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.

Figure 1. Geographical distribution of MVEV cases in Australia 2011.

Legend: Each circle with a dot inside represents a case who survived; Each black cross represents a case who died. Note, cases 11 and 12; 2 and 7; and 8 and 14 occurred in the same location, and the symbols are shown side by side.

Figure 2. Age distribution and Aboriginal status of MVEV cases and deaths 1978–2011.

Comparison of the distribution of age and Aboriginal status of MVEV cases between 1978–1999 and 2000–2011. Legend: The white unfilled areas in the graph represent either Adult or Non-Aboriginal cases. The black filled areas in the graph represent either Child or Aboriginal cases.