Interactions of cold stress and Pasteurella haemolytica in the pathogenesis of pneumonic pasteurellosis in calves: method of induction and hematologic and pathologic changes.
Six healthy neonatal calves were chilled with cold water and had focal tracheitis induced by spraying 5% acetic acid into the tracheal lumen. Subsequently, 20 ml of sterile saline solution was injected intratracheally. The effects of these interventions on total and differential white cell counts, plasma cortisol, histamine, and bradykinin, hematocrit, total plasma solids, and indices of the erythrocyte size and hemoglobin content were determined over the subsequent 12 hours. Cold stress increased plasma cortisol levels for less than 1 hour, but did not alter any other variable. This group of calves served as a control group for a second series of neonatal calves which were given 2 X 10(9) organisms of Pasteurella haemolytica intratracheally immediately following an identical period of chilling and acetic acid exposure. Calves given P haemolytica became neutropenic. There were increased numbers of circulating band neutrophils by 12 hours after exposure, and serum cortisol values were maintained at the same or greater than cold stress concentrations for all measurement periods subsequent to exposure. Infected calves had acute fibrinous pneumonia from which P haemolytica was isolated. Contrary to previous reports, these data may indicate a role for the neutrophil in the pathogenesis of early lesions of pasteurellosis. Although the association of circulating corticosteroids with stress and subsequent infection is clear, our data provide no evidence to indicate that circulating histamine or bradykinin are involved in the pathogenesis of the acute lesions of Pasteurella pneumonia.