Associations Between Delousing Practices and Pasteurellosis in Farmed Atlantic Salmon

Infections with bacteria of the genus Pasteurella have increased in occurrence in Atlantic salmon (Salmo salar) farms in Norway since 2018. This increase coincides with increased use of non-medicinal treatments against the parasitic salmon louse, Lepeophtheirus salmonis, in the farms. Here, we analysed the statistical association between the use of non-medicinal delousing methods and pasteurellosis in salmon farming in western Norway, from 2018 to 2023. The analysed data covered 1161 production cycles from 356 farming localities, in which Pasteurella was detected in 166 production cycles from 115 localities. Results showed that in months with one or more thermal delousing using heated water to remove the lice, the odds for detection of Pasteurella were 2.5 times as high as in months with no delousing (95% credible interval, c.i.: 1.8, 3.6). In months with one or more mechanical delousing involving brushing or flushing, the odds were 1.8 times as high as in months with no delousing (c.i.: 1.04, 2.9). Delousing with freshwater was not associated with increased odds for Pasteurella. The odds for Pasteurella increased with increasing salmon weight and following a Pasteurella diagnosis in the previous production cycle. These results indicate that the use of thermal and mechanical delousing may play a role in maintaining the current pasteurellosis situation in western Norway, and that freshwater delousing may be a favourable alternative in this respect.