Risk factors for the introduction of avian influenza virus into commercial layer chicken farms during the outbreaks caused by a low-pathogenic H5N2 virus in Japan in 2005

Avian influenza outbreaks caused by a low-pathogenic H5N2 virus occurred in Japan from June to December 2005. All 41 affected farms housed layer chickens. Therefore, we conducted a case-control study targeting all commercial layer chicken farms within the movement restriction areas in Ibaraki prefecture, where most outbreaks were detected, to investigate the risk factors for the introduction of avian influenza virus (AIV). Four variables were identified as risk factors associated with the introduction of AIV by multivariate logistic regression: 'introduction of end-of-lay chickens ' (odds ratio (OR) = 36.6), 'sharing of farm equipment among farms' (OR = 29.4), 'incomplete hygiene measures of farm visitors on shoes, clothes and hands' (OR = 7.0), and 'direct distance to the nearest case farm' (0-500 m, OR = 8.6; 500-1000 m, OR = 0.8; 1000-1500 m, OR = 20.1; referenced more than 1500 m). We fully believe that strict biosecurity measures should be applied against any incursion points so as not to introduce AIV into more farms.