Wild birds are a huge problem around large, confinement facilities. As feed is delivered to these operations, there is spillage around the feed tanks. In addition, most feed tanks leak at the auger boot leaving feed on the concrete pads under the tanks. Workers can easily track in bird droppings on their boots as they enter the facilities. Also, confinement facilities are not bird-proof and birds can get into these buildings very easily. The other area that leads to contamination into large facilities is via the feed mill and feed trucks. There is no way to bird-proof feed facilities and grain trucks/trains entering and leaving these feed mills. Many common diseases are spread wild bird to domestic farm animals (TGE, Avian Influenza, Swine Influenza, PRRS, etc.).
I'm not sure I understand your big IF...other strains of bird flu/swine flu (H1N1, H2N1, etc.) have entered the US and caused problems throughout the swine and poultry industries. The difference between these strains and the new H5N1 strain is the others while causing some human infections were neither lethal nor able to mutate to a flu that could be transmitted human to human. The big IF regarding H5N1 is not when it will arrive in North America or whether it will infect poultry operations but rather it's ability to mutate to a virus that can be transmitted human to human. As long as it stays in a form that can be transmitted only bird to human, the chances of a pandemic are extremely low and can be controlled through the slaughter of infected animals. If the virus mutates, all bets are off.
The reason that large confinement operations have more disease outbreak is simple:
1. More animal density per cubic foot of air space. If virus or bacteria are present, the animals are exposed to a much higher density of disease-carrying pathogens.
2. More disease vectors...feed trucks weekly that travel from farm-to-farm, dead animal/bird trucks weekly visiting the farm, transport trucks to take animals to slaughter, field service employees going from farm-to-farm.
3. Compromised immune systems due to confinement.
4. Presence of chronic, low-grade infections present in all confinement operations.
Most backyard flocks or herds, while exposed to more migratory/wild birds and animals, do not have to deal with the constant threat of disease introduction as compared to the confinement operations.
What will probably happen in regards to H5N1 Avian flu is that the disease will enter North America...large operations will be infected and control measures will be put in place to slaughter the animals/birds and the outbreak contained with no viral mutation.
The biggest risk factor is in areas with large poultry and swine operations in close proximity. Eastern NC is a prime example. Influenza viruses love to use pigs as vectors to mutate within. The scenario would be:
1. H5N1 infects large poultry operation.
2. Virus is introduced to large swine operation and mutates (no mutation, no worry).
3. Human workers on swine operation become infected.
4. These workers transmit disease throughout the population.
The control measures are the critical step once the disease enters the US...testing and culling of infected flocks/herds. Unfortunately, this testing is in the hands of corporate leaders who have the eye on their bottom-line. From personal experience, the reaction time is somewhat lacking because CEO's are loathe to spend money on monitoring procedures. Hopefully the USDA and CDC will force companies to initiate testing and culling procedures quickly. It does appear that the Germans have done this. They are nothing if not efficient.