Reducing antimicrobial usage in small-scale chicken farms in Vietnam: a 3-year intervention study by Phu, DH, Cuong, NV, Truong, DB, Kiet, BT, Hien, VB, Thu, HTV, Yen, LK, Minh, NTT, Padungtod, P, Setyawan, E, Thwaites, G, Rushton, J, Carrique-Mas, J