The use of two live attenuated vaccines to immunize egg-laying hens against Salmonella enteritidis phage type 4

Laying hens aged 24 weeks were vaccinated twice, separated by a 2-week interval, with spectinomycin-resistant mutants of either the Salmonella gallinarum 9R vaccine or a rough, aroA insertion mutant of a S. enteritidis phage type 4 strain. The 9R strain was given intramuscularly, the rough, aroA mutant both intramuscularly and orally. Two weeks after the second vaccination the immunized and a control group of unimmunized birds were challenged with a nalidixic acid-resistant mutant of a fully virulent phage type 4 strain. Full post-mortem examinations were carried out and eggs were examined bacteriologically for 3 weeks after challenge. Immunization with the 9R strain produced a marked reduction in the number of isolations of the challenge strain from a number of organs, including the ovaries. In contrast, immunization with the rough, aroA mutant produced little change in the isolation rate from the ovaries with small reductions for the liver and spleen. Both candidate vaccines produced a reduction in the number of isolations from laid eggs. The 9R strain was itself isolated from the ovaries throughout the period of examination whereas the aroA strain was not. Sera taken from the birds immediately prior to challenge were examined by slide agglutination using live S. enteritidis cells. Sera from 9R vaccinated birds contained agglutinins whereas sera from most of the chickens immunized with the rough, aroA mutant did not.