Roosters play with a rubber ball in Shenyang
A pair of roosters play with a rubber ball in Shenyang, capital of northeast China's Liaoning province May 9, 2007. The roosters have been playing regularly with the ball since the owner found the ball three months ago, according to China Daily. Reuters

A rise in the number of reported cases of avian flu, coupled with the appearance of a mutation that could be resistant to vaccines, has led the United Nations to warn of a potential bird flu resurgence.

A mutant strain of the virus has already infiltrated parts of Vietnam and China. Fears of its spread are particularly acute in Cambodia, where eight people died of bird flu -- officially known as H5N1 -- this year.

There has also been a steady rise in the number of reported cases in poultry populations since 2008, reversing a sharp decline between 2003 and 2008. The World Health Organization estimates that there have been 331 human deaths from 565 confirmed bird flu cases since 2003, and the disease is still considered endemic in Bangladesh, China, Egypt, India, Indonesia and Vietnam.

The general departure from the progressive decline in 2004-2008 could mean that there will be a flare-up of H5N1 this fall and winter, with people unexpectedly finding the virus in their backyard, United Nations chief veterinary officer Juan Lubroth said in a statement.

Lubroth said that shifting migratory patterns have brought the disease to previously unaffected countries such as Israel, the Palestinian territories, Bulgaria, Romania, Nepal and Mongolia. But he faulted the agricultural industry for facilitating its spread.

Wild birds may introduce the virus, but people's actions in poultry production and marketing spread it, Lubroth said.

The Centers for Disease Control and Prevention's website notes that humans rarely contract avian flu, but in those cases when they do the symptoms can range from conjunctivitis to respiratory disease to death.