Immunogenicity and Efficacy of Formaldehyde-Inactivated and γ-Irradiated Vaccines Against Hemorrhagic Septicemia in Mice

Hemorrhagic septicemia (HS) is an acute and fatal disease of livestock caused by the bacterium Pasteurella multocida. Prophylactic vaccination is the most effective method for controlling this disease. Although formaldehyde-inactivated HS vaccines are commonly used, γ-irradiated vaccines also show potential in enhancing the immunogenic properties of HS vaccines. This study aimed to figure out the immune efficiency and protective immunity of formaldehyde-inactivated and γ-irradiated vaccine preparations for hemorrhagic septicemia in mice. To achieve these aims, mouse bioassays were conducted to compare the vaccine formulations. Two types of oil-based experimental vaccines were prepared, each having 6.4 x 10^8 CFU/ml, to potentiate the immune response. Both physical (gamma radiation) and chemical (formaldehyde) methods were used to inactivate the bacterial cultures. For this purpose, a total of 80 mice were divided into four groups, with twenty mice in each group. Groups 1 and 2 were vaccinated with γ-irradiated and formaldehyde-inactivated vaccine, respectively, while Group 3 was inoculated with the reference HS vaccine (NIAB HS vaccine), and Group 4 served as the negative control. Post-vaccination, swelling at the injection site, mortality rate, and mean body temperatures of all mice were recorded from days 1 to 14. The antibody titers of both vaccines were assessed using the indirect hemagglutination assay (IHA) to evaluate the efficacy of the HS vaccines. To further assess the efficacy, all vaccinated animals were challenged with a virulent strain of P. multocida. The results showed that the experimental vaccines had no adverse effects on the mice, except for slight swelling at the injection site, which subsided within a few days. The IHA results showed that the irradiated vaccine formulation produced an early rise in antibody levels, which was more pronounced compared to the formaldehyde-treated vaccines. In contrast, the oil-based vaccine formulations showed a slower but steady increase in immune response, which continued throughout the experiment. The observed challenge protection rates were 95% for Group 1 (19/20 mice), 80% for Group 2 (16/20 mice), 85% for Group 3 (17/20 mice), and 0% for the control group (0/20 mice). A chi-square test was conducted to determine if there were significant differences in immunity across the distinct groups. The results showed that the p-value was significantly less than the alpha level of 0.05, suggesting that the observed differences in immunity among the groups are statistically significant. Based on these findings, the efficacy and safety of the irradiated vaccine were found to be superior and suitable candidate to that of the formaldehyde inactivated vaccines. However, vaccine trials in cattle or buffaloes must be set up so that the safety and protective properties seen in mice are also applicable to the target species.