Salmon lice - resistance to chemotherapeutants

The Norwegian Veterinary Institute has a surveillance program on resistance in salmon lice towards medicines used for salmon lice treatments. 

The aim of the program is to describe resistance towards and give advice on the use of these medicines. From 2019 fresh water was included in the program and a field-studies of fresh water sensitivity has been conducted in areas with low or higher usage of fresh water salmon lice treatments. 


The number of prescriptions for anti-salmon lice medicines were the same in 2022 as in 2021. The number of prescriptions has 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 use of this agent, with the active substance imidacloprid, was moderate in both 2021 and 2022, with 29 and 54 prescriptions respectively.

The level of resistance seen in salmon lice towards most anti-salmon medicines remained high in 2022.

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