ORIGINAL ARTICLE Pathogenicity of Pasteurella sp. in lumpsuckers (Cyclopterus lumpus L.) Rebecca Marie Ellul 1 | Cecilie Walde 2 | Gyri Teien Haugland 1 | Heidrun Wergeland 1 | Anita Rønneseth 1 1 Department of Biological Sciences, University of Bergen, Bergen, Norway 2 Department of Aquatic Animal Health, Norwegian Veterinary Institute, Bergen, Norway Correspondence Rebecca Marie Ellul, Department of Biological Sciences, University of Bergen, Bergen, Norway. Email: rebecca.ellul@uib.no Funding information University of Bergen; Norwegian Seafood Research Fund (FHF), Grant/Award Number: 900818; NFR, Grant/Award Number: 244148 Abstract The incidence of disease caused by Pasteurella sp. in farmed lumpsuckers in Norway has been steadily increasing in recent years, causing significant economic losses and fish welfare issues. The disease affects all life stages, both in hatcheries and after release into salmon cages. Therefore, it is important to establish robust challenge models, to be used for vaccine development. Exposure experiments via intramuscu- lar and intraperitoneal injection underlined the high virulence of the bacteria, whereas the cohabitation and bath models allowed the chronic symptoms of the disease to be studied more accurately. Skin lesions and haemorrhage at the base of fins were observed in the more acute cases of the disease. Symptoms including white spots over the skin, especially around the eyes, characterized the chronic cases. The latter were most prominent from the bath challenge model. Histopathol- ogy indicated a systemic pattern of disease, whereas qPCR analysis from head kid- ney showed that bacteria may be present in survivor fish at the end of the challenges. In all the challenge models investigated, Pasteurella sp. was re‐isolated from the fish, thus fulfilling Koch's postulates. These findings highlight the impor- tance of screening of lumpsuckers prior to transfer to minimize the risks of carrying over asymptomatic carriers. KEYWORDS challenge, cleaner fish, infection, lumpfish, pasteurellosis, pathology 1 | INTRODUCTION Prior to their recent introduction as cleaner fish, the biology and immune system of lumpsuckers were not very well known or not well studied. As is common with most fish species introduced in aquaculture, bacterial diseases encountered during farming were rapidly recognized. The mortality spikes seem to occur soon after hatching, after vaccination procedures and often following transfer to salmon cages (Johansen, 2013). So far, the most common bacterial species identified from disease outbreaks include Vibrio anguillarum (Marcos‐López, 2013), atypical Aeromonas salmonicida, Tenacibaculum spp., Moritella viscosa, and a Pasteurella‐like isolate (Alarcón et al., 2016; Bornø and Gulla, 2016; Rønneseth, Haugland, Colquhoun, Brudal, & Wergeland, 2017). The first confirmed outbreak of pasteurellosis caused by Pas- teurella sp. in lumpsuckers was reported by the Norwegian Veteri- nary Institute from a farm in southern Norway in 2012 (Johansen, 2013). Additional cases have since been reported from sites along the western coast of Norway (Alarcón et al., 2016). Pasteurellosis has also been recorded in production facilities where the affected fish ranged from juveniles (6–8 g) to broodstock (340 g), as well as in lumpsuckers deployed to salmon cages where disease occurred within the first few weeks following transfer (Alarcón et al., 2016). More recently, pasteurellosis was diagnosed in lumpsuckers from 14 Received: 26 June 2018 | Revised: 31 August 2018 | Accepted: 5 September 2018 DOI: 10.1111/jfd.12905 J Fish Dis. 2019;42:35–46. wileyonlinelibrary.com/journal/jfd © 2018 John Wiley & Sons Ltd | 35