it would therefore appear that the observed differences in virulence is related to a higher capacity to multiply in the lungs and induce a more severe inflammatory response. The differences in the capacity to multiply in vivo may be related to the Hillcrest strain having an intrinsically faster growth rate compared with the Beaufort strain. The capacity to multiply in the lungs may also be affected by the susceptibility of M. hyopneumoniae
isolates to the inhibitory effects of host immune defenses