Even before the boat drifts into sight, all senses are alerted to its cargo - a chorus of raucous quacking mixed with an unmistakable stench.
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 grains in newly harvested rice fields across Vietnam's southern Mekong Delta. It's an age-old practice that has always benefited the area's duck farmers and crops. Now, it's 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 yearlong lull. Any birds caught in transit can be seized and destroyed, whether they're 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 onto 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 a month and a half before going home, ridding the fields of unwanted pests and saving Hanh about $1,500 in feed costs.
This time, he is returning from Vinh Thuan district in Kien Giang province, about 12 miles from his village. Vinh Thuan has been inundated with visiting ducks in recent weeks - more than 300,000 coming from miles 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 the winter months when temperatures drop. The H5N1 virus has recently surfaced in South Korea, Japan, Thailand, Nigeria, China and Hong Kong. It's continued to kill poultry and people in Indonesia, the world's hardest hit country with 62 deaths. It has also killed four humans since last month in Egypt.
Animal health officials in Vietnam saw the latest wave of bird flu begin ripping 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 isn't 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's not easy to scour the muddy spider 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 that has worked so well for generations, 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 Dr. Jeff Gilbert, an animal health expert at the U.N. 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 here. No poultry outbreaks were reported in 2006, and there have been no human cases since November 2005.
For nearly two years, the government has banned the restocking of ducks because no vaccine exists for day-old ducklings as it does for day-old chicks. The first inoculation cannot be given until ducklings are 14 days old and most, by that age, would have already left the hatcheries for markets, where they are susceptible to infection.
``We cannot control the restocking of waterfowl. It is very difficult for us to enforce this ban,'' says Khuc Van Hung, animal health director for Vinh Thuan. Farmers have ``been doing this for decades, and there's no alternative for them.''
Last winter, the virus swept across countries in Africa, the Middle East, Central Asia and into Europe. The World Health Organization has warned that a repeat is possible this year, encouraging countries to remain on high alert.
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, potentially sparking a global pandemic. So far, most human cases have been traced to contact with infected birds.
The virus' spread has been blamed on migratory birds and the vast movement of poultry, including the smuggling of domestic birds and their parts across borders. Ducks are a particular concern because they can carry the virus without showing symptoms, spreading it through their droppings.
It's something Nguyen Van Muon thinks about occasionally as he walks barefoot through a soupy mixture of mud, feathers and duck manure in an empty rice field. He's paid about $31 a month to live with a flock of 900 ducks that have traveled 30 miles to fatten on leftovers. ``I've heard of bird flu, and I am a little afraid of it,'' Muon, 21, says shrugging his shoulders in the afternoon heat. ``But I have to do my job.''