Attack risk on infected properties during the 2007 equine influenza outbreak in New South Wales, Australia.

The aim of this preliminary study was to estimate the proportions of seropositive horses on infected premises (IPs) in order to assess the attack risk of the disease. Logistic regression analyses were conducted to evaluate the differences in attack risks between enterprise sizes and predefined spatial clusters/regions. The average attack risk experienced during the outbreak was 96.88% (median 100%), but it differed according to the size of the enterprise and other geographic and demographic conditions. The highest attack risks were observed in the Dubbo cluster/region and the lowest in the Narrabri-Northern cluster. Properties with fewer horses were generally more likely to have higher attack risks than larger enterprises, though this was not true for all cluster/regions.