Comparison of pathological signs of disease in specific-pathogen-free calves after inoculation of the respiratory tract with Ureaplasma diversum or Mycoplasma canis.

To confirm the pathogenic role of Ureaplasma diversum in respiratory disease of calves, we inoculated caesarean-delivered, colostrum-deprived calves intranasally with a dose of 10(7) colour-changing units (CCU) or endobronchially with a dose of 10(10) CCU. Clinical signs of respiratory disease were not observed, but in the endobronchially inoculated calves, thick cuffs of round cells surrounded the bronchi, bronchioli and blood vessels, and a lobular catarrhal pneumonia developed. It was concluded that the pathogenicity of U. diversum can be demonstrated after endobronchial but not after intranasal inoculation. Similar calves were inoculated endobronchially with a dose of 2 x 10(10) colony-forming units of Mycoplasma canis. Clinical signs of respiratory disease were not observed. At day 2 after inoculation, only slight pathological signs of respiratory disease were detected, and these disappeared at day 9. M. canis was not recovered from the lungs. Hence, M. canis could not be clearly identified as a pathogen in respiratory disease of calves. By comparing the results of the various experiments, we concluded that thin cuffs of round cells in the lungs can indicate mycoplasma infections, but that these are not necessarily pathognomonic.