Abstract
Organophosphates (OP) are one of the major treatments used against the salmon louse (Lepeophtherius salmonis) in Norwegian salmonid aquaculture. The use of OP since the late 1970s has resulted in widespread resistant parasites. Recently, we reported a single mutation (Phe362Tyr) in acetylcholinesterase (AChE) as the major mechanism behind resistance in salmon louse towards OP. The present study was carried out to validate this mechanism at the field level. A total of 6658 salmon louse samples were enrolled from 56 different fish farms across the Norwegian coast, from Vest Agder in the south to Finnmark in the north. All the samples were genotyped using a TaqMan probe assay for the Phe362Tyr mutation. A strong association was observed between areas with frequent use of the OP (azamethiphos) and the Phe362Tyr mutation. This was confirmed at 15 sites where results from independently conducted bioassays and genotyping of parasites correlated well. Furthermore, genotyping of surviving and moribund parasites from six bioassay experiments demonstrated a highly significant negative correlation between the frequency of resistance alleles and the probability of dying when exposed to azamethiphos in a bioassay. Based on these observations, we could strongly conclude that the Phe362Tyr mutation is a major factor responsible for OP resistance in salmon louse on Norwegian fish farms.
Highlights
Norwegian government regulations require a reduction of the salmon louse (Lepeophtheirus salmonis) burden if the abundance exceeds 0.5 adult female parasites per fish on salmon farms
This epidemiological study of genotypes associated with sensitivity towards azamethiphos in salmon louse was based on salmon louse samples submitted to the laboratory by salmon-producing companies, and bioassays conducted under the national resistance monitoring program by various fish health services along the Norwegian coast [2]
The 6658 salmon louse samples in this study were enrolled from fish farms distributed throughout the Norwegian coast, from Vest Agder in the south to Finnmark in the north (Fig 1)
Summary
Norwegian government regulations require a reduction of the salmon louse (Lepeophtheirus salmonis) burden if the abundance exceeds 0.5 adult female parasites per fish on salmon farms. Due to the limited number of chemotherapeutants available, and the shortage of other control options, antiparasitic treatments that have been used to control L.salmonis, and against which the parasite has developed resistance, are still in use. The lack of effective alternatives has—and is continues to- a growing resistance problem. Even though some surveillance programs have been conducted to educate fish farmers, fish health services and government bodies about the growing problem [1, 2], a lot still needs to be done.
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