Variation in arbovirus infection rates in species of birds sampled in a serological survey during an encephalitis epidemic in the Murray Valley of South-eastern Australia, February 1974

An integrated public health response to an outbreak of Murray Valley encephalitis virus infection during the 2022-2023 mosquito season in Victoria

Introduction

Murray Valley encephalitis virus (MVEV) is a mosquito-borne flavivirus known to cause infrequent yet substantial human outbreaks around the Murray Valley region of south-eastern Australia, resulting in significant mortality.

Methods

The public health response to MVEV in Victoria in 2022-2023 included a climate informed pre-season risk assessment, and vector surveillance with mosquito trapping and laboratory testing for MVEV. Human cases were investigated to collect enhanced surveillance data, and human clinical samples were subject to serological and molecular testing algorithms to assess for co-circulating flaviviruses. Equine surveillance was carried out via enhanced investigation of cases of encephalitic illness. Integrated mosquito management and active health promotion were implemented throughout the season and in response to surveillance signals.

Findings

Mosquito surveillance included a total of 3,186 individual trapping events between 1 July 2022 and 20 June 2023. MVEV was detected in mosquitoes on 48 occasions. From 2 January 2023 to 23 April 2023, 580 samples (sera and CSF) were tested for flaviviruses. Human surveillance detected 6 confirmed cases of MVEV infection and 2 cases of "flavivirus-unspecified." From 1 September 2022 to 30 May 2023, 88 horses with clinical signs consistent with flavivirus infection were tested, finding one probable and no confirmed cases of MVE.

Discussion

The expanded, climate-informed vector surveillance system in Victoria detected MVEV in mosquitoes in advance of human cases, acting as an effective early warning system. This informed a one-health oriented public health response including enhanced human, vector and animal surveillance, integrated mosquito management, and health promotion.

Emerging and Novel Viruses in Passerine Birds

There is growing interest in emerging viruses that can cause serious or lethal disease in humans and animals. The proliferation of cloacal virome studies, mainly focused on poultry and other domestic birds, reveals a wide variety of viruses, although their pathogenic significance is currently uncertain. Analysis of viruses detected in wild birds is complex and often biased towards waterfowl because of the obvious interest in avian influenza or other zoonotic viruses. Less is known about the viruses present in the order Passeriformes, which comprises approximately 60% of extant bird species. This review aims to compile the most significant contributions on the DNA/RNA viruses affecting passerines, from traditional and metagenomic studies. It highlights that most passerine species have never been sampled. Especially the RNA viruses from Flaviviridae, Orthomyxoviridae and Togaviridae are considered emerging because of increased incidence or avian mortality/morbidity, spread to new geographical areas or hosts and their zoonotic risk. Arguably poxvirus, and perhaps other virus groups, could also be considered "emerging viruses". However, many of these viruses have only recently been described in passerines using metagenomics and their role in the ecosystem is unknown. Finally, it is noteworthy that only one third of the viruses affecting passerines have been officially recognized.

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

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

Species Traits and Hotspots Associated with Ross River Virus Infection in Nonhuman Vertebrates in South East Queensland

Ross River virus (RRV) is a mosquito-borne zoonotic arbovirus associated with high public health and economic burdens across Australia, but particularly in South East Queensland (SEQ). Despite this high burden, humans are considered incidental hosts. Transmission of RRV is maintained among mosquitoes and many nonhuman vertebrate reservoir hosts, although the relative contributions of each of these hosts are unclear. To clarify the importance of a range of vertebrates in RRV transmission in SEQ, a total of 595 serum samples from 31 species were examined for RRV exposure using a gold-standard plaque reduction neutralization test. Data were analyzed statistically using generalized linear models and a coefficient inference tree, and spatially. RRV exposure was highly variable between and within species groups. Critically, species group ("placental mammal," "marsupial," and "bird"), which has previously been used as a proxy for reservoir hosts, was a poor correlate for exposure. Instead, we found that generalized "diet" and greater "body mass" were most strongly correlated with seropositivity. We also identified significant differences in seropositivity between the two major possum species (ringtail possums and brushtail possums), which are ecologically and taxonomically different. Finally, we identified distinct hotspots and coldspots of seropositivity in nonhuman vertebrates, which correlated with human notification data. This is the largest diversity of species tested for RRV in a single study to date. The analysis methods within this study provide a framework for analyzing serological data in combination with species traits for other zoonotic disease, but more specifically for RRV highlight areas to target further public health research and surveillance effort.

Zoonotic Viral Diseases of Equines and Their Impact on Human and Animal Health

INTRODUCTION

Zoonotic diseases are the infectious diseases that can be transmitted to human beings and vice versa from animals either directly or indirectly. These diseases can be caused by a range of organisms including bacteria, parasites, viruses and fungi. Viral diseases are highly infectious and capable of causing pandemics as evidenced by outbreaks of diseases like Ebola, Middle East Respiratory Syndrome, West Nile, SARS-Corona, Nipah, Hendra, Avian influenza and Swine influenza.

EXPALANTION

Many viruses affecting equines are also important human pathogens. Diseases like Eastern equine encephalitis (EEE), Western equine encephalitis (WEE), and Venezuelan-equine encephalitis (VEE) are highly infectious and can be disseminated as aerosols. A large number of horses and human cases of VEE with fatal encephalitis have continuously occurred in Venezuela and Colombia. Vesicular stomatitis (VS) is prevalent in horses in North America and has zoonotic potential causing encephalitis in children. Hendra virus (HeV) causes respiratory and neurological disease and death in man and horses. Since its first outbreak in 1994, 53 disease incidents have been reported in Australia. West Nile fever has spread to many newer territories across continents during recent years.It has been described in Africa, Europe, South Asia, Oceania and North America. Japanese encephalitis has expanded horizons from Asia to western Pacific region including the eastern Indonesian archipelago, Papua New Guinea and Australia. Rabies is rare in horses but still a public health concern being a fatal disease. Equine influenza is historically not known to affect humans but many scientists have mixed opinions. Equine viral diseases of zoonotic importance and their impact on animal and human health have been elaborated in this article.

CONCLUSION

Equine viral diseases though restricted to certain geographical areas have huge impact on equine and human health. Diseases like West Nile fever, Hendra, VS, VEE, EEE, JE, Rabies have the potential for spread and ability to cause disease in human. Equine influenza is historically not known to affect humans but some experimental and observational evidence show that H3N8 influenza virus has infected man. Despite our pursuit of understanding the complexity of the vector-host-pathogen mediating disease transmission, it is not possible to make generalized predictions concerning the degree of impact of disease emergence. A targeted, multidisciplinary effort is required to understand the risk factors for zoonosis and apply the interventions necessary to control it.

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.

Waiting in the wings: The potential of mosquito transmitted flaviviruses to emerge

The sudden dramatic emergence of the mosquito transmitted flavivirus Zika virus has bought to the world's attention a relatively obscure virus that was previously only known to specialist researchers. The genus Flavivirus of the family Flaviviridae contains a number of well-known mosquito transmitted human pathogenic viruses including the dengue, yellow fever, Japanese encephalitis and West Nile viruses. However, the genus also contains a number of lesser known human pathogenic viruses transmitted by mosquitoes including Wesselsbron virus, Ilheus virus, St. Louis encephalitis virus and Usutu virus. This review summarizes our knowledge of these lesser known mosquito transmitted flaviviruses and highlights their potential to emerge.

The changing epidemiology of Kunjin virus in Australia

West Nile virus (WNV) is a mosquito-borne virus responsible for outbreaks of viral encephalitis in humans and horses, with particularly virulent strains causing recent outbreaks of disease in Eastern Europe, the Middle East and North America. A strain of WNV, Kunjin (WNV_KUN), is endemic in northern Australia and infection with this virus is generally asymptomatic. However in early 2011, an unprecedented outbreak of encephalitis in horses occurred in south-eastern Australia, resulting in mortality in approximately 10%–15% of infected horses. A WNV-like virus (WNV_NSW2011) was isolated and found to be most closely related to the indigenous WNV_KUN, rather than other exotic WNV strains. Furthermore, at least two amino acid changes associated with increased virulence of the North American New York 99 strain (WNV_NY99) compared to the prototype WNV_KUN were present in the WNV_NSW2011 sequence. This review summarizes our current understanding of WNV_KUN and how the epidemiology and ecology of this virus has changed. Analysis of virulence determinants of contemporary WNV_KUN isolates will provide clues on where virulent strains have emerged in Australia. A better understanding of the changing ecology and epidemiology associated with the emergence of virulent strains is essential to prepare for future outbreaks of WNV disease in Australia.

The role of Australian mosquito species in the transmission of endemic and exotic West Nile virus strains

Recent epidemic activity and its introduction into the Western Hemisphere have drawn attention to West Nile virus (WNV) as an international public health problem. Of particular concern has been the ability for the virus to cause outbreaks of disease in highly populated urban centers. Incrimination of Australian mosquito species is an essential component in determining the receptivity of Australia to the introduction and/or establishment of an exotic strain of WNV and can guide potential management strategies. Based on vector competence experiments and ecological studies, we suggest candidate Australian mosquito species that would most likely be involved in urban transmission of WNV, along with consideration of the endemic WNV subtype, Kunjin. We then examine the interaction of entomological factors with virological and vertebrate host factors, as well as likely mode of introduction, which may influence the potential for exotic WNV to become established and be maintained in urban transmission cycles in Australia.

Experimental studies of the role of the little raven (Corvus mellori) in surveillance for West Nile virus in Australia

OBJECTIVE

To study the potential role of an Australian corvid, the little raven (Corvus mellori), in the surveillance for exotic West Nile virus (WNV) in Australia.

METHOD

In a series of trials, little ravens were infected with WNV (strain 4132 New York 1999) and Kunjin virus (strain K42886) by the intramuscular route. They were observed for 20 days during which blood and swab samples were taken for virus isolation. Tissue samples were taken from ravens humanely killed during the acute infection period, and at the termination of the trials, for virus isolation, histopathology and immunohistochemistry.

RESULTS

Ravens infected with WNV became mildly ill, but all recovered and seroconverted. Blood virus titres peaked around 3 to 4 days after inoculation at levels between 10(3.0) to 10(7.5) plaque forming units/mL. Virus or viral antigen was detected in spleen, liver, lung, kidney, intestine, testis and ovary by virus isolation and/or immunohistochemistry. WNV was detected in oral and cloacal swabs from 2 to 7 days post inoculation. The molecular and pathogenic characteristics of the inocula were consistent with them being of high virulence, as expected for this isolate. Ravens infected with Kunjin virus developed viraemia and seroconverted, although they did not develop disease.

CONCLUSIONS

Little ravens do not develop severe disease in response to virulent WNV infection and for this reason may not be important sentinel hosts in the event of an outbreak of WNV, as in North America. However, as they have relatively high viraemias, they may be able to support virus cycles.

Determination of mosquito (Diptera: Culicidae) bloodmeal sources in Western Australia: implications for arbovirus transmission

A double-antibody enzyme-linked immunosorbent assay was used to determine the bloodmeal sources of adult mosquitoes (Diptera: Culicidae) collected in encephalitis vector surveillance mosquito traps in Western Australia between May 1993 and August 2004. In total, 2,606 blood-fed mosquitoes, representing 29 mosquito species, were tested, and 81.7% reacted with one or more of the primary antibodies. Aedes camptorhynchus (Thomson) and Culex annulirostris Skuse were the most common species tested, making up 47.2% (1,234) and 35.6% (930), respectively. These species obtained bloodmeals from a variety of vertebrate hosts but particularly marsupials and cows. In contrast, Culex pullus Theobald (72.7%; 24/33), Culiseta atra (Lee) (70.0%; 7/10), Culex globocoxitus Dobrotworsky (54.5%; 12/22), and Culex quinquefasciatus Say (39.3%; 22/56) often obtained bloodmeals from birds. Although Ae. camptorhynchus and Cx. annulirostris are well established vectors of arboviruses, other mosquitoes also may have a role in enzootic and/ or epizootic transmission.

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.

Entomological investigations of an outbreak of Japanese encephalitis virus in the Torres Strait, Australia, in 1998

Japanese encephalitis (JE) virus first appeared in Australia in 1995, when three clinical cases (two fatal) were diagnosed in residents on Badu Island in the Torres Strait, northern Queensland. More recently, two confirmed human JE cases were reported in the Torres Strait Islands and Cape York Peninsula, in northern Queensland in 1998. Shortly after JE virus activity was detected in humans and sentinel pigs on Badu Island in 1998, adult mosquitoes were collected using CO2 and octenol-baited CDC light traps; 43 isolates of JE virus were recovered. Although Culex sitiens group mosquitoes yielded the majority of JE isolates (42), one isolate was also obtained from Ochlerotatus vigilax (Skuse). Four isolates of Ross River virus and nine isolates of Sindbis (SIN) virus were also recovered from members of the Culex sitiens group collected on Badu Island in 1998. In addition, 3,240 mosquitoes were speciated and pooled after being anesthetized with triethylamine (TEA). There was no significant difference in the minimum infection rate of mosquitoes anesthetized with TEA compared with those sorted on refrigerated tables (2.8 and 1.6 per 1,000 mosquitoes, respectively). Nucleotide analysis of the premembrane region and an overlapping region of the fifth nonstructural protein and 3' untranslated regions of representative 1998 Badu Island isolates of JE virus reveled they were identical to each other. Between 99.1% and 100% identity was observed between 1995 and 1998 isolates of JE from Badu Island, as well as isolates of JE from mosquitoes collected in Papua New Guinea (PNG) in 1997 and 1998. This suggests that the New Guinea mainland is the likely source of incursions of JE virus in Australia.

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.

Murray Valley encephalitis virus field strains from Australia and Papua New Guinea: studies on the sequence of the major envelope protein gene and virulence for mice

We have compared the nucleotide sequence of the gene encoding the major envelope (E) protein of a number of Murray Valley encephalitis virus (MVE) isolates from Australia and Papua New Guinea (PNG). The isolates, from widely separated geographic regions, were from four fatal human cases, a heron, and six mosquito pools and covered a period of 25 years. The sequences of the Australian strains were notable for their similarity, showing not more than 1.7% nucleotide sequence divergence in pairwise comparisons. There was 6.8% divergence in the E gene between the two available strains from PNG, and 9-10% divergence between each of the PNG strains and the Australian prototype. These data are consistent with previous conclusions based on HaeIII restriction digest analysis of cDNA to virion RNA (M. Lobigs, I. D. Marshall, R. C. Weir, and L. Dalgarno, 1986, Aust. J. Exp. Biol. Med. Sci. 64, 571-585). We conclude that a single MVE genetic type exists in Australia. Separate foci of MVE evolution appear to exist in PNG, generating greater strain variation. For all MVE isolates the deduced length of the E protein was 501 amino acids. The E protein differed at no more than three positions between any two Australian strains. The PNG strains differed from the Australian strains at 6-11 residues depending on the virus pair. Differences in amino acid sequence did not occur at a position corresponding to a previously demonstrated neutralization determinant in yellow fever virus (M. Lobigs, L. Dalgarno, J. J. Schlesinger, and R. C. Weir, 1987, Virology 161, 474-478). Thus selection for neutralization resistance may not be a major evolutionary pressure in the field situation. In comparisons between the E protein amino acid sequence of the prototype strain and those of a number of other MVE strains, 7 out of 14 differences were at residues seen at the corresponding position for Japanese encephalitis virus (JE), consistent with the close serological relationship of MVE and JE. Five Australian MVE strains and two from PNG were tested for virulence by comparing LD50 values after intraperitoneal and intracranial inoculation of 21-day-old mice; all strains were virulent by this test.