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.    

2019

The number of medicinal treatments applied against salmon lice increased slightly in 2019 compared to 2018. This discontinued a yearly trend of reduction that started in 2015. The level of resistance seen in salmon lice remained high in 2019. However, the tendency towards a reduced resistance level that started in 2017, continued in 2019 for deltamethrin, azamethiphos and hydrogen peroxide. For emamectin benzoate there was a tendency towards increased resistance in 2019, unlike the previous two years.Resistance towards deltamethrin, azamethiphos and emamectin benzoate were 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 prescriptions for medicines against salmon lice was 698 in 2019, which was a 16 percent increase compared to 2018. The number of non-medicinal treatments increased by 23 percent, to 2462 reported treatments, in the same period. Nonmedicinal methods for treatment and prevention were thereby the dominating methods for salmon lice control. Fresh water delousing accounted for 7 percent of the non-medicinal treatments in 2019 (172 reported treatments). A small field study of fresh water sensitivity was performed in the surveillance program in 2019, comparing the sensitivity levels of salmon lice from low and higher fresh water usage areas. No difference in the sensitivity levels between the farms from these two areas was detected.

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