Assessment of the proportion of under-reporting during the 2007 equine influenza outbreak in New South Wales, Australia.

During the 2007 equine influenza (EI) outbreak in Australia, there was no objective information about the possible under-reporting of cases by horse owners either so that they would avoid movement restrictions or because of their inability to detect infection. This investigation aimed to estimate the proportion of under-reporting during the outbreak based on the results of surveillance undertaken in conjunction with vaccination. The results provided improved estimates of morbidity during the outbreak and indicated the level of under-reporting likely to occur in future outbreaks of other infectious diseases in horses in Australia.