While naturally resistant ducks can be argued to have been involved in the spread of H5N1 HPAI viruses from Qinghai Lake to the rest of Eurasia, it is difficult to explain why H5N1 HPAI viruses have not spread to susceptible species in the Philippines, Australia, and the Americas, which are on the direct flyways of migratory waterfowl. More than 6AE6 million birds migrate from Eastern Asia to Alaska yearly (alaska.usgs.gov ⁄ science ⁄ biology ⁄ avian_influenza ⁄ migrants_tables.html). Despite intense surveillance in Alaska, no H5N1 HPAI viruses have been detected to date, and influenza viruses of Eurasian lineage are introduced into the Americas only rarely. 18 The major spreaders of influenza in domestic poultry are humans. As described by , from the molecular epidemiology data, transmission of H5N1 influenza in domestic poultry is perpetuated largely through movement of poultry and poultry products rather than continued reintroduction of viruses from migrating birds. 42 The alternative reservoir, the domestic duck population, has a higher likelihood of perpetuating H5N1 HPAI viruses. Prospective surveillance continued to isolate H5N1 HPAI viruses from apparently healthy ducks, geese, and chickens in Southeast Asian poultry markets during 2004-2006. Naturally resistant ducks might not be expected to show disease signs, but the absence of morbidity in highly susceptible geese and chickens is surprising. The widespread use of vaccine in chickens may explain this observation, but vaccine has been used less in geese and ducks. An alternative possibility is that the susceptible poultry had cross immunity as the result of exposure to co-circulating influenza viruses. Experimental studies have demonstrated that chickens previously infected with H9N2 virus and then inoculated with H5N1 HPAI virus become infected and shed virus but do not show disease signs. 51 The continuing co-circulation of multiple subtypes of LPAI viruses in domestic poultry could explain why a small percentage of susceptible domestic species can appear healthy while shedding transmissible levels of H5N1 HPAI virus. To provide answers to these unresolved questions about the role of domestic species, it will be necessary to establish long-term prospective surveillance in domestic poultry in the hypothetical ''epicenter zones,'' including China, Indonesia, Vietnam, Egypt, and Nigeria. It is noteworthy that in these regions, control of H5N1 HPAI virus is attempted by the continuing use of vaccines.