Safety and protective efficacy of Salmonella Pullorum spiC and rfaH deletion rough mutant as a live attenuated DIVA vaccine candidate

Salmonella enterica serovar Pullorum (S. Pullorum) causes pullorum disease (PD), which is an acute systemic disease, in chickens, and leads to serious economic losses in many developing countries because of its high morbidity and mortality rate in young chicks. The live-attenuated vaccine is considered to be an effective measure to control the Salmonella infection. In addition, the DIVA (differentiation of infected and vaccinated animals) feature without the interference of serological monitoring of Salmonella infection is an important consideration in the development of the Salmonella vaccine. In this study, we evaluated the immunogenicity and protective efficacy of a S. Pullorum rough mutant S06004 Delta spiC Delta rfaH as a live attenuated DIVA vaccine candidate in chickens. The S06004 Delta spiC Delta rfaH exhibited a significant rough lipopolysaccharides (LPS) phenotype which was agglutinated with the acriflavine, not with the O-9 mono antibody. Compared to the wildtype, 50% lethal dose (LD50) of the rough mutant increased 100-fold confirmed its attenuation. The mutant strain also showed a decreased bacterial colonization in the spleen and liver. The immunization with the mutant strain had no effect on the body weight and no tissue lesions were observed in the liver and spleen. The high level of the S. Pullorum-specific IgG titers in the serum indicated that significant humoral immune responses were induced in the immunization group. The cellular immune responses were also elicited from the analysis of lymphocyte proliferation and expression of cytokines in the spleen. In addition, the S06004 Delta spiC Delta rfaH immunized group exhibited a negative response for the serological test, while the wild-type S06004 infection group was strongly positive for the serological test showing a DIVA capability. The survival rates in the vaccinated chickens were 87% after intramuscular challenge with wild-type S. Pullorum, while the survival rates were 20% in the control groups. Overall, these results have demonstrated that the rough mutant S06004 Delta spiC Delta rfaH strain can be developed as an efficient live attenuated DIVA vaccine candidate to control the systemic S. Pullorum infection without the interference of salmonellosis monitoring program in poultry.

Automated summary

This study evaluated the immunogenicity and protective efficacy of a rough mutant strain of Salmonella enterica serovar Pullorum as a live attenuated DIVA vaccine candidate in chickens. The results showed that the mutant strain exhibited a significant rough lipopolysaccharides phenotype, increased attenuation, decreased bacterial colonization, and induced both humoral and cellular immune responses. Furthermore, the mutant strain showed a negative response for serological testing, indicating its DIVA capability. The survival rates in the vaccinated chickens were significantly higher compared to the control group. Overall, this rough mutant strain could be developed as an efficient live attenuated DIVA vaccine candidate for controlling Salmonella infection in poultry without interfering with the monitoring program.