Fowl typhoid causing Salmonella Gallinarum strain SG-18: Genomic analysis and challenge dose optimization

Salmonella Gallinarum (SG), the causative agent of fowl typhoid, significantly affects poultry health and production. This study aimed to optimize the lethal dose LD₅₀ of a virulent Salmonella Gallinarum strain (SG-18) in broiler birds through a combined evaluation of mortality, histopathology and feed conversion ratio (FCR). A total of 130 broiler birds were divided into five groups with two replicates each: Group A (1 × 10⁷ CFU), Group B (4 × 10⁷ CFU), Group C (8 × 10⁷ CFU), Group D (1 × 10⁸ CFU), and Group E (unchallenged control). The challenge experiment revealed a clear dose-dependent trend in pathogenicity: 50% mortality in Group A, 90% in Group B, and 100% in Groups C and D. The high-dose groups (C & D) also showed severe liver and spleen lesions, reduced weight gain, and impaired FCR, confirming systemic infection and metabolic disruption. Whole-genome sequencing of SG-18 identified 5,270 coding sequences, including 138 virulence genes and 72 antimicrobial resistance determinants. Key virulence factors include type III/VI secretion systems and plasmid-borne spv operons. SG-18 belonged to ST78 (clonal complex 4), and this characterization contributes to the genomic profiling of circulating SG strains. These integrated findings confirm the LD₅₀ at approximately 1 × 10⁷ CFU and support further vaccine efficacy studies against fowl typhoid using such a challenge strain.