Data on the frequency and volume of live poultry sales in northern Vietnamese LBMs were not available. Study provinces were selected based on demographic features rather than outbreak reports, given that most disease events were presumed to be undetected [23, 24, 25] : Ha Noi, the most densely populated province in northern Viet Nam [26] , and Bac Giang, a rural province with a large poultry population [27] . A snow-ball sampling approach was adopted.