In some herds, pigs are infected in the nursery especially if it is operated on a continuous-flow basis and younger pigs are commingled with older pigs. Transmission of M. hyopneumoniae primarily is through direct contact. Therefore, environmental adjustments are designed primarily to provide a comfortable living space for the pigs rather than to dilute the number of organisms suspended in the air. In a herd in which there are no clinical signs of mycoplasmal pneumonia, typical lesions may be seen in the lungs at a slaughter check. There have been occasional reports of nursing pigs being affected with mycoplasmal pneumonia, typically, signs are seen in pigs aged 6 to 10 weeks and older.
Affected pigs have a dry, nonproductive cough that is most noticeable after exercise.
Coughing may persist for 1 to 2 months. Although pigs continue to eat, feed intake is usually depressed and pigs fail to grow at a normal rate particularly if lesions are extensive due to secon- dary bacterial complications. The extent of damage to the lung and effect on growth rate are variable depending on the dose of M. hyopneumoniae, number and kind of secondary infections and degree of environmental stress. Environmental Factors that Influence Severity of Infection Researchers have identified numerous factors that influence the severity of clinical pneumonia in infected herds. These are presented in Table 1.