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This study aimed to identify the suspected pathogens causing rabbit mortality in a large-scale meat rabbit farm in Ningxia and analyze their pathogenicity. The causative pathogens and their pathogenicity were determined through necropsy of deceased rabbits， bacterial isolation and culture， 16S rRNA sequencing analysis， PCR identification， antimicrobial susceptibility testing， and challenge experiments in mice and rabbits. Results showed that 3 Gram-negative bacterial strains were isolated and identified as capsular type A Pasteurella multocida， Proteus mirabilis， and Escherichia coli through 16S rRNA sequencing and PCR identification. These strains were designated as QF24-Pm01， QF24-Ps02， and QF24-Ec03， respectively. Antimicrobial susceptibility testing by the Kirby-Bauer （K-B） method revealed that QF24-Pm01， QF24-Ps02， and QF24-Ec03 exhibited resistance to commonly used clinical drugs such as penicillin， kanamycin， and enrofloxacin， but were sensitive to ceftriaxone. Detection of resistance genes showed that all three isolates carried resistance genes aadB， strA， strB， blaTEM， and sul1. Virulence gene analysis indicated that Pasteurella multocida carried 16 virulence genes， including exbB， exbD， and hgbA； Proteus mirabilis carried ureC， zapA， atfC， atfA， and pmfA； and Escherichia coli carried eaeA and fimC. Mouse pathogenicity experiments revealed LD50 values of 9.27×10⁵ CFU， 1.43×10⁶ CFU， and 1.88×10⁷ CFU for QF24-Pm01， QF24-Ps02， and QF24-Ec03， respectively. The results of the mixed-infection pathogenicity assay in mice showed that mortality occurred at 9 h， 18 h， and 16 h post-inoculation in the QF24-Pm01， QF24-Ps02， and QF24-Ec03 groups， respectively. In contrast， the QF24-Pm01+QF24-Ec03 and QF24-Ps02+QF24-Ec03 co-infection groups exhibited mortality at 6 h and 13 h post-inoculation， respectively. Histopathological analysis revealed that both single and mixed infections resulted in tissue damage in the lungs， liver， and intestines. The primary pathological changes included pulmonary interstitial edema， capillary congestion， and neutrophil infiltration； hepatic steatosis and disordered hepatocyte cord arrangement； as well as intestinal epithelial shedding and villous rupture. In the rabbit pathogenicity assay， all rabbits in the QF24-Pm01 + QF24-Ec03 co-infection group and the QF24-Pm01 group succumbed at 24 and 36 h post-inoculation， respectively. The mortality rates at 7 days post-inoculation for the QF24-Ps02+QF24-Ec03， QF24-Ec03， and QF24-Ps02 groups were 60%， 20%， and 40%， respectively. Necropsy findings demonstrated that rabbits in the QF24-Pm01+QF24-Ps02+QF24-Ec03 co-infection group exhibited pulmonary congestion and edema with a ground-glass appearance， as well as splenic atrophy and dark brown discoloration. These pathological changes were consistent with the lesions observed in clinical necropsy examinations of affected rabbits. This study demonstrates that the pathogenicity of Pasteurella multocida is enhanced when co-infected with Escherichia coli， and that Escherichia coli and Proteus mirabilis exhibit increased virulence when co-infected. Therefore， co-infection with Pasteurella multocida，Proteus mirabilis， and Escherichia coli may be the primary cause of high mortality rates in meat rabbits at the studied facility. Consequently， in addition to preventing Pasteurella multocida infection， control measures against Escherichia coli and Proteus mirabilis should be reinforced to reduce the risk of mixed infections and associated mortality.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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