Therefore, a survey of these workers is timely and warranted, given confirmed persistence of avian influenza in Asia (13 ).
Free-grazing ducks in rice paddies are a critical factor in the spread and persistence of avian influenza.
Although persistence of avian influenza viruses in water appears to be the natural mechanism to maintain and transmit influenza viruses in wild bird populations, Brown et al. (34 ) compared 2 strains of HPAI virus (H5N1) with several wild bird origin LPAI viruses and found that HPAI virus (H5N1) does not persist in water as long as LPAI viruses, at least under experimental conditions.
The spatial spread of H5N1 avian influenza and long-term persistence of the virus in some regions has had an enormous impact on the poultry industry and presents a serious threat to the health of humans and migratory birds [ 3, 4].
Four surface glycoprotein subtypes in California isolates (H6, N8, N1, and N2), representing 162626%) segments, yielded phylogenetic trees that suggest reassortment between continental gene pools of avian influenza viruses, followed by persistence and evolution of these mixed lineage clades in Asia and North America.
One of these rare cases demonstrated the potential transmission of the bacteria from an avian reservoir through occupational exposure and its persistence in humans.
This virologic evolution may be a factor in the persistence of H5N1 viruses in the avian populations.
For example, animals from diverse origins with varying immune status that are housed together in abattoirs before slaughter may provide opportunity for virus amplification and persistence, analogous to that seen with avian influenza virus in live poultry markets (13 ).
The spatial spread of H5N1 avian influenza, significant ongoing mutations, and long-term persistence of the virus in some geographic regions has had an enormous impact on the poultry industry and presents a serious threat to human health.
A recent paper by Savill et al. used a detailed stochastic model to show that introduction of H5N1 avian influenza into vaccinated poultry populations could promote undetected pathogen persistence, facilitating "silent" spread to neighboring farms [6].
