The experimental study showed that not all inoculated
ducks developed either HI or neutralizing antibodies by
the end of the trial at 34 dpi. The virus may have failed
to successfully replicate at the entry point in eyes, nasal
or oral cavity of these antibody-negative ducks. Alternatively,
the virus may have replicated to some degree at
these mucosal entries, but was unable or had no chance
to stimulate a humoral response because of a strong innate
immune responses such as a rapid apoptosis mechanism
[41] or RIG-I pathways [42].