After emerging in southern China in the mid-1990s, the highly pathogenic avian influenza (HPAI) H5N1 virus spread across east and Southeast Asia, causing unprecedented epidemics in 2003-2004 [13] . As of 24 September 2009, the virus has caused 442 human cases, with 262 deaths worldwide 1 . Controlling the spread of H5N1 disease in poultry may contribute to the reduction of risk for humans [27] by preventing the emergence of a viral form with efficient human-to-human transmission capable of triggering a global pandemic [13] . Determining the factors involved in the spread of H5N1 in poultry and producing risk maps are critical to disease control as they would enable control measures to be targeted and surveillance in ''high-risk'' areas to be strengthened. The HPAI H5N1 virus is now well established in the poultry population in Asia, where the virus has been able to maintain itself and spread as well as periodically re-emerge. The main pathways that have been identified for the spread of H5N1 are the migration and trade of wild birds and the transport of poultry and poultry products [12] . However, the respective roles of these pathways at the global or national scale are still unclear [12, 18] . The persistence of HPAI H5N1 virus in Southeast Asia has been linked to a specific agro-ecosystem [9] that associates freegrazing ducks with rice cultivation. A separate study found that the risk of HPAI outbreaks was reduced in areas with agricultural activities other than rice farming [10] . Free-grazing ducks form a reservoir of HPAI H5N1 in Asia [29] and may contribute to the spread of the virus when they are moved among rice fields which also constitute a habitat for wild waterfowl [8] . In addition to free-grazing ducks, it is likely that backyard poultry raised in low biosecurity systems and fighting cocks are involved in the diffusion of the virus [8] ; however, their roles are still unclear. Research to date suggests that the spread of HPAI H5N1 is influenced primarily by human activities related to poultry production and poultry trading [18] , however, little is known about the underlying processes involved. Live poultry markets probably play a role in the maintenance of the virus in Asia [26, 28] and the movement of poultry within trade chains may have facilitated the spread of the HPAI H5N1 virus.