|Recently, HPAI H5N1 appeared again in Vietnam, after relatively successful efforts to eradicate it with culling and vaccinations. Is this re-introduction from commercial activity or migratory birds, or had the virus never left, but had been hidden in a reservoir and is now re-emerging? Current signs are while there might be some introduction due to trading, re-emergence is probably a bigger problem. According to Robert Webster, the virus is found in domestic ducks which do not get sick, but become long-term shedders. More importantly, the virus is undergoing ongoing antigenic drift, with multiple variant strains being commonly found in one duck now. Crucially, the HPAI virus has not been found in migratory birds in recent months.
The role of the domestic duck in perpetuating HPAI is now increasingly recognized. The following maps from this paper Free-grazing Ducks and Highly Pathogenic Avian Influenza, Thailand, Gilbert et al, 2006 illustrate the correlation between HPAI outbreak and free-grazing domestic ducks. Notice how both chicken and duck outbreaks correspond geographically to breeding of domestic ducks but not chickens.
In addition, when compared to the production of rice crop, HPAI outbreak areas also correspond to areas producing 2 or more crops a year, while areas producing single crops did not experience any outbreaks.
What does that tell us? Rice growing needs a lot of water, as the paddies need to be flooded for much of the crop cycle. Fields that are only rainfed can only yield one crop a year, while irrigated plots can support 2 or more crops. Now, if you are raising ducks, you can give them commercial feed, or you can turn the ducks out onto the fields, and they can feed naturally on the pests during the growing period and on the left-over grains after the harvest. If you live in areas with year-round rice cultivation, it is possible to breed ducks entirely by `herding' them onto paddies and letting them graze there, hence the name `grazing ducks'.
Lest you imagine that ducks are just grazing at paddies down the road, in reality, the pressure for profit means that farmers often herd their ducks over long distances to graze at paddies. Here is an example from Domestic Ducks and H5N1 Influenza Epidemic, Thailand Songserm et al, EID, 2006, showing how ducks are transported over long distances.
(Caption for above: Example of grazing-duck movement. A single flock of ducks was moved 3 times by truck in 1 season in 2004. The size of the flock is 3,000-10,000. The time spent at each site depends on the availability of rice fields at the site: an acre of rice could support 3,000 ducks for 1 to 2 days. The duck owners have agreements with the landowners regarding the time of harvest and the acreage available. One flock could spend as long as 1 month at a single site before being moved to the next.)
The above studies tell us that HPAI outbreaks are associated with the breeding of grazing ducks, and places with multiple rice crops per year are more likely to support these ducks than single crop areas. The movement of ducks over long distances of course perpetuates the problem.
What this means, interestingly, is that you can more-or-less predict where HPAI is more likely to occur by looking at charts of the distribution of double-crop areas. There are now various technologies available to do satellite remote sensing of landscape to monitor the use of land, including in particular the presence of rice paddies. Here is an interesting study Combining remote sensing and ground census data to develop new maps of the distribution of rice agriculture in China, Frolking et al, Global Biogeochemical Cycles, 2002.
Which brings us to the question of the emergence of HPAI H5N1 in southern China in the past decade, which I've been wondering about for a long time. The question that I had was why southern China? This may be giving us a clue. In addition, look at the following chart showing the tremendous growth of duck meat production in China.
(From Avian influenza, domestic ducks and rice agriculture in Thailand Gilbert et al, Agriculture, Ecosystems and Environment 119 (2007) 409-415)
What about other countries? Webster says that the HPAI virus is now endemic in domestic ducks in Indonesia, China, Vietnam, Egypt, Nigeria. Which brings me to a point made by Okieman in this diary The Rice - H5N1 Connection, where he posted several links showing how countries HPAI outbreaks are all major rice producers. In addition, in Egypt both avian and human outbreaks are concentrated along the Nile, which comprises of flooded lowlands (which, btw, was also found to be correlated to outbreaks in Thailand, as opposed to higher altitudes) suitable for rice production. Indeed, the major rice producing region of Egypt is concentrated at the Nile delta, which is where we are seeing most of the human cases.
This same pattern is also seen in Indonesia. This map is from Mapping paddy rice agriculture in South and Southeast Asia using multi-temporal MODIS images Xiao et al, Remote Sensing of Environment 100 (2006) 95 - 113.
The text of this paper says "While each of Indonesia's five main islands has some areas of intense rice production, heavily populated Java is the most productive rice area".
Finally, I come to a specific problem with this system of raising ducks on paddy fields. For the past decade, this system has been actively advocated as part of sustainable agriculture. Called rice-duck, the idea is to encourage and teach farmers to breed ducks on their paddies in order for pest control, which reduces the use of chemicals, increase rice yield, while of course bringing extra income and protein in the form of ducks and eggs.
This is part of the Global Integrated Production and Pest Management (IPPM) development program by the FAO. Check out this The Duck Ranger: Rice-Duck Systems to the Rescue, a World Bank report, which includes a comment that the leader of this project to teach rice-duck farming in the Philippines was one of the winners of the World Bank's Young Scientist Award 2005.
One question that has been asked numerous times is why is there no HPAI in the Philippines? It could be that they are just lucky, or they did not import the same poultry stock as Indonesia. However, the fact that the World Bank has a program in the Philippines to teach rice-duck integrated farming tells me that the method is probably not indigenous there. Not being able to find any information, I asked a Philippino friend who, as soon as I mentioned ducks, said "But you can't keep ducks on paddies cos they will walk on the crop and ruin it." Interestingly, I found from this document Rice-duck farming: new opportunities for farmers in Bangladesh that Bangladeshi farmers have similar beliefs "Most farmers farm rice and ducks separately, fearing that ducks would harm their rice crops."
To conclude, there are reasons to suggest that the following specific agricultural practices in combination may be contributing to the endemicity, re-emergence, and spread of HPAI H5N1:
- Multiple rice crops per year
- breeding of free-grazing ducks on paddies
- large flock size
- movement of flocks over long distances
Whether we can find solutions or alternatives to these practices, I would suggest that at least in countries where rice-duck is not indigenous practice, that development programs promoting such practice should probably be halted pending further research.
UPDATE Mar 17
This is a composite chart made from FAO maps for vietnam showing (from left to right)
1st row: HPAI outbreaks 2003-04, HPAI outbreaks 2004, chicken density, poverty density, share of rural household raising poultry
2nd row: Mean poultry holding area, raw poultry density, share of commune area irrigated paddy, accessibility, chicken to duck/goose ratio.
Notice which of the various maps correspond to the HPAI outbreaks.