Some chickens are raised in backyard flocks or as pets, and remain a greater risk because they can mingle with migrating birds.

They are a popular choice for backyard flocks because of their egg laying abilities and hardiness.

An effective vaccination may not have much impact on backyard flocks, which tend to be outside the GOE vaccination system in any case.

The disease has been found among fighting birds in backyard flocks in Nevada, New Mexico and Texas.

However, with a fairly quick rate of maturation and the ability to lay a respectable number of white eggs, it is still occasionally kept in backyard flocks.

While the outbreak took place on a commercial poultry farm, he said, the virus may well have been percolating for months in backyard flocks.

Therefore, USDA now tracks these detections in wild birds, backyard flocks, commercial flocks and live bird markets.

Africa will face a particular challenge because of the weak health infrastructure and the proliferation of what Dr. Lee called "backyard flocks" rather than industrialized poultry farms.

The flu is ubiquitous in thousands of backyard flocks, and it appears to be killing more birds every month, increasing the likelihood of human cases.

The disease impact is likely to be most felt by smallholders and owners of backyard flocks which form their main food supply.