Azamethiphos resistance in salmon lice (Lepeophtheirus salmonis) in Norway 2014–2022: A study of farm level resistance and treatment intensity

Organophosphates have been used as bath treatment against salmon lice in aquaculture since the 1970s. Resistance to organophosphates was first suspected in salmon lice in the 1990s, while the exact resistance mechanism was described in 2015. Genetic testing of lice for this mechanism is commercially available in Norway and provides information used to make treatment choices. This study describes the resistance development in salmon lice against organophosphates in Norway 2014–2022, using a dataset from commercial testing. It includes a farm-level regression analysis to gain deeper insight into the relationship between treatments and resistance level. The main findings from this study are that the resistance level on a farm is associated with the geographic area the farm belongs to and that increased treatment frequency on the same farm and on neighbouring farms are associated with more resistance. The findings in the current study can be used for better resistance management, both for medicines where resistance is established and for medicines that are novel and where resistance is not established. They can, for example, be incorporated into models for resistance development used to simulate the effect of management practices on resistance development. • The organophosphate resistance development in salmon lice in Norway 2014–2022 is described. • Resistance data and treatment data are combined for analysis. • The resistance level on a farm is associated with the production zone the farm belongs to. • The resistance level on a farm is associated with the treatment frequency of the farm.