Murray Valley encephalitis and Australian X disease

The Complex Epidemiological Relationship between Flooding Events and Human Outbreaks of Mosquito-Borne Diseases: A Scoping Review

BACKGROUND

Climate change is expected to increase the frequency of flooding events. Although rainfall is highly correlated with mosquito-borne diseases (MBD) in humans, less research focuses on understanding the impact of flooding events on disease incidence. This lack of research presents a significant gap in climate change-driven disease forecasting.

OBJECTIVES

We conducted a scoping review to assess the strength of evidence regarding the potential relationship between flooding and MBD and to determine knowledge gaps.

METHODS

PubMed, Embase, and Web of Science were searched through 31 December 2020 and supplemented with review of citations in relevant publications. Studies on rainfall were included only if the operationalization allowed for distinction of unusually heavy rainfall events. Data were abstracted by disease (dengue, malaria, or other) and stratified by post-event timing of disease assessment. Studies that conducted statistical testing were summarized in detail.

RESULTS

From 3,008 initial results, we included 131 relevant studies (dengue n = 45 , malaria n = 61 , other MBD n = 49 ). Dengue studies indicated short-term (< 1  month ) decreases and subsequent (1-4 month) increases in incidence. Malaria studies indicated post-event incidence increases, but the results were mixed, and the temporal pattern was less clear. Statistical evidence was limited for other MBD, though findings suggest that human outbreaks of Murray Valley encephalitis, Ross River virus, Barmah Forest virus, Rift Valley fever, and Japanese encephalitis may follow flooding.

DISCUSSION

Flooding is generally associated with increased incidence of MBD, potentially following a brief decrease in incidence for some diseases. Methodological inconsistencies significantly limit direct comparison and generalizability of study results. Regions with established MBD and weather surveillance should be leveraged to conduct multisite research to a) standardize the quantification of relevant flooding, b) study nonlinear relationships between rainfall and disease, c) report outcomes at multiple lag periods, and d) investigate interacting factors that modify the likelihood and severity of outbreaks across different settings. https://doi.org/10.1289/EHP8887.

Exploiting the Legacy of the Arbovirus Hunters

In recent years, it has become evident that a generational gap has developed in the community of arbovirus research. This apparent gap is due to the dis-investment of training for the next generation of arbovirologists, which threatens to derail the rich history of virus discovery, field epidemiology, and understanding of the richness of diversity that surrounds us. On the other hand, new technologies have resulted in an explosion of virus discovery that is constantly redefining the virosphere and the evolutionary relationships between viruses. This paradox presents new challenges that may have immediate and disastrous consequences for public health when yet to be discovered arboviruses emerge. In this review we endeavor to bridge this gap by providing a historical context for the work being conducted today and provide continuity between the generations. To this end, we will provide a narrative of the thrill of scientific discovery and excitement and the challenges lying ahead.

Confronting the Emerging Threat to Public Health in Northern Australia of Neglected Indigenous Arboviruses

In excess of 75 arboviruses have been identified in Australia, some of which are now well established as causative agents of debilitating diseases. These include Ross River virus, Barmah Forest virus, and Murray Valley encephalitis virus, each of which may be detected by both antibody-based recognition and molecular typing. However, for most of the remaining arboviruses that may be associated with pathology in humans, routine tests are not available to diagnose infection. A number of these so-called 'neglected' or 'orphan' arboviruses that are indigenous to Australia might have been infecting humans at a regular rate for decades. Some of them may be associated with undifferentiated febrile illness-fever, the cause of which is not obvious-for which around half of all cases each year remain undiagnosed. This is of particular relevance to Northern Australia, given the Commonwealth Government's transformative vision for the midterm future of massive infrastructure investment in this region. An expansion of the industrial and business development of this previously underpopulated region is predicted. This is set to bring into intimate proximity infection-naïve human hosts, native reservoir animals, and vector mosquitoes, thereby creating a perfect storm for increased prevalence of infection with neglected Australian arboviruses. Moreover, the escalating rate and effects of climate change that are increasingly observed in the tropical north of the country are likely to lead to elevated numbers of arbovirus-transmitting mosquitoes. As a commensurate response, continuing assiduous attention to vector monitoring and control is required. In this overall context, improved epidemiological surveillance and diagnostic screening, including establishing novel, rapid pan-viral tests to facilitate early diagnosis and appropriate treatment of febrile primary care patients, should be considered a public health priority. Investment in a rigorous identification program would reduce the possibility of significant outbreaks of these indigenous arboviruses at a time when population growth accelerates in Northern 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.

The importance of males: larval diet and adult sugar feeding influences reproduction in Culex molestus

Culex molestus is an obligatory autogenous mosquito that is closely associated with subterranean habitats in urban areas. The objective of our study was to investigate the influence of larval and adult nutrition on the role of males in determining the expression of autogeny in Cx. molestus. Mosquitoes raised at low and high larval diets had sex ratio, wing length, mating rates, autogenous egg raft size, and hatching rates recorded. There was a higher ratio of males to females when raised at a low larval diet. Mean wing lengths of both males and females were significantly greater when raised at the high larval diet regime. Regardless of larval or adult diet, males mated with only a single female. Mosquitoes raised at the higher larval diet regimes developed significantly more autogenous eggs. However, the egg raft size was reduced when adult females were denied access to sugar. The results of this study indicate that the performance of males in the reproductive process is influenced by both larval diet and adult sugar feeding.

Is the expression of autogeny by Culex molestus Forskal (Diptera: Culicidae) influenced by larval nutrition or by adult mating, sugar feeding, or blood feeding?

Culex molestus Forskal is suspected to have been introduced into southern Australia during the 1940s. Investigations to determine factors influencing the expression of autogeny, the response of this mosquito to potential blood meals, and the subsequent influence on oviposition were undertaken. Immature mosquitoes raised at five feeding regimes had mortality rates, development rates, wing length, and autogenous egg raft size measured. All surviving female mosquitoes laid autogenous eggs but there was a significant difference between the mean number of eggs per raft. For mosquitoes raised at each of the feeding regimes, there was a significant linear relationship between the number of eggs per autogenous egg raft and wing length. Newly emerged mosquitoes were offered a blood meal (i.e., rodent) daily but no blood feeding occurred until the autogenous egg raft was laid. There was no statistical difference in the rate of autogenous oviposition or post-oviposition blood feeding between control or treatment groups. The results of this study indicate that Cx. molestus is perfectly adapted to subterranean habitats in close association with human habitation, but their preference to delay blood feeding until up to day 8 following emergence may reduce their relative importance as a vector of arboviruses.

The contribution of rodent models to the pathological assessment of flaviviral infections of the central nervous system

Members of the genus Flavivirus are responsible for a spectrum of important neurological syndromes in humans and animals. Rodent models have been used extensively to model flavivirus neurological disease, to discover host-pathogen interactions that influence disease outcome, and as surrogates to determine the efficacy and safety of vaccines and therapeutics. In this review, we discuss the current understanding of flavivirus neuroinvasive disease and outline the host, viral and experimental factors that influence the outcome and reliability of virus infection of small-animal models.

Waterbird movement across the Great Dividing Range and implications for arbovirus irruption into southern Victoria

BACKGROUND

  Waterbirds are the major hosts of various arboviruses. Murray Valley encephalitis virus (MVEV) is an arbovirus native to northern Australia, the major hosts of which are Phalacrocoraciformes (cormorants), Ciconiiformes (herons) and other waterbirds. MVEV is transmitted to humans by mosquitoes and can cause acute encephalomyelitis. In Victoria, MVEV is restricted to the northern side of the Great Dividing Range (GDR), suggesting that waterbirds cannot cross the high country.

METHODS AND RESULTS

  We tested this hypothesis by analysing data on waterbird banding and recovery and discovered that 12 species can cross the GDR.

CONCLUSION

  Waterbirds have the potential to carry arboviruses, including MVEV, into southern Victoria.

Chapter 48: history of neurology in Australia and New Zealand

In comparison with most Western countries, neurology as a recognized medical specialty has a relatively brief history in Australia: the national body for neurologists, the Australian (since 2006: and New Zealand) Association of Neurologists, was founded only in 1950. The development of neurology in both countries was heavily influenced by British neurology, and until recently a period in the National Hospital for Neurology and Neurosurgery, Queen Square, London was regarded as essential to specialist training in neurology. Nevertheless, Australians have made significant contributions to international neurology since the early 20th century, commencing with the neuroanatomical research of the colorful expatriate Grafton Elliot Smith (1871-1937). Other Australian physicians who attracted early international attention through their work in clinical neuroscience included William John Adie (1886-1935), the anatomist John Irvine Hunter (1898-1924) and the surgeon Norman Royle (1888-1944). The first Australian physician to unambiguously commit himself to neurology was Alfred Walter Campbell (1868-1938), a remarkable personality who established an imposing reputation as neurocytologist and neuropathologist. The chapter provides a concise overview of the development of neurology as a clinical and academic specialty in Australia and New Zealand.

Australian X disease, Murray Valley encephalitis and the French connection

Epidemics of a severe encephalitis occurred in eastern Australia between 1917 and 1925, in which over 280 cases were reported with a fatality rate of 68%. The disease had not been described previously and was called Australian X disease. The next epidemic occurred in south-east Australia in the summer of 1950-51. The disease was given its name of Murray Valley encephalitis as this was the area from which most cases were reported. A virus was isolated by Eric French in Victoria, and about the same time by John Miles and colleagues in South Australia. The virus Murray Valley encephalitis (MVE) virus, was shown to be a Group B arbovirus (flavivirus) which was related to, but distinct from, Japanese encephalitis virus. Early seroepidemiological studies showed that the most likely vertebrate hosts were water birds. MVE virus was first isolated from Culex annulirostris mosquitoes in 1960. The most recent epidemic of Murray Valley encephalitis occurred in 1974, at which time it was renamed Australian encephalitis. Since 1974, however, all cases have been confined to northern Australia, particularly the north of Western Australia. Indeed, the Kimberley region of Western Australia contains the only confirmed enzootic foci of virus activity. A closely related flavivirus, Kunjin virus, has also been shown to be an aetiological agent of Australian encephalitis. Since the first isolation of MVE and Kunjin viruses, considerable information has been accumulated on their ecology and epidemiology, some aspects of which are briefly described.

Australian encephalitis in Western Australia, 1978-1991

OBJECTIVE

To review the various clinical manifestations of Murray Valley encephalitis (MVE) or Kunjin virus encephalitis in patients in Western Australia.

DESIGN

Review of clinical records, 1978 to 1991.

PATIENTS

Of 26 reported cases of Australian encephalitis, four were excluded from study because the patient's symptoms were not definitely associated with MVE virus or Kunjin virus infection. Two further cases of MVE were not reviewed as case records were not available. Of the remaining 20 patients, 18 had MVE and two had Kunjin virus encephalitis.

RESULTS

Sixteen cases were in the Kimberley, a tropical region where the viruses are endemic. Four were in the subtropical Pilbara and Gascoyne regions. Thirteen of the 20 cases were in Aborigines, of whom 11 were children. The seven non-Aboriginal patients were adults. Seventeen of 20 cases were in males. The range of neurological disease and outcome was similar to that in previously reported cases, with convulsions, brainstem disease or respiratory failure in severe and fatal cases, and involvement of the spinal cord, cranial nerve or cerebellum in the moderate cases. One mild cases without neurological involvement was caused by Kunjin virus.

CONCLUSIONS

The poor outcome in young Aboriginal children indicated that disease resulting from exposure early in life was more likely to be severe. The disease in adults, irrespective of facial background, was similar to that in cases reported previously from south-eastern Australia, but generally milder.

Australia's contribution to tropical health: past and present

Australian research workers have made important contributions to tropical medicine and tropical public health. Recognised high points of international significance (for example, Joseph Bancroft and filariasis, 1876; Thomas Bancroft and dengue fever, 1906; Burnet and Australian X disease, 1934; Derrick and Q fever, 1937; and Fairley and malaria, 1947) must be seen in the context of much detailed work of national relevance by institutions and individuals. The directions of Australian research can be related to several major themes: the large extent of Australia that is tropical or subtropical; interactions with neighbouring countries, especially Papua New Guinea; and concern for the health of Australian Aborigines.

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.

Weather, host and vector--their interplay in the spread of insect-borne animal virus diseases

The spread of insect-borne animal virus diseases is influenced by a number of factors. Hosts migrate, move or are conveyed over long distances: vectors are carried on the wind for varying distances in search of hosts and breeding sites; weather and climate affect hosts and vectors through temperature, moisture and wind. As parasites of host and vector, viruses are carried by animals, birds and insects, and their spread can be correlated with the migration of hosts and the carriage of vectors on winds associated with the movements of the Intertropical Convergence Zone (ITCZ) and warm winds to the north and south of the limits of the ITCZ. The virus is often transmitted from a local cycle to a migratory cycle and back again.Examples of insect-borne virus diseases and their spread are analysed. Japanese, Murray Valley, Western equine, Eastern equine and St Louis encephalitis represent viruses transmitted by mosquito-bird or pig cycles.THE AREAS EXPERIENCING INFECTION WITH THESE VIRUSES CAN BE DIVIDED INTO A NUMBER OF ZONES: A, B, C, D, E and F. In zone A there is a continuous cycle of virus in host and vector throughout the year; in zone B, there is an upsurge in the cycle during the wet season, but the cycle continues during the dry season; there is movement of infected vectors between and within zones A and B on the ITCZ and the virus is introduced to zone C by infected vectors on warm winds; persistence may occur in zone C if conditions are right. In zone D, virus is introduced each year by infected vectors on warm winds and the arrival of the virus coincides with the presence of susceptible nestling birds and susceptible piglets. The disappearance of virus occurs at the time when migrating mosquitoes and birds are returning to warmer climates. The virus is introduced to zone E only on occasions every 5-10 years when conditions are suitable. Infected hosts introduced to zone F do not lead to circulation of virus, since the climate is unsuitable for vectors. Zones A, B and C correspond to endemic and zones D and E to epidemic conditions.Similar zones can be recognized for African horse sickness, bluetongue, Ibaraki disease and bovine ephemeral fever - examples of diseases transmitted in a midge-mammal cycle. In zones A and B viruses are transported by infected midges carried on the wind in association with the movement of ITCZ and undergo cycles in young animals. In these zones and in zone C there is a continual movement of midges on the warm wind between one area and another, colonizing new sites or reinforcing populations of midges already present. Virus is introduced at times into fringe areas (zones D and E) and, as there is little resistance in the host, gives rise to clinical signs of disease. In some areas there is persistence during adverse conditions; in others, the virus is carried back to the endemic zones by infected midges or vectors.Examples of viruses maintained in a mosquito/biting fly-mammal cycle are Venezuelan equine encephalitis and vesicular stomatitis. These viruses enter a migratory cycle from a local cycle and the vectors in the migratory cycle are carried over long distances on the wind. Further examples of virus spread by movement of vectors include West Nile, Rift Valley fever, yellow fever, epizootic haemorrhagic disease of deer and Akabane viruses.In devising means of control it is essential to decide the relationship of host, vector and virus and the nature of the zone in which the area to be controlled lies. Because of the continual risk of reintroduction of infected vectors, it is preferable to protect the host by dipping, spraying or by vaccination rather than attempting to eliminate the local population of insects.

Association of Australian arboviruses with nervous disease in horses

An outbreak of Murray Valley encephalitis (MVE) occurred in New South Wales during the first five months of 1974. Specimens from 52 horses with nervous disease collected January to May 1974 were examined histopathological or virologically. Although MVE virus was not isolated, 13 horses had serological evidence of recent infection with MVE virus. Another 4 horses had evidence of recent infection with Ross River virus. Two animals had histological evidence of viral infection of the central nervous system. Attempts to experimentally infect 2 horses with a low dose of MVE virus were not successful by intravenous, intramuscular and subcutaneous routes.

Isolation of Murray Valley encephalitis virus from sentinel chickens

Sentinel chickens were established in 3 centres along the Murray Valley on 20 December 1973. The demonstration of antibody in the serum of chickens in the Mildura and Kerang areas indicated Sindbis virus activity late in December 1973 and early in January 1974. Tests for antibody to MVE virus were negative until blood collected from one chicken at Echuca on 27 February 1974 and several chickens at Mildura and Kerang on 14 March 1974, showed positive HI reactions. Murray Valley encephalitis virus was isolated from blood collected from chickens at Echuca and Kerang respectively on 31 January 1974 and 27 February 1974.

Epidemiology of the arthropod-borne encephalitides

Since the recognition that louping-ill, known for well over 100 years as an epizootic disease of sheep in Scotland, was caused by a virus transmitted by arthropods, many other arthropod-borne viruses capable of causing encephalitis in domestic animals or man have been discovered. The author reviews here the knowledge at present available on these viruses, originally termed "arthropod-borne encephalitides viruses" but now often referred to as "arbor viruses".In this discussion of the host and vector relationships of the two broad groups of arbor viruses - the mosquito-borne and the tick-borne-and of the distribution, epidemiology and control of the various diseases they cause, the author includes an outline of the types of investigation likely to provide the most useful information, stressing in this connexion the value of ecological surveys.