Temporal Changes in Antimicrobial Resistance, SpvB Polyproline Linker Length, and Pulsed-Field Gel Electrophoresis Types of Salmonella Gallinarum from Chickens in South Korea: A 25-Year Retrospective Study (2002–2026)

Fowl typhoid, caused by Salmonella Gallinarum (SG), remains an important poultry disease. We characterized 136 SG isolates, one per farm, from diagnostic submissions of diseased chickens to the national veterinary laboratory in South Korea from 2002 to 2026, comparing two periods (2002–2010 and 2011–2026). Antimicrobial susceptibility was determined by broth microdilution, the spvB polyproline linker (PPL; the region joining the two functional domains of the SpvB virulence factor) was sequenced, and genetic relatedness was assessed by XbaI–pulsed-field gel electrophoresis (PFGE). Resistance to nalidixic acid was most frequent (93.4%), followed by streptomycin (83.1%) and ciprofloxacin (71.3%). Resistance to ampicillin, tetracycline, and nalidixic acid was less frequent in 2011–2026 (false discovery rate-adjusted p < 0.05), and these associations persisted after adjustment for chicken type. Multidrug resistance was detected in 52.2% of isolates, without significant change between periods. However, 13 of the 34 multidrug-resistant isolates from 2002–2010 were resistant to four or more antimicrobial classes, whereas all 37 from 2011–2026 were resistant to exactly three classes. Five PPL lengths of 13–21 prolines were identified, and their distribution differed between periods (p = 0.016). Isolates carrying ≥17 prolines were more frequent in 2011–2026 than in 2002–2010 (p < 0.01). PFGE yielded 61 pulsotypes in 27 clusters, with two dominant clusters detected across years and chicken types. These findings indicate that the PFGE-defined SG population showed temporal phenotypic changes within Korean poultry flocks, underscoring the need for continued surveillance of resistance and genotype.