Mycoplasma bovis pneumonia and arthritis in feedlot cattle
- Identify evidence of damage mediated by hydrogen peroxide or other oxygen radicals in the characteristic caseonecrotic lesions caused by Mycoplasma bovis.
- Determine whether Ontario strains of Mycoplasma bovis produce hydrogen peroxide in differing amounts and whether these differences correlate with the presence or absence of disease.
Mycoplasma bovis causes chronic pneumonia in feedlot cattle and is particularly problematic as it fails to respond well to antibiotic therapy, leading to chronic suffering and in many cases euthanasia. M. bovis infects the lungs of most feedlot cattle but is enigmatic because the majority of these calves never develop disease. An explanation is that some strains of M. bovis are more virulent than others, or alternately there is the premise that calves may require some other abnormality for M. bovis to cause disease. It was hypothesized that hydrogen peroxide production is an important virulence factor of M. bovis, that causes oxidative injury to lung tissue that is distinct and identifiable using immunohistochemistry when compared to other forms of pneumonia. The second hypothesis was that strains which cause pneumonia have an increased capacity for hydrogen peroxide production. Immunohistochemical markers for oxidative damage--malondialdehyde, 4-hydroxynonenal, and nitrotyrosine--were used to compare the lungs of calves with caseonecrotic pneumonia consistent with M. bovis, or with bronchopneumonia lacking caseous necrosis. Oxidative damage was identified in these lung lesions of M. bovis bronchopneumonia, although it mainly affects foci of coagulative necrosis and to a lesser extent those of caseous necrosis, and was less extensive than seen in cases of Mannheimia haemolytica pneumonia (shipping fever). Thus, oxidative damage does indeed occur in the lung lesions caused by Mycoplasma bovis. Hydrogen peroxide production by M. bovis strains isolated from the lungs of calves with pneumonia was measured. Strains of M. bovis that cause caseonecrotic bronchopneumonia produced higher amounts of hydrogen peroxide in comparison to other strains isolated from the lungs of feedlot calves, indicating that isolates of Mycoplasma bovis from differing forms of pneumonia do differ in their capacity to produce hydrogen peroxide.
These findings show, for the first time, a specific way in which this bacterium causes damage to the lung (oxidative injury), that this bacterium is able to produce substances (hydrogen peroxide) that are known to cause this type of injury, and furthermore that isolates of this bacterium from lesions of mycoplasmal pneumonia seem to produce more of this hydrogen peroxide compared to isolates that seem not to be causing disease (that is, isolates from other forms of pneumonia). The findings of this study will be of relevance to Ontario veterinarians and producers by leading to a diagnostic test that distinguishes disease-associated from harmless strains of Mycoplasma bovis, and in the longer term to design more appropriate interventions such as a vaccine to prevent this disease and the associated death losses.
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