Primary and secondary bacterial pathogens behave
differently, especially during the compromised period
between waning passive and protective active immunity,
for several reasons. First, A. pleuropneumoniae and
M. hyopneumoniae have a higher pathogenicity than H.
parasuis and M. hyorhinis. Second and very importantly,
H. parasuis and M. hyorhinis infect the nasal and tracheobronchial
epithelium very early in the piglet’s life
(Ross 1984). This might facilitate a gradual development
of active immunity under the cover of humoral
colostral antibodies, which is beneficial to the host