Administration of a live attenuated Salmonella vaccine using an inactivated oil-emulsion vaccine as a vehicle for commercial chicken flocks

Since thefinding that inoculating an aroA-deletion live Salmonella Typhimurium vaccine parenterally provides improved and longer-lasting protection against Salmonella colonisation of the laying-hen intestine, this administration route has been adopted by the industry. To make this method practicable and economical, mixing the live bacterial vaccine with an inactivated viral vaccine has become popular. In vitro and in vivo studies were performed designed to assess the effect on the survival of the live salmonellae and the ability to stimulate serum antibody when mixed into oil-emulsion vaccines, compared with more traditional diluents. A rapid decline in viable salmonellae was observed when mixing with an inactivated Riemerella/Pasteurella bacterin. Mixing with an inactivated viral vaccine produced a less severe and more gradual decline in viable salmonellae over time; however, there was a surprising resuscitation of the bacteria 60 min after mixing. Serum antibody 14 days after inoculation of vaccine diluted in a universal diluent rose significantly, compared with sham vaccinated birds. Birds receiving the vaccine diluted in an inactivated vaccine at the time of preparation did not show a significant serological response; however, when given 60 min post-preparation, serum antibody was significantly increased. There appeared to be a correlation of the magnitude of serum antibody produced with the number of viable salmonellae inoculated. The use of the live vaccine incorporated into an inactivated vaccine may give variable results and needs assessment before adoption.