Evaluation of clinical, histological and immunological changes and qPCR detection of Mycoplasma hyopneumoniae in tissues during the early stages of mycoplasmal pneumonia in pigs after experimental challenge with two field isolates.

Differences in Mycoplasma hyopneumoniae strain virulence and infection patterns will affect experimental challenge systems used to evaluate vaccine efficacy. Two strains (Hillcrest and Beaufort) were assessed by experimental pig challenge for their ability to induce clinical and pathological lesions and cytokine responses. Tracheobronchial lavage fluid (TBLF) was collected before and 17-18 days after challenge with Hillcrest (n=8), Beaufort (n=8) or no organisms (n=3). Coughing was assessed twice daily, and at slaughter 21 (n=9) or 28 (n=10) days post-challenge, gross and histopathology of lungs were quantified and a quantitative PCR (mhp183 qPCR) was applied to detect M. hyopneumoniae DNA in tissues and TBLF. Hillcrest was clearly superior to Beaufort in its ability to induce coughing and pneumonic lesions. At 17-18 days, interleukin (IL)-1β and IL-6 concentrations in TBLF were only significantly higher (8.7 and 5.1 fold respectively) than controls (P<0.001) in Hillcrest-challenged pigs. Lungs of all Hillcrest-challenged pigs were qPCR positive at either slaughter date, but only at day 28 in Beaufort-challenged pigs. M. hyopneumoniae DNA was highest in concentration in lungs 21 days after Hillcrest challenge, and was detected in the spleen, kidney and/or liver of Hillcrest-challenged pigs, but not in Beaufort pigs. While M. hyopneumoniae DNA concentration in TBLF was elevated following Hillcrest and Beaufort challenge, there was no significant difference in mean mycoplasmal DNA concentration detected in TBLF from pigs challenged with either isolate (P>0.05). Thus a suitable challenge strain, coupled with lung pathology and cytokine assays, are valuable in assessing post-challenge responses. Assessment of M. hyopneumoniae DNA in lung and abdominal tissues by mhp183 qPCR, in conjunction with histopathology, were valuable in confirming M. hyopneumoniae infection.