Protection against experimental fowl typhoid by vaccination with strain 9R reconstituted from the freeze-dried state

Abstract

A strain of Salmonella gallinarum, strain 9R, whose value as an immunising agent against fowl typhoid is well established, was freeze-dried and stored for varying periods at room temperature. When reconstituted, different numbers of viable organisms were used to vaccinate 7 to 8-week-old pullets. The outcome of oral challenge with a standard dose of 50 million living organisms of the virulent parent strain 9, as measured by death from typical fowl typhoid up to 3 weeks after challenge, survival with lesions and/or faecal excretion of smooth S. gallinarum tended to correlate positively with the numbers of living strain 9R used in the vaccine. Although even very small viable numbers protected for some weeks, only the injection of comparatively high numbers of living strain 9R (25 or 125 millions) gave a relatively substantial protection to birds challenged 16 weeks or more after vaccination. Definite resistance to challenge was still present 32 weeks after vaccination, particularly among birds that had been given 125 million living strain 9R organisms.