Why did we focus our model efforts on China?
H5N1 first emerged in southern China and the virus, after prominently circulating in this region has become endemic. The rapid increase in livestock production and the fact that many people live in close proximity to their livestock (chickens, ducks, pigs, etc.) creates conditions for virus amplification and exchange among a wider range of hosts.( McMichael et al. 2007)
A paper following the initial outbreaks of H5N1 in poultry and the first death of a boy in Hong Kong (1997) explains these conditions and raises the question of southern China as “An Influenza Epicentre”.
The Global Debate –Can wild birds spread highly pathogenic avian influenza viruses long distances?
In the spring of 2005, an historic event occurred. Thousands of geese and lower numbers of some ducks, gulls, and other species, were found dead at a remote reserve in north-central China – the Qinghai Lake National Nature Reserve. In total, more than 6500 waterbirds died, and more than half of them were the iconic bar-headed goose (Anser indicus) – all due to highly pathogenic H5N1 avian influenza virus. A few months later, H5N1 had spread to Europe and Africa. Eventually, H5N1 had reached more than 60 countries across three continents.
The involvement of wild birds on such a large scale sparked the debate.
More info: Science news article focusing on wild bird debate.
Example position statements on wild bird debate: American Bird Conservancy Wetlands International.
While many avian influenza viruses originate in wild birds in non-lethal form, viruses that enter the poultry system find ample hosts within which to reproduce and evolve, and this is generally where viruses mutate into a highly pathogenic form. These viruses become lethal to poultry (and other birds) and can occasionally jump the species barrier to infect humans.
Even within poultry, avian influenza viruses affect species differently. For example, H5N1 is generally lethal to chickens; however domestic ducks have been shown to be silent reservoirs, meaning they can shed virus without showing symptoms. In China and other regions of Asia, chickens and ducks are not farmed in the same ways. Many ducks are free-ranging, meaning they forage in the agricultural fields and rice paddies during the day and return to the farm at night. This allows exposure to other birds – both wild and domestic – either directly or via fecal droppings in the water or moist soils where virus is known to persist for days to months.
The last photo below was taken in southeastern China and shows domestic ducks freely moving from the natural wetlands over a levee into rice paddy fields.