Aeromonas salmonicida infection levels in pre‐ and post‐stocked cleaner fish assessed by culture and an amended qPCR assay

Abstract

Due to increasing resistance to chemical therapeutants, the use of 'cleaner fish' (primarily wrasse, Labridae, species) has become popular in European salmon farming for biocontrol of the salmon louse, Lepeophtheirus salmonis (Krøyer). While being efficient de-licers, cleaner fish mortality levels in salmon cages are commonly high, and systemic bacterial infections constitute a major problem. Atypical furunculosis, caused by Aeromonas salmonicida A-layer types V and VI, is among the most common diagnoses reached in clinical investigations. A previously described real-time PCR (qPCR), targeting the A. salmonicida A-layer gene (vapA), was modified and validated for specific and sensitive detection of all presently recognized A-layer types of this bacterium. Before stocking and during episodes of increased mortality in salmon cages, cleaner fish (primarily wild-caught wrasse) were sampled and screened for A. salmonicida by qPCR and culture. Culture indicated that systemic bacterial infections are mainly contracted after salmon farm stocking, and qPCR revealed A. salmonicida prevalences of approximately 4% and 68% in pre- and post-stocked cleaner fish, respectively. This underpins A. salmonicida's relevance as a contributing factor to cleaner fish mortality and emphasizes the need for implementation of preventive measures (e.g. vaccination) if current levels of cleaner fish use are to be continued or expanded.

Pathogenicity of Pasteurella sp. in lumpsuckers (Cyclopterus lumpus L.)

“…The protocol used for this study was validated using the method of Gulla, Duodu, Nilsen, Fossen, and Colquhoun (2016), with a few modifications as follows. A late exponential phase culture of Pasteurella sp.…”

Detection and quantification of Aeromonas salmonicida in fish tissue by real‐time PCR

“…; Gulla et al . ). In the present study, four additional organs were included: spleen, intestine, gills and the brain.…”

“…An explanation for the difference in sensitivity between the present study and Balcazar et al (2007), even though the same assay is applied in both studies, could be that another real-time PCR instrument and kit was used in this study and the standard curve was generated from a cloned PCR product, while Balcazar et al (2007) used extracted DNA from pure cultures of A. salmonicida NCIMB 1102. The most frequently examined organ for A. salmonicida in previous real-time PCR studies has been the kidney (Balcazar et al 2007;Goodwin & Merry 2009;Keeling et al 2012;Gulla et al 2015). In the present study, four additional organs were included: spleen, intestine, gills and the brain.…”

“…Real-time PCR has been used in several studies to detect A. salmonicida in fish tissue (Balcazar et al 2007;Goodwin & Merry 2009;Keeling et al 2012;Gulla et al 2015). In the study by Balcazar et al (2007), an assay was developed with 100% specificity and a sensitivity of 16 CFU per reaction.…”

Cleaner fish diseases

“…The authors discuss questions arising from this as they relate to lumpfish health and also biosecurity measures on salmon farms where cleaner fish are being deployed. Work by Gulla, Duodu, Nilsen, Fossen, and Colquhoun () follows wild‐caught cleaner fish before and after their deployment in salmon netpens and found a much high prevalence of A. salmonicida following their stocking in netpens. Biosecurity concerns are also raised by the findings of Scholz et al.…”