About 1,400 Pekin ducks waddle inside four long cages within the vessel that serves as a taxi for thousands of waterfowl ferried to feed on leftover grain in newly harvested rice fields across Vietnam's southern Mekong Delta. It is an age- old practice that has always benefited the area's duck farmers and crops.

Now it has been outlawed for helping fan bird flu across eight provinces in one month.

The government last week banned the movement of all ducks after the H5N1 virus resurfaced last month following a year-long lull. Any birds caught in transit can be seized and destroyed, whether they are vaccinated or not.

"I've been roaming my ducks around for more than 30 years," says Ngo Hong Hanh, 57, standing barefoot on the riverbank near his boatload of noisy ducks. "I don't think I can abandon this practice, because it is my main income."

Hanh loads his flock on to boats three or four times a year and travels to vacant fields littered with grains of rice left amid the dry stubble of recently cut stalks. For a small fee, the ducks forage for a month and a half before going home, ridding the fields of unwanted pests and saving Hanh about US$1,500 (HK$11,700) in feed costs.

This time, he is returning from Vinh Thuan district in Kien Giang province, 20 kilometers from his village. Vinh Thuan has been inundated with visiting ducks in recent weeks - more than 300,000 coming from all around to be released into the freshly harvested paddies. Normally, the district is home to only about 19,000 waterfowl.

Bird flu typically flares during winter months, when temperatures drop.

The H5N1 virus has recently surfaced in South Korea, Japan, Thailand, Nigeria, China and Hong Kong. It has continued to kill poultry and people in Indonesia, the world's hardest hit country with 62 deaths. It has also killed four humans in Egypt since last month.

Animal health officials in Vietnam saw the latest wave of bird flu begin rippling across the Delta last month, killing or forcing the slaughter of 40,000 birds, but they were helpless to stop it. They blame the flare-ups on unvaccinated birds and the mass arrival of free-range ducks. Vaccination helps to decrease the spread, but even that is not foolproof because ducks must receive multiple shots each year to ensure immunity.

"If we can successfully prevent ducks from roaming from one place to another, we can stop the spread of the virus," says Dinh Cong Than, director of Kien Giang's provincial animal health department.

"Our government policy is to change this practice, but I don't think you can do it overnight."

Authorities have set up road and water checkpoints to try to stop poultry from coming in from outside provinces. Four boatloads of about 5,400 ducks have been intercepted by night patrols in Vinh Thuan, but officials say it is not easy to scour the muddy web of canals and rivers that snake across the country's rice basket.

International experts say it might not be necessary to stop the Mekong practice, as long as the ducks are closely monitored and vaccinated against the H5N1 virus.

"It's a nice little ecosystem, a good farming practice, but because of its risk with respect to [avian influenza], then it does have to be reviewed and it would be higher risk," says Jeff Gilbert, an animal health expert at the UN Food and Agriculture Organization in Hanoi.

Vietnam had been hailed a success story for beating back the virus that began ravaging Asian poultry stocks in late 2003. A nationwide mass poultry vaccination program, coupled with strong political will, brought the virus under control after it killed 42 people in the country. No poultry outbreaks were reported in 2006, and there have been no human cases since November 2005.

The virus has killed at least 163 people worldwide but remains hard for humans to catch. Experts fear it could mutate into a form that spreads easily among people, sparking a global pandemic.

ASSOCIATED PRESS