Salmon lice - resistance to chemotherapeutants

The number of prescriptions for anti-salmon lice medicines increased slightly in 2021 compared to 2020. The number of prescriptions has however been relatively stable since 2017. This is in contrast to the period 2014 to 2017, during which the number decreased by 78 percent.

In 2021, for the first time in more than two decades, a new anti-salmon lice medicine from a new substance class was approved in Norway. The level of resistance seen in salmon lice towards most anti-salmon medicines remained high in 2021.

There was however a tendency of reduced resistance towards deltamethrin and azamethiphos. This is despite an increased use of azamethiphos over the last three years. For hydrogen peroxide and emamectin benzoate a more stable resistance situation was seen. Resistance towards deltamethrin, azamethiphos and emamectin benzoate was generally widespread along the Norwegian coast.

Less resistance was found towards hydrogen peroxide than towards the other medicines, but loss of sensitivity was indicated in several areas. The number of reported farm treatment-weeks using non-medicinal treatments decreased by five percent, to 2822 weeks, from 2020 to 2021. This was the first decrease seen in the number of non-medicinal treatments per year. Non-medicinal methods for treatment and prevention were still the dominating methods for salmon lice control.