Ducks and Rice Play Key Role in Bird Flu Outbreaks
26/03/08

Ducks, people and rice paddies – rather than chickens – are the major factors behind outbreaks of H5N1 highly pathogenic avian influenza in Thailand and Viet Nam, and are probably behind outbreak persistence in other countries of the region such as Cambodia and Lao PDR.

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In "Mapping H5N1 highly pathogenic avian influenza risk in Southeast Asia: ducks, rice and people", just published in the latest issue of the Proceedings of the National Academy of Sciences of the United States (PNAS), a group of experts from FAO and associated research centres looked at the series of waves of H5N1 highly pathogenic avian influenza (HPAI) in Thailand and Viet Nam between early 2004 and late 2005.

Initiated and coordinated by FAO senior veterinary officer Jan Slingenbergh, the researchers applied a modelling technique to establish how different factors contributed to spread of the virus, including the numbers of ducks, geese and chickens, human population size, rice cultivation and local geography. The numbers of ducks and people, and the extent of rice cultivation emerged as the most significant factors, even though the two countries had fought outbreaks in two different ways.

Strong link

The paper notes that there is a strong link between duck grazing patterns and rice cropping intensity. Ducks feed mainly on leftover rice grains in harvested paddy fields, so free-ranging ducks in both countries are moved to many different sites in line with rice harvest patterns,

In Thailand, for example, the proportion of young ducks in flocks was found to peak in September-October; these rapidly growing young ducks can therefore benefit from the peak of the rice harvest in November-December. Meat ducks are slaughtered around the Chinese New Year, a time when the volume of sales-related duck movement rises considerably.

These peaks in congregation of ducks indicate periods in which there is an increase in the chances for virus release and exposure, and rice paddies often become a temporary habitat for wild bird species.

Defining this pattern was made possible through the use of satellite mapping of rice paddy agriculture over time, cropping intensity and duck grazing locations. The intersections among these, together with the chronology of disease outbreaks, helped the scientists pinpoint critical situations in time when HPAI risk was greatest.

Virus evolution may become easier to predict

According to Slingenbergh, "we now know much better where and when to expect H5N1 flare-ups, and this helps to target prevention and control. In addition, with virus persistence becoming increasingly confined to areas with intensive rice-duck agriculture in eastern and southeastern Asia, evolution of the H5N1 virus may become easier to predict."

FAO estimates that approximately 90 percent of the world’s 1.044 billion domestic ducks are in Asia. China and Viet Nam account for the bulk of this – 775 million or 75 percent. Thailand has about 11 million ducks.

In Thailand during 2005, long-distance duck travelling greatly diminished because farmers and traders had to provide a health certificate for the animals. The local movements of ducks decreased when the government started to support in-door keeping of ducks, offering feed subsidies and construction of enclosures. Together, these measures stopped the H5N1 transmission cycle and since late 2005 Thailand has suffered only sporadic outbreaks.

Viet Nam started nationwide vaccination of all poultry at the end of 2005, including the Mekong delta which is home to 50 million ducks. This large-scale vaccination was repeated in 2006/07. Initially, human infections disappeared and levels of disease in poultry fell noticeably. Only gradually did H5N1 viruses re-appear, mostly in unvaccinated ducks and particularly in the Mekong delta.

Now, says Slingenbergh, interventions based on knowledge of hotspots and local rice-duck calendars is called for, in order to target disease control and replace indiscriminate mass vaccination.

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