Potential effectiveness of public health interventions during the equine influenza outbreak in racehorse facilities in Japan, 2007.

An outbreak of equine influenza (H3N8) occurred among fully vaccinated racehorses in Japan from August to September, 2007. To assess the potential effectiveness of public health interventions other than vaccination (i.e. movement restriction, isolation and quarantine), which started immediately on the date of detection of the first febrile case, a simple epidemiological model was developed and applied to the observed data. The epidemic curves in five racehorse facilities revealed consistent temporal patterns: (i) a sharp increase in symptom onset of cases during the first 3 days, which is thought to reflect the incubation period before interventions and (ii) thereafter, a continuous decline in incidence reflecting successful control. Whereas the reproduction number before interventions was 2.4-24.7, the estimate declined to 0.1-0.3 following interventions. The effectiveness of all the countermeasures was assessed by the relative reduction in the reproduction number and ranged from 88.0% to 99.5%. The combined effect of vaccination prior to the outbreak and other public health interventions is thought to have helped control the outbreak in 2007.