Abstract
The study was conducted in the Abai Region of Kazakhstan to diagnose bovine pasteurellosis using a mouse bioassay. Pathological samples of liver, spleen, lungs, and colon were collected from 35 affected cattle on Lana Farm, where recent mortalities had occurred. Samples were transported to the laboratory at 4°C within three hours and prepared as 10% tissue suspensions for bioassay. Specific-pathogen-free (SPF) white mice (20–25 g, 4–6 weeks old) were divided into control and experimental groups (n=5 per group). Experimental mice received 0.5 mL of the tissue suspension via intraperitoneal injection, while controls received sterile saline. Mice were observed for 72 hours for clinical signs, including lethargy, tachypnea, ruffled fur, and decreased activity. Within 12–18 hours post-inoculation, experimental mice exhibited initial signs of infection. By 24–48 hours, 80–90% of mice had died, whereas control mice remained healthy. Necropsy revealed lesions characteristic of acute septicemia, including petechial and suffusive hemorrhages on the heart, enlarged and pasty spleen, hepatized lungs, and blood-filled liver. Impression smears from tissues stained with Romanowsky–Giemsa and Gram methods showed numerous bipolar, Gram-negative coccobacilli typical of Pasteurella multocida. High-power microscopy revealed bacterial clusters around blood vessels and phagocytized bacteria within neutrophils, confirming systemic infection. These results indicate the high virulence of the isolated strain and confirm the effectiveness of the mouse bioassay as a sensitive diagnostic method, particularly when culture-based identification is challenging. Combining bioassay with smear microscopy enables reliable detection of the pathogen and allows for timely implementation of control measures on the farm. The study highlights the importance of in vivo approaches in comprehensively evaluating the pathogenicity and dissemination of P. multocida in livestock populations.