Mosquito-borne Murray Valley encephalitis: Re-emergence in Western Australia

<p dir="ltr">Murray Valley encephalitis virus (MVEV) is a zoonotic flavivirus, transmitted by Culex annulirostris mosquitoes, and is endemic in Northern Australia and Papua New Guinea with sporadic outbreaks occurring. Occasionally cases occur in south-eastern Australia. MVEV was first identified in 1917 in the South-eastern region of Australia along the Murray River which crosses 3 states (Victoria, New South Wales, and South Australia). Following heavy rainfall and flooding during the summer season, Murray River, Australia's longest river, experienced surges in MVEV cases, leading to local endemicity. As such, major MVEV outbreaks in south-eastern states of Australia include 45 cases in 1951, 58 cases in 1974 and 17 cases in 2011 with cases in intervening years predominantly restricted to Northern Australia. During the Australian summer of 2022–2023, 26 human cases were reported, attributed to a surge in mosquito populations and water birds following prolonged flooding events. Ciconiiformes water birds, such as cormorants and herons, serve as hosts in the bird-mosquito-bird cycle, with transmission to humans occurring through infected mosquitoes. In humans, infection with MVEV results in an illness known as 'Australian X′ characterised by symptoms of fever, headache, nausea, stiff neck, and light sensitivity. However, severe cases can manifest with confusion, seizures, paralysis encephalitis, and even death.</p>