Summary
The most common forms of inflammation of the lower respiratory tract in lambs are acute enzootic pneumonia, caused mainly by Mannheimia haemolytica, chronic enzootic pneumonia (defined as ‘atypical’ in lambs), the aetiological of which is Mycoplasma ovipneumoniae and viral inflammation principally caused by parainfluenza virus type 3. The authors conducted anatomopathological and histopathological studies of the most commonly encountered spontaneous lung inflammations in lambs slaughtered in the Abruzzo region of Italy, with special attention to ‘atypical pneumonia’. Microbiological isolations and a histopathological and immunohistochemical analysis were performed to reveal any possible correlations between causal agents and lesion patterns. Positive results for M. ovipneumoniae were compared to those for Mycoplasma isolation to evaluate the sensitivity of the two techniques. Of a total of 156 samples, 31 (19.8%) demonstrated involvement of M. ovipneumoniae, 15 (9.6%) were positive on microbiological isolation confirmed by typing with biomolecular methods and, finally, histological lesions (atypical pneumonia) were observed in the remaining 16 cases (10.2%). Of these 31 samples, 23 (14.7% of the total) demonstrated postive antigen in alveolar macrophages and giant cells on immunohistochemical testing. These data revealed the presence of chronic enzootic pneumonia in the Abruzzo area and the importance of immunohistochemistry (in combination with isolation and anatomopathological and histopathological examination) for the diagnosis of pneumonia caused by M. ovipneumoniae, as well as the high sensitivity shown by antigen marker expression, even in samples where bacterial load was limited.

Keywords
Histopathology, Immunohistochemistry, Lambs, Mannheimia haemolytica, Mycoplasma ovipneumoniae, Parainfluenza virus type 3, Pneumonia.