Also, the lesions induced by secondary pathogens like
H. parasuis and M. hyorhinis occur in sites (pleura, pericardium,
peritoneum, meninges, and joints) outside
the respiratory tract. There is a clear physical barrier
that exists between the infection site and the effector
site of the microbe. In contrast, primary pulmonary
pathogens like A. pleuropneumoniae and M. hyopneumoniae
are easily delivered to the lungs directly
from the environment or from the nasal and tonsillar
epithelium.