Human Seroprevalence for Dengue, Ross River, and Barmah Forest viruses in Australia and the Pacific: A systematic review spanning seven decades

Background

Dengue (DENV), Ross River (RRV) and Barmah Forest viruses (BFV) are the most common human arboviral infections in Australia and the Pacific Island Countries and Territories (PICTs) and are associated with debilitating symptoms. All are nationally notifiable in Australia, but routine surveillance is limited to a few locations in the PICTs. Understanding the level of human exposure to these viruses can inform disease management and mitigation strategies. To assess the historic and current seroprevalence of DENV, RRV and BFV in Australia and the PICTs we conducted a systematic literature review of all published quantitative serosurveys.

Methodology and principal findings

The Preferred Reporting of Items for Systematic Reviews and Meta-Analyses procedures were adopted to produce a protocol to systematically search for published studies reporting the seroprevalence of DENV, RRV and BFV in Australia and the PICTs. Data for author, research year, location, study population, serosurvey methods and positive tests were extracted. A total of 41 papers, reporting 78 serosurveys of DENV, RRV and BFV including 62,327 samples met the inclusion criteria for this review. Seroprevalence varied depending on the assay used, strategy of sample collection and location of the study population. Significant differences were observed in reported seropositivity depending on the sample collection strategy with clinically targeted sampling reporting the highest seroprevalence across all three viruses. Non-stratified seroprevalence showed wide ranges in reported positivity with DENV 0.0% – 95.6%, RRV 0.0% – 100.0%, and BFV 0.3% – 12.5%. We discuss some of the causes of variation including serological methods used, selection bias in sample collection including clinical or environmental associations, and location of study site. We consider the extent to which serosurveys reflect the epidemiology of the viruses and provide broad recommendations regarding the conduct and reporting of arbovirus serosurveys.

Conclusions and significance

Human serosurveys provide important information on the extent of human exposure to arboviruses across: (1) time, (2) place, and (3) person (e.g., age, gender, clinical presentation etc). Interpreting results obtained at these scales has the potential to inform us about transmission cycles, improve diagnostic surveillance, and mitigate future outbreaks. Future research should streamline methods and reduce bias to allow a better understanding of the burden of these diseases and the factors associated with seroprevalence. Greater consideration should be given to the interpretation of seroprevalence in studies, and increased rigour applied in linking seroprevalence to transmission dynamics.

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.

Ross River Virus Infection of Horses: Appraisal of Ecological and Clinical Consequences

Ross River virus (RRV) is a mosquito-borne arbovirus of the genus Alphavirus that causes disease in humans and horses in Australia. A temporal association of RRV infection in horses with clinical signs including pyrexia, malaise, and polyarthralgia has been reported, along with reduced athletic performance, often for extended periods. Despite these reports, disease due to RRV remains somewhat controversial as experimental infection of horses has resulted in obvious viraemia yet minimal signs of clinical disease. The relatively high viraemia demonstrated by horses infected with RRV has led to speculation that they could act as an important reservoir host of the virus, although this remains unclear. This review sought to appraise the existing literature relating to RRV infection of horses and to summarize the ecological and clinical consequences of RRV of relevance to the equine industry and to public health more broadly.

Neglected Australian Arboviruses Associated With Undifferentiated Febrile Illnesses

Infections with commonly occurring Australian arthropod-borne arboviruses such as Ross River virus (RRV) and Barmah Forest virus (BFV) are diagnosed routinely by pathology laboratories in Australia. Others, such as Murray Valley encephalitis (MVEV) and Kunjin (KUNV) virus infections may be diagnosed by specialist reference laboratories. Although Alfuy (ALFV), Edge Hill (EHV), Kokobera (KOKV), Sindbis (SINV), and Stratford (STRV) viruses are known to infect humans in Australia, all are considered 'neglected.' The aetiologies of approximately half of all cases of undifferentiated febrile illnesses (UFI) in Australia are unknown and it is possible that some of these are caused by the neglected arboviruses. The aims of this study were to determine the seroprevalence of antibodies against several neglected Australian arboviruses among residents of Queensland, north-east Australia, and to ascertain whether any are associated with UFI. One hundred age- and sex-stratified human plasma samples from blood donors in Queensland were tested to determine the prevalence of neutralising antibodies against ALFV, BFV, EHV, KOKV, KUNV, MVEV, RRV, SINV, and STRV. The seroconversion rates for RRV and BFV infections were 1.3 and 0.3% per annum, respectively. The prevalence of antibodies against ALFV was too low to enable estimates of annual infection rates to be determined, but the values obtained for other neglected viruses, EHV (0.1%), KOKV (0.05%), and STRV (0.05%), indicated that the numbers of clinical infections occurring with these agents are likely to be extremely small. This was borne out by the observation that only 5.7% of a panel of 492 acute phase sera from UFI patients contained IgM against any of these arboviruses, as detected by an indirect immunofluorescence assay. While none of these neglected arboviruses appear to be a cause of a significant number of UFIs in Australia at this time, each has the potential to emerge as a significant human pathogen if there are changes to their ecological niches.

Ross River virus evolution: implications for vaccine development

Ross River virus is a mosquito-borne alphavirus that causes approximately 5000 cases of epidemic polyarthritis in Australia each year and has direct medical-associated costs of approximately US$15 million annually. While mosquito control programs are able, at best, to contain rather than prevent this disease, natural infection with Ross River virus confers lifelong protection against subsequent clinical infection. A killed-virus vaccine has been developed, which is in Phase III clinical trials. Analyses of intra-host genetic diversity and of long-term evolutionary changes in Ross River virus populations suggest that antigenic variation is unlikely to pose a threat to the efficacy of this vaccine.

Evaluation of an inactivated Ross River virus vaccine in active and passive mouse immunization models and establishment of a correlate of protection

Ross River Virus has caused reported outbreaks of epidemic polyarthritis, a chronic debilitating disease associated with significant long-term morbidity in Australia and the Pacific region since the 1920s. To address this public health concern, a formalin- and UV-inactivated whole virus vaccine grown in animal protein-free cell culture was developed and tested in preclinical studies to evaluate immunogenicity and efficacy in animal models. After active immunizations, the vaccine dose-dependently induced antibodies and protected adult mice from viremia and interferon α/β receptor knock-out (IFN-α/βR(-/-)) mice from death and disease. In passive transfer studies, administration of human vaccinee sera followed by RRV challenge protected adult mice from viremia and young mice from development of arthritic signs similar to human RRV-induced disease. Based on the good correlation between antibody titers in human sera and protection of animals, a correlate of protection was defined. This is of particular importance for the evaluation of the vaccine because of the comparatively low annual incidence of RRV disease, which renders a classical efficacy trial impractical. Antibody-dependent enhancement of infection, did not occur in mice even at low to undetectable concentrations of vaccine-induced antibodies. Also, RRV vaccine-induced antibodies were partially cross-protective against infection with a related alphavirus, Chikungunya virus, and did not enhance infection. Based on these findings, the inactivated RRV vaccine is expected to be efficacious and protect humans from RRV disease.

Environmental monitoring to enhance comprehension and control of infectious diseases

In a world of emerging and resurging infectious diseases, dominated by zoonoses, environmental monitoring plays a vital role in our understanding their dynamics and their spillover to humans. Here, we critically review the ecology, epidemiology and need for monitoring of a variety of directly transmitted (Sin Nombre virus, Avian Influenza) and vector-borne (Ross River virus, West Nile virus, Lyme disease, anaplasmosis and babesiosis) zoonoses. We focus on the valuable role that existing monitoring plays in the understanding of these zoonoses, the demands for new monitoring, and how improvements can be made to existing monitoring. We also identify the fruitful outcomes which would result from implementation of the monitoring demands we have highlighted. This review aims to promote improvements in our understanding of zoonoses, their management, and public health by encouraging discussion among researchers and public health officials.

Influence of hosts on the ecology of arboviral transmission: potential mechanisms influencing dengue, Murray Valley encephalitis, and Ross River virus in Australia

Ecological interactions are fundamental to the transmission of infectious disease. Arboviruses are particularly elegant examples, where rich arrays of mechanisms influence transmission between vectors and hosts. Research on host contributions to the ecology of arboviral diseases has been undertaken within multiple subdisciplines, but significant gaps in knowledge remain and multidisciplinary approaches are needed. Through our multidisciplinary review of the literature we have identified five broad areas where hosts may influence the ecology of arboviral transmission: host immunity; cross-protective immunity and antibody-dependent enhancement; host abundance; host diversity; and pathogen spillover and dispersal. Herein we discuss the known and theoretical roles of hosts within these topics and then apply this knowledge to three epidemiologically important mosquito-borne arboviruses that occur in Australia: dengue virus (DENV), Murray Valley encephalitis virus (MVEV), and Ross River virus (RRV). We argue that the underlying mechanisms by which hosts influence arboviral activity are numerous and attempts to delineate these mechanisms further are needed. Investigations that focus on hosts of vector-borne diseases are likely to be rewarding, particularly where the ecology of vectors is relatively well understood. From an applied perspective, enhanced knowledge of host influences upon vector-borne disease transmission is likely to enable better management of disease burden. Finally, we suggest a framework that may be useful to identify and determine host contributions to the ecology of arboviruses.

The preclinical testing of a formaldehyde inactivated Ross River virus vaccine designed for use in humans

Ross River virus was grown in industrial facilities in vaccine-certified Vero cells in the absence of serum, inactivated using standard formalin-inactivation protocols, treated with Benzonase to digest host cell DNA and purified on a sucrose gradient. Mice given two subcutaneous injections of 0.625 microg of this vaccine or two doses of 0.156 microg vaccine with aluminium hydroxide adjuvant failed to develop a detectable viraemia after intravenous challenge with 10(6)TCID50 of the prototype strain of Ross River virus (T48). Guinea pigs immunised with one or two10 microg doses of vaccine with adjuvant also failed to develop a detectable viraemia following a similar challenge. The levels of neutralising antibody (neutralisation index 1.9-3.1) in the mice protected against challenge with 10(6)TCID50 Ross River virus were similar to those in 16 former epidemic polyarthritis patients (1.1-3.5) who had not experienced a second clinical infection with Ross River virus in the 20 years following their initial infection.

Detection of Ross River virus in clinical samples using a nested reverse transcription-polymerase chain reaction

BACKGROUND

Ross River virus (RRV) is a mosquito borne alphavirus that has been found in Australia, Papua New Guinea and the Pacific Islands. It is aetiological agent of epidemic polyarthritis, a debilitating illness whose symptoms are arthritis, arthralgia, lethargy, rash and fever which may persist for weeks or months. Diagnosis is made on a serological basis, but in many cases is presumptive rather than definite.

OBJECTIVES

To apply the polymerase chain reaction (PCR) to detection of RRV in human sera to assess its suitability for application in disease diagnosis.

STUDY DESIGN

Sensitivity of the nested RT-PCR assay was determined by detection of virus of known titre diluted in uninfected serum. Clinical serum samples from patients serologically diagnosed of having RRV infection were tested by nested RT-PCR to assess its diagnostic value.

RESULTS

Sensitivity of the nested RT-PCR assay was determined to be detection of 0.01 PFU of virus stock in 100 mul serum. Clinical samples tested showed that 10 of 26 (38%) serum samples with low or negative (non-diagnostic) virus-specific antibody titres were PCR-positive, whereas all 22 specimens with high antibody titres were PCR-negative. PCR positivity was unaffected by repeated freezing and thawing of samples.

CONCLUSIONS

While PCR cannot replace serology as a means of RRV diagnosis, it may be useful in conjunction with serological testing, particularly for forming definitive diagnoses in those samples with low (inconclusive) antibody titres. It is faster and more sensitive than virus isolation by tissue culture, and could also prove useful in investigations of disease pathogenesis.

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.

Specific enzyme immunoassays for the rapid detection of Ross River virus in cell cultures inoculated with infected mosquito homogenates

BACKGROUND

Ross River (RR) virus is a mosquito-borne alphavirus and one of the aetiological agents of epidemic polyarthritis in humans. Early detection of increased virus activity in mosquito populations enables public health authorities to implement measures to reduce the number of human infections during epidemics. However, current surveillance techniques require a minimum of four weeks for viruses to be isolated and identified.

OBJECTIVES

This study was carried out to assess the use of enzyme immunoassays (EIA) as rapid alternatives to traditional cell culture techniques for detection of RR virus in mosquitoes.

STUDY DESIGN

Enzyme immunoassays and immunoperoxidase assays were developed using RR-specific monoclonal antibodies and compared to traditional methods for detection of RR virus in field-caught mosquito samples.

RESULTS

By inoculation of C6/36 cell cultures with mosquito homogenates and testing monolayers and culture supernatant by EIA, RR virus was detected and identified in all infected samples within 6 days.

CONCLUSIONS

The use of EIA provides a rapid, sensitive and specific alternative to traditional methods for the detection of RR virus in mosquito vectors.

Ross River virus disease in Australia, 1886-1998, with analysis of risk factors associated with outbreaks

Ross River virus (RR) is a mosquito-borne arbovirus responsible for outbreaks of polyarthritic disease throughout Australia. To better understand human and environmental factors driving such events, 57 historical reports on RR outbreaks between 1896 and 1998 were examined collectively. The magnitude, regularity, seasonality, and locality of outbreaks were found to be wide ranging; however, analysis of climatic and tidal data highlighted that environmental conditions act differently in tropical, arid, and temperate regions. Overall, rainfall seems to be the single most important risk factor, with over 90% of major outbreak locations receiving higher than average rainfall in preceding months. Many temperatures were close to average, particularly in tropical populations; however, in arid regions, below average maximum temperatures predominated, and in southeast temperate regions, above average minimum temperatures predominated. High spring tides preceded coastal outbreaks, both in the presence and absence of rainfall, and the relationship between rainfall and the Southern Oscillation Index and La Niña episodes suggest they may be useful predictive tools, but only in southeast temperate regions. Such heterogeneity predisposing outbreaks supports the notion that there are different RR epidemiologies throughout Australia but also suggests that generic parameters for the prediction and control of outbreaks are of limited use at a local level.

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.

Enhancement or modulation of the vector competence of Ochlerotatus vigilax (Diptera: Culicidae) for ross river virus by temperature

Two different doses of Ross River virus (RR) were fed to Ochlerotatus vigilax (Skuse), the primary coastal vector in Australia; and blood engorged females were held at different temperatures up to 35 d. After ingesting 10(4.3) CCID50/mosquito, mosquitoes reared at 18 and 25 degrees C (and held at the same temperature) had higher body remnant and head and salivary gland titers than those held at 32 degrees C. although infection rates were comparable. At 18, 25, and 32 degrees C, respectively, virus was first detected in the salivary glands on days 3, 2, and 3. Based on a previously demonstrated 98.7% concordance between salivary gland infection and transmission, the extrinsic incubation periods were estimated as 5, 4, and 3 d, respectively, for these three temperatures. When Oc. vigilax reared at 18, 25, or 32 degrees C were fed a lower dosage of 10(3.3) CCID50 RR/mosquito, and assayed after 7 d extrinsic incubation at these (or combinations of these) temperatures, infection rates and titers were similar. However, by 14 d, infection rates and titers of those reared and held at 18 and 32 degrees C were significantly higher and lower, respectively. However, this process was reversible when the moderate 25 degrees C was involved, and intermediate infection rates and titers resulted. These data indicate that for the strains of RR and Oc. vigilax used, rearing temperature is unimportant to vector competence in the field, and that ambient temperature variations will modulate or enhance detectable infection rates only after 7 d extrinsic incubation. Because of the short duration of extrinsic incubation, however, this will do little to influence RR epidemiology, because by this time some Oc. vigilax could be seeking their third blood meal, the latter two being infectious.

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.

O'nyong-nyong fever in south-central Uganda, 1996-1997: clinical features and validation of a clinical case definition for surveillance purposes

O'nyong-nyong (ONN) fever, caused by infection with a mosquito-borne central African alphavirus, is an acute, nonfatal illness characterized by polyarthralgia. During 1996-1997, south-central Uganda experienced the second ONN fever epidemic ever recognized. Among 391 persons interviewed and sampled, 40 cases of confirmed and 21 of presumptive, well-characterized acute, recent, or previous ONN fever were identified through active case-finding efforts or during a household serosurvey and by the application of clinical and laboratory criteria. Among confirmed cases, the knees and ankles were the joints most commonly affected. The median duration of arthralgia was 6 days (range, 2-21 days) and of immobilization was 4 days (range, 1-14 days). In the majority, generalized skin rash was reported, and nearly half had lymphadenopathy, mainly of the cervical region. Viremia was documented in 16 cases, primarily during the first 3 days of illness, and in some of these, body temperature was normal. During this epidemic, the combination of fever, arthralgia, and lymphadenopathy had a specificity of 83% and a sensitivity of 61% in the identification of cases of ONN fever and thus could be useful for surveillance purposes.

Aspergillosis in children with cancer: A 34-year experience

A retrospective review of medical records, microbiology and pathology laboratory records, and nosocomial infection surveillance data was undertaken to describe the experience with culture-documented aspergillus infection in pediatric cancer patients at our facility. Sixty-six patients were identified from a 34-year period. The most common underlying diagnosis was leukemia. Risk factors included neutropenia, immunosuppression, and prior antibiotic therapy. On the basis of clinical presentation, 23 patients were believed to have disseminated disease and 43 to have localized disease. The lung was the most frequently affected organ. Despite aggressive medical and surgical management, overall mortality was 85% within the first year after diagnosis. Patients who presented with disease in sites other than the lungs fared better than patients with initial pulmonary involvement (P=.0014). Aspergillosis continues to be associated with poor outcome. Development of improved medical and adjuvant therapies, including surgery, is warranted.

Dissemination barriers to Ross River virus in Aedes vigilax and the effects of larval nutrition on their expression

Effects of larval nutrition on vector competence of the mosquito Aedes vigilax (Skuse) (Diptera: Culicidae) from Townsville, north Queensland, for Ross River virus (RR) were examined. Larvae were reared on three different diets to create three significantly different size classes of adult mosquito. These were fed on serial dilutions of RR and then sampled on alternate days so the progression of the virus through the mosquito could be examined. No differences of vector competence could be attributed to larval nutrition. Barriers to infection and the correlation between infection rate, viral titre and transmission of RR by Ae. vigilax were also examined. The mesenteronal barrier was the only infection barrier expressed. No correlation between viral titre and transmission was detected, but a strong correlation was found between salivary gland infection and transmission rate. From this it was possible to estimate that 98.7+/-1.3% of Ae. vigilax with infected salivary glands transmit RR.

Clinical and subclinical Barmah Forest virus infection in Queensland

Barmah Forest virus is a mosquito-borne agent (alphavirus) reported to cause both clinical and subclinical infections in New South Wales. This report describes 29 cases of clinical Barmah Forest virus infection diagnosed between July 1988 and March 1989 (21 from Queensland, six from New South Wales and two from Victoria) and provides evidence of extensive subclinical infection with this virus (0.23% of the population per annum) throughout Queensland. It also includes a description of the first isolation of Barmah Forest virus from a patient. Data obtained in the course of the study suggest that Barmah Forest virus infections may not be diagnosed correctly in many instances because of the similarity of the symptoms of this disease to those of epidemic polyarthritis and the small number of laboratories providing the necessary serological services.

Arbovirus infection in a Murray Valley community

Serum antibodies to Ross River virus and Murray Valley encephalitis virus were measured during 1974-1975 in residents of Echuca, an urban Murray Valley community. A representative group of volunteers was obtained by random selection of households. The prevalence of antibodies to both viruses increased progressively with age. Prevalence was equal in both sexes for both viruses in all age groups, indicating that the risk of infection was mainly determined by geography rather than by personal activities. Antibody levels remained unchanged in the following year when there was no disease activity in the area. The stability of antibody levels permitted retrospective estimates of mean rates of infection. These were approximately 0.4% per annum for both viruses when age was used as the index for years of exposure. With allowance for other factors, the best estimate for both virus infections is probably closer to 1%. The morbidity rate for Ross River virus infection appeared to be low. It is concluded that infection with Ross River virus and with Murray Valley encephalitis-related viruses is endemic in the Murray Valley.

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.

Detection of Ross River virus immunoglobulin M antibodies by enzyme-linked immunosorbent assay using antibody class capture and comparison with other methods

An enzyme-linked immunosorbent assay based on antibody class capture was developed for the detection of Ross River virus-specific immunoglobulin M antibodies (RRV IgM). The assay was specific, reproducible and precise. When compared with conventional tests for the detection of RRV IgM, such as hemagglutination inhibition following sucrose density gradient centrifugation and indirect enzyme-linked immunosorbent assay, the class capture assay was more sensitive. In 186 sera which were collected from 39 patients with RRV infection over a period of 1-4 yr from onset of initial symptoms, RRV IgM persisted for at least 1-2 yr. Sera were tested both at a single dilution from which the results were expressed as a binding index and in a dilution series in which they were expressed as an antibody titre. Binding index values gave better discrimination between sera collected during acute and later phases of the disease and may be of greater value than antibody titres in the diagnosis of RRV infection.