Here is one account from Promedmail, an excellent site maintained by the International Society for Infectious Diseases:
This is from one of my colleagues (a doctor) who is working in Indonesia. He visited a pig and chicken farm in Bali.
"There were 1000 chickens living over 20 pig pens with just slats between them. It is a farming system called tampung sari. This way the chickens' faeces falls directly to the pigs and they eat it. Apparently pigs can get up to 40 percent of the RDA [nutritional recommended daily allowance ] from chicken faeces, and it is common practice to feed chicken faeces to pigs.
To my knowledge culled chickens are not used to feed pigs (at least in tampung sari), but it is possible. It is well known that chicken faeces [i.e., feces from infected chickens] contain H5N1, so the exposure from chicken faeces is enough to explain the finding.
Likewise, in China, ducks and pigs live closely together so that it is easy to explain the finding of H5N1 in pigs in China in 2004."
[Integrated farming systems are meant to utilize the products of one sub-system by another sub-system, generating useful materials like protein biomass in the process. Various combinations are known. In Southeast Asia, integrated systems combining chickens/ducks, pigs and fish are widely applied. - Mod.AS]
For further reading, here is a paper on integrated fish-livestock systems, and an older article on the possible link between agricultural practices and transmission/mutation in viruses.
