While the mouse model has been used for immunogenicity studies, the murine disease does not have the same pathogenicity as human infection and is not ideal for H5N1 infection studies. These differences may be due to species changes in HA receptor specificity and distribution, as well as differential immunopathogenicity [13] [14] [15] [16] [17] . In contrast, it is generally accepted that ferrets exhibit pathology more similar to humans after H5N1 infection, including severe lethargy, fever, weight loss, transient lymphopenia, and viral replication in the upper/lower respiratory tracts and multiple systemic organs [13] . Furthermore, human isolates of influenza virus have been shown to attach and infect ferret airways [49, 50] , indicating that humans and ferrets share similar HA receptor specificity [51] .