The role of duck species in the westward spread of the Qinghai-H5N1 virus remains controversial. Circumstantial evidence from global wildlife surveillance supports the hypothesis that migratory birds, including wild ducks, have contributed to the current Eurasian endemic of H5N1 HPAI viruses. 45 Surveillance studies in Thailand in 2004 showed that most domestic grazing ducks infected with H5N1 HPAI viruses were asymptomatic 4 and that the initial spread of H5N1 HPAI viruses to chickens and humans corresponded to the movement of grazing ducks. 4, 46 In fact, infected domestic ducks grazing on man-made wetlands (e.g., harvested rice fields and irrigation canals) may maintain the infection and spread it to wild birds that feed at the same sites. If these wild birds are migratory and experience limited morbidity, they in turn can disperse HPAI viruses widely (Figure 2) , as suggested by the high genetic similarity of HPAI isolates from Africa, Europe, and the Middle East to the Qinghai-H5N1 virus. 37 