Salmonella Enteritidis (SE) and, to a lesser extent, Salmonella Typhimurium (ST) are associated with egg-related outbreaks in people. Recently, monophasic strains of ST (mST; lacking one phase of the flagellar antigen) have been described, and they have officially been
classified as variants of ST and thus may contribute to human exposure to contaminated eggs. Currently used vaccination programmes are licensed for use against biphasic variants of ST, and their efficacy against mST has not yet been investigated. In this study, the effectiveness of four vaccination
programmes currently in use in the UK poultry industry was evaluated against challenge with one SE strain, one ST strain and two mST strains. A Bayesian model was used to estimate the impact of vaccination on the rate of faecal shedding and on egg contamination. For the majority of vaccine/challenge
strain combinations, there was little or no effect of vaccination on the proportion of birds shedding Salmonella for either biphasic or monophasic strains. However, vaccination was effective at reducing egg contamination. A significantly lower proportion of eggshells were positive for
the vaccinated birds compared with non-vaccinated birds, including the mST strains (vaccination resulted in a 55% and 21% reduction for the two mST strains). Calculated across all strains, the estimated rate of positive egg contents was lower in vaccinated birds (Bayesian median was 0.13%
for vaccinated birds versus 0.27% for non-vaccinated birds). For both vaccinated and unvaccinated birds, there was also an apparent difference in the proportion of positive organs between strains, with the SE strain resulting in a lower proportion of positive organs at post-mortem examination
compared with the other strains.
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Document Type: Research Article
Animal Health and Veterinary Laboratories Agency, Biomathematics and Statistics Unit, Loughborough, UK
Animal Health and Veterinary Laboratories Agency, Department of Bacteriology and Food Safety, New Haw, Addlestone, UK
March 4, 2014
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