Fig. 2.

Pathways of emergence of AIVs in commercial poultry operations. Red arrows indicate transitions between the different processes in emergence: AIV spillover from wildlife to AIV spread within poultry flocks on a single operation to AIV spread between poultry operations. (A) Spillover mechanisms from wildlife (adapted from Franklin, 2008). Arrows represent movement of AIVs. Bold arrows indicate transmission links that are strongly supported by empirical studies, thin arrows indicate connections that are supported by limited studies and dotted arrows indicate pathways that remain unexplored. Indirect AIV transmission pathways from wild waterfowl (I and II) to poultry (VI) include: drinking contaminated water (III) or contacting non-waterfowl bird species (V) or wild mammals (IV) that were infected through III. IV may also be infected by scavenging infected waterfowl carcasses by wild mammals. *Note that the importance of direct transmission routes from waterfowl to poultry is well-supported in mixed-species backyard flocks but the importance of this connection in transmission to commercial poultry remains to be determined. It is possible that any of these links involve an intermediate link such as human shoes, etc. (B) AIV spread within poultry operations. Once AIV infects poultry in a farm, it can be transmitted directly to other individuals or indirectly through contaminated water, fomites or air. (C) AIV spread between poultry operations. The mechanisms are variable and currently uncertain, particularly for airborne and local spread. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)