Seasonal and Regional Dynamics of Streptococcus agalactiae and Francisella orientalis Infections in Brazilian Tilapia: A Six‐Year National Study

Streptococcus agalactiae and Francisella orientalis are major bacterial pathogens affecting Nile tilapia ( Oreochromis niloticus ) worldwide, leading to substantial economic losses in aquaculture. This study investigated the spatial and temporal distribution of these pathogens in tilapia production units of Brazil and identified risk factors associated with their occurrence. Between 2019 and 2024, 257 tilapia production units across Brazil were monitored, resulting in 395 bacteriological examinations and 10,161 fish evaluated. Water temperature and fish weight were recorded, and bacterial isolates were identified by MALDI‐ToF and serotyping. Spatial and temporal analyses were conducted, alongside risk prediction modeling. S. agalactiae was detected in 69.2% of production units, with serotype Ib widely distributed nationwide and serotype III restricted to the Northeast, where its occurrence increased over time. F. orientalis was detected in 23.3% of production units, predominantly during colder months in the South and Southeast regions. Seasonal analysis revealed that S. agalactiae occurred year‐round with minimal detection in August, whereas F. orientalis peaked in July. Spatial clustering revealed high‐risk areas overlapping with major tilapia production zones. Water temperature and fish weight were significant predictors for all three pathogens ( S. agalactiae Ib/III and F. orientalis ). Coinfections at the fish level were rare, although concurrent pathogens presence within production units were observed. Overall, the study provides a comprehensive assessment of the seasonal and regional dynamics of S. agalactiae and F. orientalis in tilapia production units of Brazil. These findings underscore the need for strengthened biosecurity measures and region‐specific disease surveillance strategies to mitigate the impact of these pathogens.