Investigations into the possible role of Mycoplasma arginini in ovine respiratory disease.

The inoculation of eight five- to seven-month-old sheep by the respiratory route with a culture of Mycoplasma arginini, administered simultaneously with or two days before a culture of Pasteurella haemolytica A2, did not lead to the pulmonary establishment of either organism. Minor lung changes found at slaughter seven days later were therefore considered not to have been induced by the inocula. Two other groups of seven sheep each were initially inoculated intratracheally with an ampicillin-treated lung lesion homogenate in which only M ovipneumoniae was detectable. After seven days one group was inoculated intranasally and intratracheally with mixed cultures of M arginini and P haemolytica A2, and one with P haemolytica A2 alone. In the M arginini-treated group pyrexia peaked earlier and one animal died, but no macroscopic or microscopic differences were apparent between the two groups at necropsy 10 to 11 days later; six sheep from each group had lung lesions indistinguishable from ovine atypical pneumonia. M arginini was isolated in high titre from the respiratory tract of two animals in the M arginini-treated group, including the sole fatality. However, an adventitious parainfluenza type 3 virus infection, identified in four animals from the M arginini-treated group and one from the other, may have been responsible for the inter-group clinical differences. It was concluded that the strain of M arginini used was capable neither of predisposing the lung to secondary invasion by P haemolytica A2, nor of exacerbating the pneumonia and effects elicited with M ovipneumoniae and P haemolytica.