Ultrastructural study of ovine pulmonary pasteurellosis: involvement of neutrophils and macrophages

Pasteurellosis is a common infectious disease characterised by fibrinous pn eumonia and involving neutrophils and macrophages. This study aimed to dete rmine the timing and extent of the pathogenic involvement of these cell ele ments in lesions induced in experimentally-infected lambs. A concentration of approximately 3x10(8) bacteria/ml. was inoculated into 3 0 two-month-old disease-free Merino lambs. Five lambs were assigned to each of five experimental batches, slaughtered on days 1, 3, 7, 11 and 15 follo wing intratracheal inoculation, and to one control batch inoculated with a sterile solution. One control animal was slaughtered at the same time as ea ch experimental batch. More characteristic lesions occur in bronchioles, peribronchial tissue and alveoli and are characterised by fibrinous processes. From the start of the experiment, epithelial-cell disruption and loss of microvilli were apparen t; cell debris, desquamate cells and bacterial elements were observed in br onchiolar lumina, embedded in a fibrillar granular material. Alveolar struc tures displayed fewer neutrophils and macrophages, containing phagocytic va cuoles. Laminar bodies were apparent in type II pneumocytes. The intersepta l area contained similar cell types, as well as abundant edema. In the cour se of the experiment, macrophage numbers increased in all the areas involve d, with signs of intense phagocytic activity. The final phase of the experi ment was characterised by a mild interseptal infiltrate and by clear alveol ar lumina.