The present study describes the gross, histopathology and ultrastructural changes in the respiratory, gastrointestinal and urinary tracts of buffalo calves experimentally inoculated with wild-type Pasteurella multocida B:2. Six 8-month old buffalo calves were divided into two groups. Calves of Group 1 were inoculated subcutaneously with 5 ml the inoculum containing 10⁹ colony-forming unit (cfu)/mL of live wild-type P. multocida B:2 while calves of Group 2 were similarly inoculated with 5 ml of sterile phosphate buffered saline. All buffaloes were observed daily for clinical signs before surviving buffaloes were killed on day 3 post-inoculation for gross and histology examinations of the respiratory, gastrointestinal and urinary tracts. All infected calves of Group 1 were dead between 6 and 12 h post-inoculation with typical gross lesions of hemorrhagic septicemia. The respiratory, gastrointestinal and urinary tracts showed moderate to severe congestion and hemorrhages. Frothy fluid and fibrin were found in the respiratory tract while the content of gastrointestinal tract appeared soft, watery and occasionally blood-tinged. The urinary tract contained blood-tinged urine. Histologically, the organs of respiratory, gastrointestinal and urinary tracts showed varying degrees of congestion and hemorrhages with endothelial destruction. The right middle lung lobe, the small intestines and kidneys showed significantly more severe histological lesions. Ultrastructure examination revealed the presence of P. multocida B:2 on the erythrocytes and the endothelium of the respiratory, gastrointestinal and urinary tracts. Similarly, P. multocida B:2 was successfully isolated from all samples collected from these tracts, suggesting the involvement of these body systems in transmission of the disease.

Key words: Gastrointestinal tract, Hemorrhagic septicemia, Respiratory tract, Urinary tract