Evaluation Of The Immune Protection Efficacy Of The Pasteurella Multocida GatA Mutant

Pasteurella multocida can cause fowl cholera with high morbidity and mortality,which could cause huge economic losses to worldwide poultry industry.The commercial inactivated vaccines against FC possess only partial protection and have certain adverse effect,thus a novel attenuated live vaccine with safety and high protection is urgently required.Our previous study demonstrated that the strain with deletion of the gatA gene that is involved in synthesis of lipopolysaccharide outer core oligosaccharides is highly attenuated in virulence.Here,we further systematically evaluated the safety,immunogenicity and immune protection effect of the gatA gene mutant strain PMZ2（ΔgatA）.1.SafetyThe median lethal dose of the PMZ2（ΔgatA）is much higher than the wild type stain PM0818 in intramuscular,intranasal,or intraoral route in ducklings.Seven-days-old ducklings were inoculated with 10⁸CFU of the PMZ2 via nasal or oral route.We found that the weight gains and temperature changes in the infection group were similar with that of the PBS group.Meanwhile,when ducks were inoculated with 10⁹CFU of the bacteria,the bacterial loads of the PMZ2 and the pathological lesions in liver,lung and spleen caused decreased significantly than that of the WT strain in ducks.2.The immune protective effect caused by single immunizationWe immunized seven-days-old ducklings with 10⁸CFU of the PMZ2 via nasal or oral route.And 14 days later,ducks were challenged with at least 100folds of LD₅₀ of the wild type strain.Results showed that ducks were all dead in the nasal group,while 50%of ducks in the oral group survived post-challenge.Also,serum Ig G antibody level in ducks did not increase in the nasal group,but increased significantly in the oral group after immunization.The pathological test showed that the lesions in the liver,spleen and lungs of ducks were alleviated in the oral but not the nasal group compared with the PBS control.3.The immune protective effect caused by boost immunizationWe immunized seven-days-old ducklings with 10⁸CFU of the PMZ2 via nasal or oral route twice at an interval of 14-days,and then ducks were challenged with at least 100 folds of LD₅₀ of the wild type strain 14 days after the second immunization.Results showed that all animals in the two immunization groups survived the challenge,while 70%of the animal in the PBS group succumbed to the challenge.Also,the serum Ig G antibody level in both groups increased significantly after second immunization.The pathological test showed that no or only slight tissue injuries were found in the liver,lung and spleen in both immunization groups after challenge.To sum up,the Pasteurella multocida gatA mutant exhibited safety and good immunogenicity and could provide high level of protection against lethal challenge after twice immunization,indicating that this mutant strain is promising to be a good live vaccine candidate.