gens impact the wide variety of hosts they can infect. This in turn will influence the potential for viral persistence and potentially spill over into humans.
Of the Anseriformes, the dabbling (or puddle) ducks have the greatest prevalence and mallards in particular have the highest prevalence (Olsen et al., 2006). The age structure of these populations is also important in that juvenile birds are more likely to have an infection than adults, and this likely is influenced by immunological status. Other migratory birds including the Charadriiformes (shorebirds, gulls, terns, and waders) can be infected but typically only at very low levels (Krauss et al., 2004; Olsen et al., 2006). However, this does not mean they are unimportant in the transmission cycle or maintenance of the virus. More intensive long-term data on how these viruses circulate and transmit between these birds are needed.
Finally, the potential for newly reassorted strains to emerge is probably heightened now because of the widespread circulation of 2009-H1N1 influenza A. We hope that our Fogarty International Center-funded program will both help identify the risk of co-infections (e.g., regions with high poultry and hog farm density) and actually find evidence in the testing that our groups will be doing. The recent report of hog farms in Indonesia with high prevalence of H5N1 (Cyranoski, 2005) and two very recent suspected cases of humans passing 2009-H1N1 influenza A onto hogs highlight this risk.
We conclude that there are a growing number of strategies being developed to predict the origin and spread of novel emerging pathogens. These strategies meld ecological, virological, and mathematical approaches to identify high-risk regions, activities, and behaviors, and they have some potential for prevention and control. At the same time, far more detailed and structured studies are needed to truly get to the underlying causes of zoonotic influenza emergence and help prevent the next human-to-human high-pathogenicity pandemic. To do these studies effectively will require some capital investment, likely within the range of a few tens of millions of dollars. However, we believe the potential reduction in pandemic risk would be a wise investment because the predicted pandemic mortality and associated economic costs are within the tens of billions of dollars (Meltzer et al., 1999).
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