Safety and efficacy of a virulence gene-deleted live vaccine candidate for fowl typhoid in young chickens.

The safety and efficacy of a live lon-and-cpxR-deleted Salmonella enterica serovar Gallinarum (SG) vaccine candidate (JOL916) was evaluated in young layer chickens. Vaccinated (n=25) and unvaccinated (n=25) groups were organized, respectively, at 1, 2, 3, and 4 weeks of age. One-week-old and 2-week-old chickens were orally inoculated with 2×10(7) colony-forming units of JOL916, and orally challenged with 2 x 10(6) colony-forming units of a wild-type SG strain at the third week post vaccination (w.p.v.). Doses of vaccination and challenge were increased 10-fold for 3-week-old and 4-week-old chickens. SG-antigen-specific peripheral lymphocyte proliferation response and concentrations of plasma IgG and secretary IgA in the intestine were examined at the second w.p.v. Gross lesions of the liver and spleen and recovery of the vaccine strain from the spleen were also examined at the second w.p.v. No evidence of side effects was detected by observation of general condition and body weight gain in all vaccinated groups. No, or very mild, gross lesions in the chickens were observed in the liver and/or spleen after vaccination. Significant cellular immune responses and systemic IgG responses were induced after vaccination in all age groups. Elevation of secretary IgA concentration was significant in the group, vaccinated at the age of 1 week. Depression scores after challenge were significantly lower in the vaccinated groups, as compared with the corresponding control groups. Significant reductions of death rates were observed in all vaccinated groups, as compared with the equivalent unvaccinated groups. Thus, the oral vaccination of young chickens with JOL916 was demonstrated to be safe. Moreover, it offered efficient protection against fowl typhoid.