24. Virus de l'encéphalite de la Murray Valley

Murray Valley encephalitis virus (MVEV) is a zoonotic flavivirus endemic to northern Australia and Papua New Guinea, and the causative agent of Murray Valley encephalitis (formerly known as Australian encephalitis or Australian X disease) ^[1][3]. It is closely related to Kunjin virus, sharing similar ecology but typically causing higher morbidity ^[3]. Human infection is acquired exclusively through the bite of an infected mosquito, as the virus is not transmitted from person to person ^[3].

The great majority of MVEV infections are asymptomatic, with serological surveys indicating that only about one in 800–1000 infections results in clinical disease ^[3]. A minority of those infected experience a mild illness with fever, headache, nausea and vomiting, and only a small fraction of these progress to encephalitis ^[3]. When Murray Valley encephalitis develops, it can cause permanent neurological damage or death, and infection is believed to confer lifelong immunity thereafter ^[3].

MVEV is endemic to northern Australia and Papua New Guinea, with occasional southward incursions into south-eastern Australia during heavy summer monsoon rainfall and Murray–Darling River flooding, producing outbreaks separated by decades of very few or no identified cases ^[3]. Historical epidemics were recorded in 1917 and 1918 in south-eastern Australia following high-rainfall years, and the virus was first isolated from humans during a 1951 Murray Valley epidemic ^[3]. In Victoria during the 2022–2023 mosquito season, integrated surveillance detected MVEV in mosquitoes on 48 occasions across more than 3,000 trapping events and identified 6 confirmed human cases alongside additional unspecified flavivirus infections, with one probable equine case detected through enhanced animal surveillance ^[1]. Other endemic Australian mosquito-borne viruses such as Ross River and Barmah Forest viruses are reviewed together with MVEV in describing the country's zoonotic arboviral landscape ^[4].

MVEV is maintained in an enzootic bird–mosquito–bird cycle, with waterbirds of the order Ciconiiformes—including herons and cormorants—serving as the natural reservoir ^[3]. The principal mosquito vector is Culex annulirostris, and spread to southern Australia is thought to occur when infected birds or mosquitoes migrate from endemic northern regions during flood conditions ^[3]. Humans and horses are incidental, dead-end hosts; human infection occurs only through the bite of an infected mosquito, and person-to-person transmission does not occur ^[3].

Effective prevention of flavivirus infections depends on interrupting the principal transmission route, which is arthropod-borne via mosquito or tick vectors, together with targeted vaccination where vaccines are available. For mosquito-borne flaviviruses such as dengue, transmission occurs between humans and Aedes mosquitoes, so vector control measures aimed at reducing Aedes populations and limiting human–vector contact are central to prevention ^[2]. Japanese encephalitis virus, the most important cause of epidemic encephalitis and a flavivirus whose geographical area continues to expand, can be prevented through vaccination, which is available for at-risk populations, as well as through personal protection against mosquito bites and broader vector management; the availability of vaccines, despite ongoing geographical expansion of the virus, supports the feasibility of this combined approach ^[2]. For tick-borne flaviviruses such as tick-borne encephalitis virus, which occurs across large parts of Eastern Europe and the Commonwealth of Independent States, prevention strategies focus on avoiding tick bites, including the use of protective clothing, repellents, and tick checks, together with vaccination of at-risk groups living in or travelling to endemic areas ^[2]. Yellow fever, an arboviral haemorrhagic disease that resurged in the late 20th century after mid-century control, is preventable through vaccination, which has historically been the cornerstone of preventing outbreaks, alongside continued surveillance and vector control in affected regions ^[2]. Taken together, prevention of flavivirus disease rests on three complementary pillars: (1) vector control to reduce populations of mosquitoes such as Aedes or to reduce exposure to ticks in endemic areas; (2) personal protective measures to minimize bites by infected arthropods; and (3) vaccination where an effective vaccine exists, notably for Japanese encephalitis and yellow fever ^[2].

1. 1 Braddick M et al. An integrated public health response to an outbreak of Murray Valley encephalitis virus infection during the 2022-2023 mosquito season in Victoria. Front Public Health. 2023. PMID: 37860808. doi: 10.3389/fpubh.2023.1256149. PubMed: https://pubmed.ncbi.nlm.nih.gov/37860808/
2. 2 Solomon T et al. Dengue and other emerging flaviviruses. J Infect. 2001 Feb. PMID: 11531316. doi: 10.1053/jinf.2001.0802. PubMed: https://pubmed.ncbi.nlm.nih.gov/11531316/
3. 3 Wikipedia contributors. Murray Valley encephalitis virus - Wikipedia [Internet]. Wikipedia. cited 3 Sept 2026. Available from: https://en.wikipedia.org/wiki/Murray_Valley_encephalitis_virus
4. 4 Ong OTW et al. Mosquito-Borne Viruses and Non-Human Vertebrates in Australia: A Review. Viruses. 2021 Feb 9. PMID: 33572234. doi: 10.3390/v13020265. PubMed: https://pubmed.ncbi.nlm.nih.gov/33572234/