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.
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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