The use of live vaccines in experimental Salmonella gallinarum infection in chickens with observations on their interference effect

1. A good immunity was produced in chickens against oral infection with Salm. gallinarum by the use of either of two live attenuated vaccines, one smooth, 9S and one rough, 9R. Dead vaccines had little or no effect.2. The immunity possessed by chickens vaccinated with 9R at 7 weeks of age commenced to wane 12 weeks later. Chickens similarly vaccinated with 9S were completely immune 4, 12 and 34 weeks later. When tested during the active laying period (at 20 weeks after vaccination) their immunity, although substantial, was not complete.3. Vaccine 9R did not produce demonstrable agglutinins against smooth Salm. gallinarum in chickens. It was non-lethal to 1-day-old chicks and was never found to revert to the smooth virulent form in vitro or in vivo. By contrast, vaccine 9S produced agglutinins and was lethal for 1-day-old chicks, its virulence not being increased by passage. No deaths were ever observed when this vaccine was used in either chickens of 5–12 weeks old or laying hens.4. Although both vaccines produced a good immunity when employed in laying hens, vaccination with 9S was accompanied by a marked reduction in egg production; this was not the case with 9R.5. An immunity of the interference type was also evoked by both live vaccines. This immunity was still produced to some extent in chickens injected with 9S, 2 days after they were infected with Salm. gallinarum by mouth.6. The interference effect could not be produced by the use of either dead Salm. gallinarum vaccines or certain non-specific substances. The intravenous injection of indian ink had a marked adverse effect on the course of the disease. Injecting chickens with a virulent strain of Bacterium coli at the same time as they were infected with Salm. gallinarum resulted in the latter infection running a milder course than would normally have been expected.