Etiology and Pathogenesis:
M haemolytica and B trehalosi are distributed worldwide, and diseases caused by them are common in sheep and goats of all ages, although the prevalence of serotypes may vary by region and flock. M haemolytica, B trehalosi, and P multocida are common commensal organisms of the tonsils and nasopharynx of healthy sheep and goats. The presence of multiple Pasteurella spp may
serve to keep the bacterial populations in check, because there appears to be some interference with growth when multiple species are present. For these organisms to cause infection, a combination of stressors, including heat, overcrowding, exposure to inclement weather, poor ventilation, handling, and transportation, leaves sheep and goats susceptible to respiratory viral infections. Parainfluenza 3, adenovirus type 6, respiratory syncytial virus, possibly bovine adenovirus type 2, ovine adenovirus types 1 and 5, and reovirus type 1 cause primary respiratory infections that are rarely life threatening but predispose to secondary M haemolytica infections. Respiratory infections with Mycoplasma ovipneumoniae and Bordetella parapertussis have also been reported to be associated with secondary M haemolytica infections. The combination of stressors and primary infections are thought to break down the mucosal barrier integrity of the lower respiratory tract, allowing M haemolytica to colonize, proliferate, and induce significant tissue damage. The virulence of M haemolytica and B trehalosi is mediated by the action of several factors, including endotoxin, leukotoxin, and capsular polysaccharide, that afford the bacteria advantages over host immunity. The leukotoxin is particularly important in the pathogenesis, because it is specifically toxic to ruminant leukocytes, resulting in fibrin deposition in lungs and on pleural surfaces. The lipopolysaccharide endotoxin contributes to adverse reactions in the lungs and also leads to systemic circulatory failure and shock. The capsular polysaccharide prevents the phagocytosis of the bacteria and assists in attachment to the alveolar epithelial surface. Survival of the acute phase of pneumonic pasteurellosis depends on the extent of lung involvement and damage in the lower respiratory tract. Sheep and goats that recover may have chronic respiratory problems, including reduced lung capacity and weight gain efficiency if ≥20% of the lung was damaged. In one review, there was no association between virulence and the presence of hemolysis on blood agar culture plates. Clinical Findings and Lesions:
B trehalosi mainly causes septicemia and systemic pasteurellosis in sheep
