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.  

2020

The number of medicinal treatments applied against salmon lice decreased slightly in 2020 compared to 2019. The number of treatments has been relatively stable since 2017. This is in contrast to the period 2014 to 2017, during which the number decreased by 78 percent. The level of resistance seen in salmon lice remained high in 2020.

The tendency towards a reduced resistance level seen from 2017 to 2019 for deltamethrin, azamethiphos and hydrogen peroxide, were exchanged in more flattened curves for 2020. For emamectin benzoate the tendency towards increased resistance seen in 2019, continued in 2020. 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 non-medicinal treatments increased by 21 percent, to 2983 reported treatment weeks, from 2019 to 2020. Non-medicinal methods for treatment and prevention were thereby the dominating methods for salmon lice control. Fresh water delousing, alone or in combination with other treatments, accounted for 9 percent of the non-medicinal treatments in 2020 (281 reported treatments). A field study of fresh water sensitivity was performed for the second time in the surveillance program in 2020, comparing the sensitivity levels of salmon lice from areas with low and higher frequency of fresh water treatments. The results showed higher fresh water tolerance in lice from farms located in the higher fresh water usage areas.

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