibodies to Barmah Forest, Sindbis and Trubanaman viruses in animals and humans in the south-west of Western Australia

A study was undertaken in the south-west of Western Australia to investigate potential vertebrate hosts of Barmah Forest virus (BFV), Sindbis virus (SINV) and Trubanaman virus (TRUV) following isolation of these viruses from mosquitoes collected during routine surveillance for arboviruses. Over 3000 animal and human sera collected between 1979 and 1995 were tested for the presence of neutralising antibodies to each of the viruses. The overall prevalence of antibodies to BFV, SINV and TRUV was 0.4%, 0.3% and 1.6%, respectively. Antibodies to BFV were detected only in quokkas (3.2%), horses (1.2%) and humans (0.9%). No definitive evidence of infection with BFV was detected in samples collected prior to 1992, supporting previous suggestions that BFV was introduced into the region after this time. Antibodies to SINV were detected in western native cats (16.7%), emus (4.5%), rabbits (0.8%) and horses (0.7%), and evidence of TRUV infection was most common in western grey kangaroos (21.1%), feral pigs (3.6%), rabbits (2.4%), foxes (2.3%), quokkas (1.6%) and horses (1.6%).

Causative agent of Pogosta disease isolated from blood and skin lesions

Pogosta disease is a mosquito-borne viral disease in Finland, which is clinically manifested by rash and arthritis; larger outbreaks occur in 7-year intervals. The causative agent of the disease has been suspected of being closely related to Sindbis virus (SINV). We isolated SINV from five patients with acute Pogosta disease during an outbreak in fall 2002 in Finland. One virus strain was recovered from a whole blood sample and four other strains from skin lesions. The etiology of Pogosta disease was confirmed by these first Finnish SINV strains, which also represent the first human SINV isolates from Europe. Phylogenetic analysis indicates that the Finnish SINV strains are closely related to the viral agents isolated from mosquitoes and that cause clinically similar diseases in nearby geographic areas.

Isolation of arboviruses from mosquitoes (Diptera: Culicidae) collected from the Gulf Plains region of northwest Queensland, Australia

As part of investigations into Japanese encephalitis (JE) virus and related flaviviruses in northern Australia, 153,529 mosquitoes were collected and processed for virus isolation from the Gulf Plains region of northwest Queensland. Collections from within 30 km of each of the townships of Croydon, Normanton and Karumba yielded 3,087 (2.0%), 66,009 (43.0%), and 84,433 (55.0%) mosquitoes, respectively, from which 16 viruses were isolated. Four isolates of Murray Valley encephalitis (MVE), two of Kunjin (KUN), three of Ross River (RR), and one of Sindbis (SIN) viruses were obtained from Culex sitiens subgroup mosquitoes. Molecular identification of the mosquito species composition of these virus positive pools revealed that most isolates were from pools containing mainly Culex annulirostris Skuse and low numbers of Culex palpalis (Taylor). Only three pools, one each of MVE, KUN, and RR, were from mosquitoes identified exclusively as Cx. annulirostris. Other viruses isolated include one Edge Hill virus from Ochlerotatus normanensis (Taylor), an isolate of SIN from Anopheles meraukensis Venhuis, two isolates of RR from Anopheles amictus Edwards, and single isolates of RR from Anopheles bancroftii Giles andAedes lineatopennis (Ludlow). The isolate of RR from Ae. lineatopennis was the first reported from this species. The public health implications of these isolations in the Gulf Plains region are discussed briefly.

The risk of Ross River and Barmah Forest virus disease in Queensland: implications for New Zealand

OBJECTIVES

To describe the incidence of Ross River (RR) and Barmah Forest (BF) virus disease in Queensland and determine the risk of importation of RR virus from Queensland into New Zealand (NZ) via viraemic travellers.

METHODS

Based on routine RR and BF virus notification data of seven major urban tropical and subtropical Queensland populations, incidence rates adjusted for age, sex, season and a baseline level of immunity were used to examine the annual and seasonal risk of disease in the specific populations and selected subgroups. The risk for NZ was determined by estimating the number of infections among major visitor groups travelling from Queensland to NZ, using seroconversion rates.

RESULTS

In Queensland, annual rates of RR and BF virus disease ranged between 31.5-288.3 and 3.4-37.4/100,000 person years respectively and increased to between 48.4-423.5 and 3.8-40.4/100,000 person years at risk when adjusted for immunity. Our estimates indicate that more than 100 viraemic travellers may enter NZ from Queensland each year. Estimates were greatest among New Zealanders returning home.

CONCLUSIONS AND IMPLICATIONS

Usefulness of notification data could be maximised by presenting more detailed information to the local governments responsible for the control and public health awareness of these pathogens. Given the high number of viraemic persons entering NZ, the abundance of possums and the emergence of Oc. camptorhynchus, transmission of RR virus within NZ is probable. Health authorities should prepare for a virgin soil epidemic of RR virus by initiating serological and clinical surveillance in key areas, enhance public and professional awareness and elevate national resources necessary to invoke emergency vector control and case management.

Ross River virus: ecology and distribution

Ross River virus is the most common mosquito-borne pathogen in Australia, and approximately 5000 human cases are reported annually. The infection is not fatal, but there is considerable morbidity associated with a debilitating polyarthritis that is the major symptom. The virus is annually active in most regions of Australia, but exists as strains that vary in virulence. Native macropods are thought to be the natural vertebrate hosts, although horses and humans may be involved during epidemic activity, and vertical transmission of the virus occurs in mosquitoes. Different mosquito species are involved as vectors in various regions and in different seasonal and environmental conditions. In coastal areas the saltmarsh mosquitoes Aedes camptorhynchus and Ae. vigilax are the most important vectors in southern and northern regions, respectively, whereas in inland areas Culex annulirostris is the most important vector, although various Aedes species can be involved depending on region and conditions, and the epidemiology of the disease and vector control imperatives vary with circumstance concomitantly.

Ross River virus transmission, infection, and disease: a cross-disciplinary review

Ross River virus (RRV) is a fascinating, important arbovirus that is endemic and enzootic in Australia and Papua New Guinea and was epidemic in the South Pacific in 1979 and 1980. Infection with RRV may cause disease in humans, typically presenting as peripheral polyarthralgia or arthritis, sometimes with fever and rash. RRV disease notifications in Australia average 5,000 per year. The first well-described outbreak occurred in 1928. During World War II there were more outbreaks, and the name epidemic polyarthritis was applied. During a 1956 outbreak, epidemic polyarthritis was linked serologically to a group A arbovirus (Alphavirus). The virus was subsequently isolated from Aedes vigilax mosquitoes in 1963 and then from epidemic polyarthritis patients. We review the literature on the evolutionary biology of RRV, immune response to infection, pathogenesis, serologic diagnosis, disease manifestations, the extraordinary variety of vertebrate hosts, mosquito vectors, and transmission cycles, antibody prevalence, epidemiology of asymptomatic and symptomatic human infection, infection risks, and public health impact. RRV arthritis is due to joint infection, and treatment is currently based on empirical anti-inflammatory regimens. Further research on pathogenesis may improve understanding of the natural history of this disease and lead to new treatment strategies. The burden of morbidity is considerable, and the virus could spread to other countries. To justify and design preventive programs, we need accurate data on economic costs and better understanding of transmission and behavioral and environmental risks.

Arboviruses associated with human disease in Australia

Mosquito-borne arboviruses are an important public health issue in Australia. The alphaviruses Ross River and Barmah Forest virus are widespread and active annually, and cause debilitating polyarthritis. The flaviviruses Murray Valley encephalitis, Kunjin and Japanese encephalitis virus are restricted in distribution and activity but may cause life-threatening illness, and dengue viruses are active in some areas.

Mosquito-borne arboviruses in Australia: the current scene and implications of climate change for human health

Of the mosquito-borne arboviruses, the encephalitic Murray Valley encephalitis and Kunjin viruses are a major public health concern, but the arthritides Ross River and Barmah Forest viruses are more important in a public health sense, being responsible for a far greater number of infections. Reported cases of Ross River totalled approximately 30,000 during 1991-1996; there have been several widely separated outbreaks of Barmah Forest in recent years and case reports are increasing annually. Surveillance programmes have increased our understanding of the geographic regions, climatic conditions and vector factors associated with viruses. Virus activity is widespread but is often localised, is driven primarily by mosquito abundance and various species are involved; host factors are involved also, but are not well understood. Typically, mosquito populations are governed by availability of habitat and environmental conditions. Models of climate change predict increases in rainfall, tides and temperature for parts of Australia, and such changes have the potential to increase the risk of arbovirus transmission by increasing the distribution and abundance of vectors, and duration of mosquito and arbovirus seasons. However, the amplitude of climate change is uncertain and the ecology of arbovirus transmission is complex. It is likely that some areas will have increases in arbovirus activity and human infection with predicted climate change, but risk of increased transmission will vary with locality, vector, host and human factors.

Ross River virus infection on the North Coast of New South Wales

A retrospective follow-up survey was undertaken of residents of the North Coast of New South Wales infected with Ross River virus in 1992. The aims of the study were to describe the epidemiology and acute symptomatology of Ross River virus infection, its natural history during the first 12 months of infection, and its effects on those infected. Questionnaires were distributed to both cases and their medical practitioners. Of 129 people infected, aged between six and 85 years, 81 (63 per cent) were male and 48 (37 per cent) were female. The peak age-specific incidence was in the age group 50 to 59 years. The most common symptoms were arthralgia (95 per cent) and tiredness (91 per cent). Over 60 per cent took time off work. At 12 months follow-up, over 50 per cent reported persistent arthralgia, 35 per cent reported persistent tiredness and 15 per cent were still unable to carry out their normal activities. The median duration of symptoms was in the range 7 to 12 months, and of incapacity was in the range five weeks to three months. There were some differences from previous reports of Ross River virus outbreaks, in the incidence of major symptoms and the duration of illness and incapacity. These are likely to be at least partly due to inconsistent measurement methods. In this study, there were systematic differences between medical practitioners' and patients' estimates of periods of incapacity. Previous estimates of the direct economic costs and indirect human costs of infection based on data obtained from medical practitioners, although alarming, are almost certainly underestimates.

Viral infections: alpha-viral arthropathy

Six different mosquito-borne viruses (Chikungunya, O'nyong-nyong, Mayaro, Ross River, Sindbis and Barmah Forest) have been associated with arthritis in humans. These viruses are prevalent in the tropics and subtropics and they produce similar symptoms, consisting of fever, joint pains and rash. The symptoms are usually of short duration, around 1 week; complete recovery is the rule apart from exceptional cases of Chik infection. Precise diagnosis requires a serological service which is not available in many parts of the tropics these days. Treatment is symptomatic and there is no vaccine currently available. With an increasing number of visitors to the tropics being exposed to potential infection and with rapid air transport it is possible that visitors may return home during the viraemic incubation stage, infect the local mosquito populations and then develop clinical disease.

Arboviruses causing human disease in the Australasian zoogeographic region

Over 65 arboviruses have been reported from countries in the Australasian zoogeographic region, but only a few have been implicated in human disease. These include the flaviviruses Murray Valley encephalitis (MVE), Kunjin (KUN), Kokobera (KOK), and dengue, particularly types 1 and 2; the alphaviruses Ross River (RR), Barmah Forest (BF), and Sindbis (SIN); and the bunyaviruses, Gan Gan and Trubanaman. In this paper recent epidemiological and clinical results pertaining to these viruses are reviewed, with major emphasis on MVE and RR viruses. The extensive early studies of Australian arboviruses have been reviewed by Doherty [49, 50], and their ecology and vectors more recently by Kay and Standfast [87]. In addition, the biology of MVE and KUN [113] and RR [87, 114] viruses have been the subjects of more detailed reviews. The Australasian zoogeographic region is defined as countries east of the Wallace and Weber lines, two hypothetical lines in the Indo-Australian archipelago where the fauna of the Australasian and Oriental regions meet. Seroepidemiological studies of human arboviral infections have suggested that the Japanese encephalitis flavivirus and the chikungunya alphavirus occur only in the Oriental region, whereas the related MVE and RR viruses, respectively, are restricted to the Australasian region [85, 148]. Serological results from Wallacea, the zone between the Wallace and Weber lines, are not so clear-cut [85]. This review is therefore restricted to countries east of Wallacea, specifically New Guinea and Australia.

Arbovirus infections of humans in high-risk areas of south-eastern Australia: a continuing study

OBJECTIVES

To determine the current immune status of high-risk populations of New South Wales and Victoria to the arboviral pathogens, Murray Valley encephalitis (MVE) and Kunjin (KUN) viruses, which are associated with Australian encephalitis (AE), and Ross River (RR) and Kokobera (KOK) viruses which are associated with polyarthritis. Further, to estimate seroconversion rates to these viruses in high-risk populations over the 10-year period 1981-1991.

DESIGN AND STUDY POPULATION

Blood was taken from 2873 permanent residents, children and adults from previously identified high-risk areas in western NSW and northern Victoria. Samples were tested by the haemagglutination-inhibition (HI) test for antibodies to the four viruses. All sera were also tested for MVE and KUN antibodies by the more specific neutralisation test (NT). Ninety-five of the subjects had been seronegative when sampled 10 years previously.

RESULTS

Age standardised prevalence rates for flavivirus HI antibodies (MVE, KUN, KOK) ranged from 66% (Bourke) to 15% (Forbes), and were similar to those observed 10 years previously. However, specific NT antibodies to MVE and KUN were uncommon in all districts except Bourke, indicating a very high level of susceptibility to Australian encephalitis, should a fresh epidemic occur. Whereas KUN virus seems enzootic in NSW and Victoria, MVE did not appear to have been present since the last outbreak in 1974, even in Bourke. Flavivirus antibody rates (as detected by the broadly reactive HI test) greatly exceeded those specifically attributable to MVE and KUN (NT test) or KOK, leading to the speculation that unidentified flaviviruses are responsible for most human infections. Ross River virus antibody prevalence rates exceeded those of flaviviruses in all districts, ranging from 72% (Bourke) to 25% (Cohuna), and were uniformly higher than those observed in 1981. Ten-year seroconversion rates in seronegative panels were 8.5% for flaviviruses and 24.2% for RR virus, and are broadly consistent with the cross-sectional study.

CONCLUSIONS

Although flavivirus and alphavirus infections have occurred at a "steady rate"in western NSW and northern Victoria, there is a general lack of immunity to the agents of Australian encephalitis in all centres except Bourke. This needs to be considered in public health policy in these areas.

An outbreak of epidemic polyarthritis (Ross River virus disease) in the Northern Territory during the 1990-1991 wet season

OBJECTIVE

To describe the epidemiology of a large outbreak of epidemic polyarthritis in the Northern Territory during the wet season of 1990-1991.

DESIGN, SETTING AND PARTICIPANTS

Arbovirus cases notified to the Northern Territory Department of Health and Community Services by general practitioners and local laboratories between 1 July 1990 and 30 June 1991.

MAIN OUTCOME MEASURES

Date and place of infection, age, sex and symptoms.

RESULTS

Doctors in the Northern Territory notified 368 cases; another 14 were infected interstate. The epidemic started in September, peaked in January and tailed off in April. The highest attack rates occurred in the rural areas of Jabiru, Litchfield Shire and Katherine. Those most affected were 30-34 year olds. Children, the elderly and Aboriginal people were under-represented.

CONCLUSIONS

Epidemic polyarthritis is a wet season problem in the Northern Territory, affecting the rural towns and districts more than the cities. Pre-planned mosquito control measures (effective water drainage and larval control) limited the extent of the 1990-1991 epidemic in Darwin City and Palmerston. The low attack rate in children reflects asymptomatic and less clinically severe infections. The under-representation of Aboriginal people may be the result of infection occurring earlier in life. A related cross-sectional seroprevalence survey has shown that rural Aboriginal people across all age groups have a significantly higher seropositive rate than urban non-Aboriginal residents.

Defined epitope blocking with Murray Valley encephalitis virus and monoclonal antibodies: laboratory and field studies

In an attempt to develop a specific serological test for Murray Valley encephalitis (MVE) virus antibodies, a panel of MVE monoclonal antibodies was utilised in defined-epitope blocking ELISA tests. In sera of mice immunised singly and in combinations of MVE, Alfuy (ALF), and Kunjin (KUN) viruses, blocking patterns usually distinguished MVE infections from those of the other flaviviruses. When blocking tests with selected MAbs were applied to 468 flavivirus antibody positive sera collected from human subjects throughout New South Wales, sera with blocking patterns consistent with previous MVE infection were found in 18 subjects. All were long-term residents of areas previously frequented by MVE, and all were of an age to have been exposed to the virus in past epidemics. No such sera were found in subjects living in coastal areas of NSW where MVE has never been reported.

Illness caused by a Barmah Forest-like virus in New South Wales

Barmah Forest virus, a recently-discovered arbovirus which belongs to the alphavirus genus of the family Togaviridae, has been shown to cause infections in humans in New South Wales. The present report documents three patients in whom Barmah Forest viral infection appears to have resulted in illness. Barmah Forest virus or a closely-related alphavirus may, as are several other alphaviruses, be pathogenic.

Barmah Forest virus infections in humans in New South Wales

Antibodies to Barmah Forest virus, a member of the alphavirus group, which was first isolated in 1974, have been found to be widespread in humans in New South Wales. Antibody studies showed a higher prevalence in the north coastal zones of the State, and lower rates in individuals who were living in all other biophysical zones. Antibody rates were significantly higher in male than in female subjects. The pathogenicity of the Barmah Forest virus is at present not known.

Illness caused by a Kokobera-like virus in south-eastern Australia

Three patients who lived in south-eastern Australia and who suffered acute polyarticular illnesses in the summer months of 1983-1984 and 1984-1985 are described. Two patients lived in the southwestern plains of New South Wales and one in Bairnsdale in eastern Victoria. Serological studies implicated Kokobera virus, a flavivirus, as the likely causative agent. This would appear to be the first report to indicate the pathogenicity of Kokobera virus.

A major outbreak of epidemic polyarthritis in New South Wales during the summer of 1983/1984

A large outbreak of epidemic polyarthritis (EPA) caused by Ross River virus (RRV) occurred in New South Wales in the summer of 1983/1984. The total number of cases was unknown, but 1196 cases were confirmed by laboratory tests. Most patients came from the area west of the dividing range, with especially large numbers in the Murrumbidgee irrigation area. Cases were reported from October 1983 to June 1984, with 60% of cases occurring in January and February. The seasonal pattern was similar throughout the state. Men and women were affected in approximately equal numbers, with the highest frequencies in the 30-39 years' age group. Clinical illness was uncommon in young children. Based on a detailed study of 118 patients from Griffith, arthralgia, lethargy, rash and headache were the most common symptoms. The average period of incapacity (defined as inability to resume normal duties) was about six weeks. On this basis, the cost of the epidemic was estimated at about $3 million. There were 257 cases of EPA in the Griffith Shire and it was calculated that approximately 340 RRV infections occurred in the shire. The implication is that, in this outbreak, most RRV infections resulted in clinical illness.