Losing the 'arms race': Multiresistant salmon lice are dispersed throughout the North Atlantic Ocean

Nothing lasts forever, including the effect of chemicals aimed to control pests in food production. As old pesticides have been compromised by emerging resistance, new ones have been introduced and turned the odds back in our favour. With time, however, some pests have developed multi-pesticide resistance, challenging our ability to control them. In salmonid aquaculture, the ectoparasitic salmon louse has developed resistance to most of the available delousing compounds. The discovery of genetic markers associated with resistance to organophosphates and pyrethroids made it possible for us to investigate simultaneous resistance to both compounds in approximately 2000 samples of salmon lice from throughout the North Atlantic in the years 2000–2016. We observed widespread and increasing multiresistance on the European side of the Atlantic, particularly in areas with intensive aquaculture. Multiresistant lice were also found on wild Atlantic salmon and sea trout, and also on farmed salmonid hosts in areas where delousing chemicals have not been used. In areas with intensive aquaculture, there are almost no lice left that are sensitive to both compounds. These results demonstrate the speed to which this parasite can develop widespread multiresistance, illustrating why the aquaculture industry has repeatedly lost the arms race with this highly problematic parasite.publishedVersio

Aquaculture-driven evolution: distribution of pyrethroid resistance in the salmon louse throughout the North Atlantic in the years 2000–2017

The parasitic salmon louse, and its documented resistance to chemotherapeutants, represents the most persistent environmental challenge to global salmonid aquaculture. We used a genetic marker associated with pyrethroid resistance to analyse ∼15 000 lice collected from the North Atlantic in the period 2000–2017. The genotype associated with resistance was not detected in lice collected from throughout the North Atlantic in the year 2000 or 2002. However, by the year 2009 onwards, it was found in lice from fish farms throughout much of the North Atlantic. It was also found in modest frequencies in lice collected from wild Atlantic salmon captured off Greenland. The most recent samples displayed very high frequencies of the genotype associated with resistance, particularly in intensive aquaculture regions of Norway (>90%) and Scotland (>70%). These results closely align with observations from the field. We suggest that pyrethroid resistance first emerged in Europe just before or around the year 2000 and was thereafter dispersed throughout much of the North Atlantic where its increased frequency was driven by extensive pyrethroid use. Although the resistant genotype was not detected in lice from Canada, it is likely to occur in very low frequencies that would quickly increase if pyrethroids were to be used in that region.publishedVersio

Identification and Characterization of Viral Structural Proteins of Infectious Salmon Anemia Virus

Infectious salmon anemia virus (ISAV) is an unclassified Orthomyxovirus that has been shown to contain a segmented genome with eight single-stranded RNA species coding for 10 viral proteins. Four major structural proteins were characterized in the present study: two glycosylated proteins with estimated molecular masses of 42 and 50 kDa, one 66-kDa phosphoprotein, and one 22-kDa protein. Examination of lysed virions revealed the two glycoproteins and the 22-kDa protein in the soluble fraction, while the 66-kDa phosphoprotein and a minor part of the 22-kDa protein were found in the pelleted fraction. Immunofluorescence staining of infected cells demonstrated that the 22-kDa protein was a late protein accumulating in the nucleus. We conclude that the 66-kDa protein is the nucleoprotein, the 22-kDa protein is the matrix protein, and the 42- and 50-kDa proteins are the surface proteins. Radioimmunoprecipitation analysis of the 42-kDa glycoprotein, which was previously shown to represent the ISAV hemagglutinin, indicated that this protein exists at least as dimers. Further, by labeling of purified ISAV with [1,3-(3)H]diisopropyl fluorophosphate, it was also demonstrated that the viral esterase is located with the hemagglutinin. This finding was confirmed by demonstration of acetylesterase activity in affinity-purified hemagglutinin preparations. Finally, the active-site serine residue could be tentatively identified at position 32 within the amino acid sequence of the hemagglutinin of ISAV strain Glesvaer/2/90. It is proposed that the ISAV vp66 protein be termed nucleoprotein, the gp42 protein be termed HE protein, and the vp22 protein be termed matrix protein

The protective effect of the Phe362Tyr mutation in salmon lice’ (Lepeophtheirus salmonis) AChE when exposed to full-scale azamethiphos bath treatments

Organophosphates are applied for medicinal bath treatments of salmon lice (Lepeophtheirus salmonis) infested farmed salmonids. This chemical class remains important despite the development of resistant parasites, due to few available treatment options. The protective effect of the Phe362Tyr mutation in one of the acetylcholinesterase (AChE) genes of L. salmonis has previously been studied in small-scale treatments with the organophosphate azamethiphos. The current study was aimed at investigating the protective effect of this mutation in field treatments of commercial fish farms. In addition the effect of different methods of sampling on the occurrence of salmon lice with zero, one or two copies of Phe362Tyr (homozygote sensitive (SS), heterozygote (RS) and homozygote resistance (RR) respectively) were investigated. Salmon lice were collected prior to and after azamethiphos treatments in four fish farms. The resistance genotypes were determined for each parasite. No SS lice were found in any of the farms post-treatment. Both RR and RS genotypes protected the salmon lice from the effect of azamethiphos, but the protective effect of RR genotype was greater than of RS. The study thereby emphasizes the strong selection pressure towards resistant parasites imposed by an azamethiphos treatment event in farms where resistant parasites are present. The distribution of SS, RS and RR genotypes were not found to differ between lice from different net pens, sampled at different dates or picked off fish of different health statuses in a study with repeated sampling from a single farm

The protective effect of the Phe362Tyr mutation in salmon lice’ (Lepeophtheirus salmonis) AChE when exposed to full-scale azamethiphos bath treatments

Organophosphates are applied for medicinal bath treatments of salmon lice (Lepeophtheirus salmonis) infested farmed salmonids. This chemical class remains important despite the development of resistant parasites, due to few available treatment options. The protective effect of the Phe362Tyr mutation in one of the acetylcholinesterase (AChE) genes of L. salmonis has previously been studied in small-scale treatments with the organophosphate azamethiphos. The current study was aimed at investigating the protective effect of this mutation in field treatments of commercial fish farms. In addition the effect of different methods of sampling on the occurrence of salmon lice with zero, one or two copies of Phe362Tyr (homozygote sensitive (SS), heterozygote (RS) and homozygote resistance (RR) respectively) were investigated. Salmon lice were collected prior to and after azamethiphos treatments in four fish farms. The resistance genotypes were determined for each parasite. No SS lice were found in any of the farms post-treatment. Both RR and RS genotypes protected the salmon lice from the effect of azamethiphos, but the protective effect of RR genotype was greater than of RS. The study thereby emphasizes the strong selection pressure towards resistant parasites imposed by an azamethiphos treatment event in farms where resistant parasites are present. The distribution of SS, RS and RR genotypes were not found to differ between lice from different net pens, sampled at different dates or picked off fish of different health statuses in a study with repeated sampling from a single farm

Lack of evidence of vertical transmission of piscine myocarditis virus in Atlantic salmon (Salmo salar L.)

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Effects of the geoindex or the kernel density of treatments (KD) on the proportion of mutated genotypes.

<p>GAM-plots showing the non-linear relative effects of the geoindex (left panel) or kernel density of treatments (KD) on the proportion of mutated genotypes (RR or RS) in farm sampled salmon lice. The two plots represent univariate fitting of the proportion model. The relationship between farm geoindexes and their north coordinate is also shown (right panel).</p

<i>Phe362Tyr</i> in AChE: A Major Factor Responsible for Azamethiphos Resistance in <i>Lepeophtheirus salmonis</i> in Norway

<div><p>Organophosphates (OP) are one of the major treatments used against the salmon louse (<i>Lepeophtherius salmonis</i>) in Norwegian salmonid aquaculture. The use of OP since the late 1970s has resulted in widespread resistant parasites. Recently, we reported a single mutation (<i>Phe362Tyr</i>) 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 <i>Phe362Tyr</i> mutation. A strong association was observed between areas with frequent use of the OP (azamethiphos) and the <i>Phe362Tyr</i> 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 <i>Phe362Tyr</i> mutation is a major factor responsible for OP resistance in salmon louse on Norwegian fish farms.</p></div