No one knows, but the United States government has surveillance programs in place to detect the virus as soon as possible, whether it may arrive in infected poultry, imported pet caged birds, or migratory birds.

Since 2004, the United States has banned importation of poultry from countries affected by avian influenza viruses. Transportation of eggs, poultry, and poultry products is being closely monitored because the virus has spread from one country to another through the poultry trade. The commercial poultry industry is cause for the greatest concern because it involves the transportation of billions of birds every year, along with associated products such as manure, feathers, and eggs.

The virus has the greatest opportunity to mutate into more harmful forms in the crowded environment of factory farms, where huge poultry flocks are housed together in proximity to humans. Exportation of poultry and hatching eggs can spread the disease to the commercial poultry industry in other countries and to small farms.

The United States government has also banned importation of pet birds and bird products from areas affected by high pathogenicity H5N1. The USDA quarantines and tests all live birds imported from other countries. However, the illegal trade of caged birds is more difficult to monitor because smugglers continually seek ways to avoid detection. For this reason, it is possible that the illegal bird trade could be one of the most likely routes by which the virus reaches North America. This includes trade in live birds such as poultry, game species, pet birds, falcons, fighting roosters, homing pigeons, and bird parts or products such as meat, feathers, bones, and guano.

It is also possible that wild birds from Asia could carry the virus to their breeding grounds in the Pacific Islands or Alaska, or that birds from Europe could carry it into Canada or the northern United States. Most (99 percent) of North American migrants do not come from Europe or Asia, places from which they would bring the virus. However an infected bird from Asia could migrate to Alaska to breed. While there, it could pass on the virus to another species that survives the infection and migrates into the Lower 48 in autumn. The chances of this happening are probably small. In 2006, scientists will test about 100,000 birds in Alaska, Canada, and along major flyways in the United States to help detect this if it occurs.

If the highly pathogenic H5N1 virus is detected in North America, it will not signal the start of a human pandemic. In all well-documented cases, humans have contracted the virus from poultry. There are only a few cases in which the virus may have been passed from one person to another. Also, there are no confirmed reports that the virus has been transmitted from wild birds to humans so far. For these reasons, the main preventive efforts would focus on keeping wild birds away from poultry and eradicating the virus immediately from poultry flocks if found.

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