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

Linda A Selvey,Michael Lindsay,Peter G Markey,Peter G Markey,Sean Tobin,David W Smith,Ann P Koehler,Paul Armstrong,Lynne Dailey,Matthew Kasper

doi:10.1371/journal.pntd.0002656

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.

Highlights

Murray Valley encephalitis virus (MVEV) causes the most serious of the mosquito-borne virus diseases endemic to Australia
The outbreak was associated with significant flooding in many parts of Australia and most cases reported either outdoor activities where mosquito exposure was highly likely or significant mosquito exposure
Encephalitic MVEV is between 15–30%, with long-term neurological sequelae occurring in 30–50% of survivors and only around 40% recovering completely [6]

Summary

Introduction

Murray Valley encephalitis virus (MVEV) causes the most serious of the mosquito-borne virus diseases endemic to Australia. It occurs on the island of New Guinea, and little is known about its epidemiology there [1]. Spread of MVEV outside these enzootic foci is thought to be due to rainfall and flooding that allowed movement of infected water birds to previously arid environments. Persistence in desiccation-resistant mosquito eggs may contribute to outbreaks in previously arid areas, and the existence of cryptic enzootic foci has been postulated [3,4,5]

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