Aerosol vaccination of pigs against Mycoplasma hyopneumoniae infection

<jats:title>Summary</jats:title> <jats:p>Aerosol vaccination is used effectively to immunize poultry against Newcastle disease, but to the authors’ knowledge, this vaccination procedure is not well studied in other species. The efficacy of <jats:sc>im</jats:sc> and aerosol vaccination of pigs against <jats:italic>Mycoplasma hyopneumoniae</jats:italic> infection was evaluated. Twenty-one pigs from a <jats:italic>Mycoplasma</jats:italic>-free herd were randomly allotted by litter and body weight into 3 groups. One group was given aerosolized phosphate-buffered saline solution (<jats:sc>pbss</jats:sc>) by inhalation. The second group (<jats:sc>aero</jats:sc>) was given aerosolized <jats:italic>M hyopneumoniae</jats:italic> vaccine by inhalation. The third group (<jats:sc>im</jats:sc>) was given the same vaccine by <jats:sc>im</jats:sc> injection. Vaccination by <jats:sc>im</jats:sc> administration was repeated once, and aerosol vaccination was repeated twice at 2-week intervals. Two weeks after the last vaccination, all pigs were intra-tracheally challenge-exposed with 3 ml of broth culture containing 10<jats:sup>7</jats:sup> color-changing units (<jats:sc>ccu</jats:sc>) of a low-passage strain of virulent <jats:italic>M hyopneumoniae</jats:italic>. Pigs were observed daily for coughing. Four weeks after challenge exposure, all pigs were necropsied. Percentage of lung affected by gross pneumonia was measured, bronchioalveolar lavage fluid (<jats:sc>balf</jats:sc>) cells were counted, and quantitative culture for mycoplasmas was performed on lung sections. Additionally, <jats:italic>M hyopneumoniae</jats:italic>-specific antibodies were measured in prevaccination, postvaccination, and postchallenge-exposure serum and <jats:sc>balf</jats:sc> by use of indirect <jats:sc>elisa</jats:sc>. Mean prevalence of persistent coughing in pigs of the <jats:sc>aero</jats:sc> group (4.6 d/pig) was not different from that in pigs of the <jats:sc>pbss</jats:sc> group (3.7 d/pig). Prevalence of coughing in <jats:sc>im</jats:sc> vaccinated pigs (1.0 d/ pig) was lower (<jats:italic>P</jats:italic> < 0.05) than that in pigs of the <jats:sc>pbss</jats:sc> group. Mean gross lung lesion scores and <jats:sc>balf</jats:sc> cell counts were not different between the <jats:sc>aero</jats:sc> (15% pneumonia, 5,233 cells/μl) and <jats:sc>pbss</jats:sc> (11% pneumonia, 3,022 cells/μ1) groups, but were lower (<jats:italic>P</jats:italic> < 0.05) in the <jats:sc>im</jats:sc> group (1.5% pneumonia, 400 cells/μl) than in the <jats:sc>pbss</jats:sc> group. Mean lung mycoplasmal counts were not significantly (<jats:italic>P</jats:italic> < 0.05) different among the <jats:sc>pbss</jats:sc> (10<jats:sup>5.6</jats:sup> CCU/g), <jats:sc>aero</jats:sc> (10<jats:sup>5.3</jats:sup> CCU/g), and <jats:sc>im</jats:sc> (10<jats:sup>3.3</jats:sup> CCU/g) groups. Postvaccination <jats:italic>M hyopneumoniae</jats:italic>-specific IgG or IgA was not detectable in <jats:sc>balf</jats:sc> after either vaccination procedure. Postvaccination <jats:italic>M hyopneumoniae</jats:italic>-specific serum IgG concentration was not different among the 3 groups. Postchallenge exposure <jats:italic>M hyopneumoniae</jats:italic>-specific IgG and IgA were detectable in <jats:sc>balf</jats:sc> of all pigs, but were not different among the 3 treatment groups. Postchallenge exposure-specific serum IgG concentration was not different between the <jats:sc>pbss</jats:sc> (mean OD, 0.739) and <jats:sc>aero</jats:sc> (mean OD, 0.672) groups, but was higher (<jats:italic>P</jats:italic> < 0.05) in the <jats:sc>im</jats:sc> group (mean OD, 1.185) than in the <jats:sc>pbss</jats:sc> group. Aerosol vaccination failed to induce local and systemic antibody responses detectable by <jats:sc>elisa</jats:sc>, and failed to protect pigs against mycoplasmal pneumonia. Intramuscular vaccination failed to induce local and systemic antibody responses detectable by <jats:sc>elisa</jats:sc>, but substantially reduced the clinical signs and lesions caused by challenge exposure to virulent <jats:italic>M hyopneumoniae</jats:italic>.</jats:p>