The Changing Epidemiology of Murray Valley Encephalitis in Australia: The 2011 Outbreak and a Review of the Literature

Abstract

Murray Valley encephalitis virus (MVEV) is the most serious of the endemic arboviruses in Australia. It was responsible for six known large outbreaks of encephalitis in south-eastern Australia in the 1900s, with the last comprising 58 cases in 1974. Since then MVEV clinical cases have been largely confined to the western and central parts of northern Australia.In 2011, high-level MVEV activity occurred in south-eastern Australia for the first time since 1974, accompanied by unusually heavy seasonal MVEV activity in northern Australia. This resulted in 17 confirmed cases of MVEV disease across Australia. Record wet season rainfall was recorded in many areas of Australia in the summer and autumn of 2011. This was associated with significant flooding and increased numbers of the mosquito vector and subsequent MVEV activity. This paper documents the outbreak and adds to our knowledge about disease outcomes, epidemiology of disease and the link between the MVEV activity and environmental factors.Clinical and demographic information from the 17 reported cases was obtained. Cases or family members were interviewed about their activities and location during the incubation period.In contrast to outbreaks prior to 2000, the majority of cases were non-Aboriginal adults, and almost half (40%) of the cases acquired MVEV outside their area of residence. All but two cases occurred in areas of known MVEV activity.This outbreak continues to reflect a change in the demographic pattern of human cases of encephalitic MVEV over the last 20 years. In northern Australia, this is associated with the increasing numbers of non-Aboriginal workers and tourists living and travelling in endemic and epidemic areas, and also identifies an association with activities that lead to high mosquito exposure. This outbreak demonstrates that there is an ongoing risk of MVEV encephalitis to the heavily populated areas of south-eastern Australia.

Viral neuronotropism is important in the pathogenesis of Murray Valley encephalitis

“…The clinical manifestations of MVE in this and other cases described in the literature were generally nonspecific, and include fever, headache, nausea, vomiting followed by convulsions, neurological consequences of spinal cord and brainstem involvement, culminating in coma and death in about 20% of patients [2]. Long‐term sequelae in survivors or complete recovery have been reported [1].…”

“…Subsequent outbreaks have been reported in 1956, 1974 and 2011 [2,3]. Similar to other flaviviral encephalitides such as Japanese encephalitis (JE), West Nile (WN) encephalitis and Tick Borne Encephalitis (TBE) [1], MVE is often severe with significant morbidity and mortality [2]. As MVE viral tropism in the central nervous system (CNS) is still unknown, we have examined three suspected (not culture‐proven) cases of MVE, and report in one case, neuronal localization of viral antigens and viral RNA using immunohistochemistry (IHC) and in situ hybridization (ISH), respectively.…”

Encephalitic Arboviruses: Emergence, Clinical Presentation, and Neuropathogenesis

“…WNV, which was first identified in the West Nile subregion of Uganda, is now endemic in temperate and tropical regions throughout the world, causing yearly outbreaks of encephalitis, with a mortality rate of 5 % to 10 % [16]. MVEV causes similar outbreaks in Australia, New Guinea, and New Zealand, while SLEV rarely causes encephalitis in the USA (<10 cases per year) [16,17]. JEV is the most medically important member of the serogroup, causing 30,000 to 50,000 cases of encephalitis and 10,000 deaths each year in Asia [18].…”

The challenges posed by equine arboviruses

“…Global distribution, by country, of equine flaviviruses: (a) West Nile virus[95,[106][107][108][109]; (b) Japanese encephalitis virus[110,111]; (c) Murray Valley encephalitis virus[112].…”