Individual virological results. Detection of the influenza A virus genome in nasal secretions of piglets with (first column, rooms B1, B2 and B3) or without passive immunity (second column, rooms F1, F2 and F3), inoculated or in contact (direct or indirect). Grey-tinted squares match positive M gene RT-PCR results (MDA
in dark grey, MDA
and MDA− in light grey).