Characterization of a Mycoplasma hyopneumoniae aerosol infection model in pigs

The purpose of the present study was to develop and characterize an experimental aerosol model for Mycoplasma hyopneumoniae (M. hyopneumoniae) infection and respiratory disease in pigs. The experiment was carried out to determine the pathogenicity, colonization, mucosal immune response, and clinical course of disease of dose-controlled aerosols of M. hyopneumoniae. Four groups of three M. hyopneumoniae-free gilts each were individually exposed to aerosols of diluted lung homogenate containing M. hyopneumoniae strain 232 in a chamber. Each group was exposed to different doses of viable organisms (10⁵ to 10⁶ color changing units/mL during 15-20 or 30-35 min in two consecutive days). Nasal, laryngeal, and deep-tracheal secretions were collected from each gilt at 0, 7, 14, 21, and 28 days post-exposure (dpe). Blood samples were collected at 0 and 28 dpe. At necropsy, lung lesions were assessed, and bronchial secretions and bronchoalveolar lavage fluid (BALF) were collected from each lung set. Blood was used to assess seroconversion by means of an indirect ELISA, while BALF, deep-tracheal and nasal secretions were tested by modifying the ELISA to evaluate mucosal IgG and IgA production. Nasal, laryngeal, deep-tracheal, and bronchial secretions were tested by real-time PCR to evaluate bacterial load. Gilts became infected irrespective of the infectious dose, as determined by M. hyopneumoniae detection in deep-tracheal secretions from all gilts at 7 dpe. A specific local humoral immune response starting at 14 dpe was detected in all gilts. While all experimental groups presented gilts with some extent of mycoplasmal pneumonia, only the exposure of gilts to high-dose aerosols consistently reproduced typical clinical signs and severe lung lesions. These results showed that the reproduction of mycoplasmal pneumonia by means of infectious aerosols can be successfully achieved at experimental level, making this model a valuable alternative to evaluate preventive and treatment measures against M. hyopneumoniae.