In the present study, passage levels of the Hillcrest and Beaufort strains were 7 and 15
respectively. It is unclear whether the higher passage number for the Beaufort strain resulted in decreased virulence of this isolate. However, M. hyopneumoniae strains 11, 232 and 144 L used as 21st to 27th passage cultures have induced mycoplasmal pneumonia in challenged pigs