Uncontrolled, M. hyopneumoniae causes a moderate pneumonia, depresses the pig's immunodefence system, predisposes to other infections, slows growth rate and reduces feed conversion efficiency (FCE) by 5-10%.
 

However, in the field situation, nearly 40-50% of lesions are infected with secondary invaders, particularly Pasteurella multocida . These cause serious broncho-pneumonia and increase the severity of the porcine respiratory disease complex (PRDC). Growth and FCE are further reduced, while mortality and the need for additional treatments are increased.