Australian encephalitis in the Northern Territory: clinical and epidemiological features, 1987–1996

BACKGROUND: The last epidemic of Australian encephalitis occurred in 1974. Since then, cases have been reported from the Kimberley of Western Australia (WA). AIMS: To describe the epidemiology and clinical features of Australian encephalitis in the Northern Territory (NT) of Australia. METHODS: Review of cases of Australian encephalitis presenting to Royal Darwin Hospital from 1987-1996 and review of sentinel chicken surveillance for Australian encephalitis viruses. RESULTS: Sixteen patients were identified; ten from the NT and six from WA. Cases occurred in the years 1987, 1988, 1991 and 1993. Infection was acquired throughout northern NT below latitude 20 degrees S in the months March to July. All infections were due to Murray Valley encephalitis (MVE) virus. Eleven of the patients were children. Distinguishing features were spinal cord and brainstem involvement and the absence of seizures in adults. CT scanning was normal and EEG showed no focal activity. Five died (31%) and four (25%) have residual neurological disability. Sentinel chicken surveillance since 1992 shows yearly seroconversion to MVE virus throughout northern NT; human cases occurred simultaneously with chicken seroconversion in 1993. CONCLUSIONS: Australian encephalitis is endemic in the NT; the areas at risk are north of Tennant Creek. Outbreaks are seasonal and occur every few years. Young children are most at risk. Mortality and morbidity are high. Prevention of disease is by avoidance of mosquito exposure and vector control measures.