INTERACTION OF MYCOPLASMA-HYOPNEUMONIAE AND PASTEURELLA-MULTOCIDA INFECTIONS IN SWINE

To investigate the interaction between Mycoplasma hyopneumoniae and Pa steurella multocida infection, 32 pig were randomly assigned by litter , sex, and weight to 4 treatment groups. Group-1 pigs were inoculated wi th M hyopneumoniae and allowed to recover from M hyopneumoniae infe ction. Group-2 pigs were vaccinated against M hyopneumoniae and then i noculated with M hyopneumoniae. Group-3 pigs were inoculated with M hy opneumoniae and developed clinical signs of mycoplasmosis. Group-4 pig s had never been exposed to M hyopneumoniae. All pigs were initially s eronegative for M hyopneumoniae. All pig were subsequently inoculated with P multocida and euthanatized 2 weeks later. Pasteurella multocida was isolated only from the lungs of group-3 pigs, and these pig had a significantly higher median percentage of lung surface ai-ea affected by pneumonia than did pigs in the other groups. For group-3 pigs, per centage of lung surface area affected by pneumonia was positively corr elated with the number of P multocida colonies isolated. We concluded that P multocida is not a primary respiratory pathogen in pigs, but th at M hyopneumoniae infection can render the lungs susceptible to P mul tocida colonization and infection. Pigs recovered from or vaccinated a gainst infection with M hyopneumoniae were resistant to P multocida in fection.