A Salmonella Pullorum outbreak with neurological signs in adult layers and outbreak investigation using whole genome sequencing

This report describes a Salmonella enterica subspecies enterica Serovar Gallinarum Biovar Pullorum (SP) outbreak in a Dutch laying flock of 65 weeks of age (W) showing increased mortality, decreased egg production, and neurological signs. The flock was vaccinated three times with a live S. enteritis and S. typhimurium vaccine in the rearing period. At 71 W, as mortality peaked, a live SG 9R vaccine was applied. At 76 W, mortality decreased and egg production stabilized but did not return to a normal level. Post-mortem, histological, bacteriological, and serological examinations were frequently performed. Gross findings in diseased and dead birds were characterized by hepatomegaly, splenomegaly, and oophoritis. SP was frequently isolated from the liver, spleen, ovary, and bone marrow, and the isolation rate did not vary significantly between the different organs. In birds with neurological signs, histological examination and immunostaining showed cerebral granulomas in the presence of Salmonella. Previous Salmonella vaccinations did not hamper the detection of specific SP antibodies (positive at serum dilution >= 1:8) using a commercial rapid plate agglutination test. At 85 W, 40 clinically healthy birds were investigated and three had ovarian lesions, explaining egg production not returning to normal. These birds had antibodies against SP, indicating contact with the bacterium, but carrier status could not be confirmed by culture. Phylogenetic analysis showed SP isolates within this outbreak to be clonal and differing from SP isolates from previous outbreaks.RESEARCH HIGHLIGHTS center dot Cerebral granulomas are associated with nervous signs in Salmonella Pullorum outbreak.center dot Bone marrow is also a recommended tissue for isolation of Salmonella Pullorum.center dot Rapid plate agglutination test detects Pullorum antibodies in a vaccinated flock.center dot Phylogenetic analysis showed clonality of isolates within the outbreak.