Use of live and inactivated Salmonella enteritidis phage type 4 vaccines to immunise laying hens against experimental infection.

Four groups of Dekalb Delta commercial layer hens (8 hens per group) were reared individually in cages in an isolation unit. At the age of 8 weeks, groups 1 and 2 were vaccinated with 9R Salmonella gallinarum live (9R live) vaccine. At the age of 18 weeks, group 1 was re-vaccinated with 9R live S. enteritidis vaccine, while groups 2 and 3 were vaccinated with S. enteritidis bacterin. At the age of 22 weeks, groups 2 and 3 were re-vaccinated with S. enteritidis bacterin. Group 4 was not vaccinated and was kept as a control group. At 24, 27 and 30 weeks of age, the four groups were challenged using various concentrations of S. enteritidis phage type 4 and various routes of inoculation. Eggs, cloacal swabs and blood were collected weekly for bacteriological and serological examination. Isolation of the challenge organism from the eggshells and egg contents of the vaccinated groups was significantly lower than with the control group. There was no correlation between humoral antibodies and the shedding of the challenge organism in eggs or cloacal swabs. A combined vaccination programme of 9R live S. enteritidis vaccine and S. enteritidis bacterin provided better protection to laying hens than either vaccine administered alone. Vaccination should never be performed alone but in conjunction with other measures related to veterinary hygiene and good management.