An outbreak of epidemic polyarthritis (Ross River virus disease) in the Northern Territory during the 1990‐1991 wet season

OBJECTIVE: To describe the epidemiology of a large outbreak of epidemic polyarthritis in the Northern Territory during the wet season of 1990-1991. DESIGN, SETTING AND PARTICIPANTS: Arbovirus cases notified to the Northern Territory Department of Health and Community Services by general practitioners and local laboratories between 1 July 1990 and 30 June 1991. MAIN OUTCOME MEASURES: Date and place of infection, age, sex and symptoms. RESULTS: Doctors in the Northern Territory notified 368 cases; another 14 were infected interstate. The epidemic started in September, peaked in January and tailed off in April. The highest attack rates occurred in the rural areas of Jabiru, Litchfield Shire and Katherine. Those most affected were 30-34 year olds. Children, the elderly and Aboriginal people were under-represented. CONCLUSIONS: Epidemic polyarthritis is a wet season problem in the Northern Territory, affecting the rural towns and districts more than the cities. Pre-planned mosquito control measures (effective water drainage and larval control) limited the extent of the 1990-1991 epidemic in Darwin City and Palmerston. The low attack rate in children reflects asymptomatic and less clinically severe infections. The under-representation of Aboriginal people may be the result of infection occurring earlier in life. A related cross-sectional seroprevalence survey has shown that rural Aboriginal people across all age groups have a significantly higher seropositive rate than urban non-Aboriginal residents.