Pneumonic pasteurellosis induced experimentally in gnotobiotic and conventional calves inoculated with Pasteurella haemolytica

SUMMARY Experimental pneumonia caused by Pasteurella haemolytica was induced in 2-week-old gnotobiotic (n = 4) and conventional (n = 6) calves by endobronchial inoculation into the right caudal lung lobe of 7.9 × 10 10 ± 0.6 × 10 10 (mean ± sd ) colony-forming units of P haemolytica in the 6-hour log phase of growth. The calves were studied for 24 hours or less. Regression lines for the relationship between clinical index and time for the gnotobiotic group and conventional group of calves were compared, and the clinical index was found to be significantly ( P ≤ 0.005) more rapid in the gnotobiotic group. There was also a significant difference in the preinoculation, absolute segmented neutrophil count (P ≤ 0.05), and in the total serum protein, albumin, and globulin values (P ≤ 0.05). Comparison of the preinoculation and post inoculation blood cell and blood chemical values revealed a significant increase ( P ≤ 0.05) in the numbers of band neutrophils and fibrinogen in conventional calves, and a significant decrease ( P ≤ 0.05) in the total wbc count in gnotobiotic calves. Necropsy of both groups of calves revealed a circular to oblong lesion that was congested, edematous, and firm, and which occupied 20% to 100% of the right caudal lung lobe and involved the remaining lung lobes to a more minor degree. When mean lesion scores of the 2 groups of calves were compared, no significant difference ( P ≤ 0.05) was found. Microscopic examination of the lungs revealed edema of the perivascular and interlobular septa and hemorrhage in the alveoli of both groups, although the conventional group had more fibrinopurulent inflammation.