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Gill health is one of the main health challenges for Atlantic salmon (Salmo salar L.) mariculture worldwide, and amoebic gill disease (AGD), caused by the marine ectoprotozoan Neoparamoeba perurans, is currently one of the most significant diseases in terms of prevalence and economic impact. This review describes the host response of Atlantic salmon to the disease, focusing on the pathological changes, immune response and mechanisms underlying the prominent epithelial proliferation and mucus hypersecretion occurring in affected fish. Health management strategies and risk factors are also discussed.

Amoebic gill disease (AGD) is one of the main diseases affecting Atlantic salmon (Salmo salar L.) mariculture. Hallmarks of AGD are hyperplasia of the lamellar epithelium and increased production of gill mucus. This study investigated the expression of genes involved in mucus secretion, cell cycle regulation, immunity and oxidative stress in gills using a targeted 21-gene PCR array. Gill samples were obtained from experimental and natural Neoparamoeba perurans infections, and sampling points included progressive infection stages and post-freshwater treatment. Up-regulation of genes related to mucin secretion and cell proliferation, and down-regulation of pro-inflammatory and pro-apoptotic genes were associated with AGD severity, while partial restoration of the gill homeostasis was detected post-treatment. Mucins and Th2 cytokines accoun ted for most of the variability observed between groups highlighting their key role in AGD. Two mucins (muc5, muc18) showed differential regulation upon disease. Substantial up-regulation of the secreted muc5 was detected in clinical AGD, and the membrane bound muc18 showed an opposite pattern. Th2 cytokines, il4/13a and il4/13b2, were significantly upregulated from 2 days post-infection onwards, and changes were lesion-specific. Despite the differences between experimental and natural infections, both yielded comparable results that underline the importance of the studied genes in the respiratory organs of fish, and during AGD progression., Irish Research Council (Employment Based Postgraduate Programme Project ID EBPPG/2013/2), European Commission under the TNA programme (project ID AE030036) within AQUAEXCEL2020 project (652831)., CSIC PIE project no. 201740E013.

Amoebic gill disease (AGD), caused by the protozoan parasite Neoparamoeba perurans, is one of the most significant infectious diseases for Atlantic salmon (Salmo salar L.) mariculture. The present study investigated the humoral immune response (both local in gill mucus and systemic in serum) of farmed Atlantic salmon naturally infected with N. perurans in commercial sea pens, at two different stages of the disease and after freshwater treatment. Parameters analysed included activity of immune related enzymes (i.e. lysozyme, peroxidase, protease, anti-protease, esterase, alkaline phosphatase), IgM levels, and the terminal carbohydrate profile in the gill mucus. Overall, greater variations between groups were noted in the immune parameters determined in gill mucus than the equivalent in the serum. In gill mucus, IgM levels and peroxidase, lysozyme, esterase and protease activities were decreased in fish showing longer exposure time to the infection and higher disease severity, then showed a sequential increase after treatment. Results obtained highlight the capacity of gills to elicit a local response to the infection, indicate an impaired immune response at the later stages of the disease, and show partial reestablishment of the host immune status after freshwater treatment. In addition to providing data on the humoral response to AGD, this study increases knowledge on gill mucosal humoral immunity, since some of the parameters were analysed for the first time in gill mucus.

Jellyfish can cause significant losses to the aquaculture industry. This report describes the clinical presentation and pathology caused by the mauve stinger jellyfish Pelagia noctiluca (Forsskal, 1775) in cultured Atlantic salmon Salmo salar L. in Ireland and highlights the potential consequences and need for descriptive pathology of such interactions. P. noctiluca is a member of the cnidarian class Scyphozoa (Order Semaeostomeae; Family Pelagiidae) and has worldwide distribution through the warm and temperate waters of the marine environment. This golden brown to purple jellyfish species lacks the benthic polyp stage and has a primarily pelagic habitat, although ocean currents bring it closer to coastal waters. P. noctiluca is frequently observed from the west coast of Ireland especially during the autumn months, but large aggregations are not commonly reported. In a jellyfish survey carried out around Ireland in 2009, P. noctiluca was the most abundant species found, accounting for 93% of the overall catch (total wet weight of gelatinous species) (Bastian et al. 2011). Its distribution and density were found to be heterogeneous, with great variation between regions (apparent North-South gradient in density) and sampling points (Bastian et al. 2011). When in contact with a fish farm, small jellyfish can pass through the nets, while bigger individuals tend to break up into pieces still capable of stinging the fish. In jellyfish blooms, affected fish can suffer from hypoxia, mechanical damage and toxicity via nematocysts discharge (Baxter et al. 2011a; Rodger, Henry & Mitchell 2011). In 2013, large swarms of P. noctiluca were observed intermittently in several Northern and Southern farms in the west coast of Ireland from the beginning of August to mid-November. Jellyfish presence was determined by visual observations from boat and cages and by divers during removal of mortalities. Zooplankton sampling through vertical net tows and posterior species identification was also carried out. The diagnosis of the jellyfish species involved was based on the morphological features described in detail by Russell (1970). Jellyfish appeared in the vicinity and inside cages (Fig. 1a), and associated mortalities ranged from

A Piscirickettsia salmonis infection was diagnosed in lumpfish (Cyclopterus lumpus L.) juveniles held in a marine research facility on the west coast of Ireland. The main clinical signs and pathology included marked ascites, severe multifocal liver necrosis and severe diffuse inflammation and necrosis of the exocrine pancreas and peri-pancreatic adipose tissue. Numerous Piscirickettsia-like organisms were observed by histopathology in the affected organs, and the bacterial species was characterized by molecular analysis. Sequencing of the partial 16S rDNA gene and internal transcribed spacer region showed the lumpfish sequences to be closely related to previously identified Atlantic salmon (Salmo salar L.) sequences from Ireland. To the authors' knowledge, this is the first detection of P. salmonis in lumpfish worldwide. The infection is considered potentially significant in terms of lumpfish health and biosecurity.

A microsporidian species with 98.3-98.4% nucleotide identity to Tetramicra brevifilum (Journal of Fish Diseases, 3, 1980, 495) was diagnosed in lumpfish (Cyclopterus lumpus, L.) broodstock held at a breeding and rearing facility in western Ireland. The fish were wild-caught from the west coast of Ireland, and the first case was diagnosed one year after capture. Clinical signs included severe bloating, lethargy, exophthalmos, anorexia, white patches on the cornea and externally visible parasitic cysts on skin and fins. Necropsy revealed severe ascites, white nodules and vacuoles in all the internal organs and partial liquefaction of the skeletal muscle. On histological examination, microsporidian xenomas were observed in all internal organs, the skin, skeletal muscle, gills and the eyes. The microsporidian species was identified by molecular analysis and transmission electron microscopy. This is the first record of T. brevifilum infecting lumpfish, and the disease is considered to be of potential significance to the rising aquaculture industry of this species.

The lumpsucker, Cyclopterus lumpus , is a scorpaeniform marine fish inhabiting the rocky seabed of the North Atlantic and southern parts of the Arctic ocean. The species is highly valued by the Icelandic fishing industry and its eggs are used for human consumption. In recent years, the aquaculture industry has become interested in this species due to its potential use as a cleaner fish for biological control of sea lice on Atlantic salmon, Salmo salar . As for the majority of wild fish species, knowledge of optimal husbandry and diseases affecting lumpsucker is scarce. Its new use in intensive aquaculture highlights the need for further knowledge of its susceptibility to infections; for welfare, management and biosecurity reasons. This short communication describes a clinical vibriosis caused by Vibrio anguillarum in wild caught captive lumpsucker in Scotland. The affected individuals, with a mean weight of 36 g, were being kept in tanks prior to intended transfer to offshore salmon cages. The water temperature in the tanks was 6°C and the fish were fed a commercial marine fish diet. Mortality levels ranged from 3.6 per cent to 16.8 per cent between tanks in the week prior to sampling, and affected fish exhibited a number of gross signs including congestion and haemorrhage …

A risk framework has been developed to examine the influence of climate change on disease emergence in the United Kingdom. The fish immune response and the replication of pathogens are often correlated with water temperature, which manifest as temperature ranges for infection and clinical diseases. These data are reviewed for the major endemic and exotic disease threats to freshwater fish. Increasing water temperatures will shift the balance in favour of either the host or pathogen, changing the frequency and distribution of disease. A number of endemic diseases of salmonids (e.g. enteric red mouth, furunculosis, proliferative kidney disease and white spot) will become more prevalent and difficult to control as water temperatures increase. Outbreaks of koi herpesvirus in carp fisheries are likely to occur over a longer period each summer. Climate change also alters the threat level associated with exotic pathogens. The risk of viral haemorrhagic septicaemia (VHSV), infectious haematopoietic necrosis virus (IHNV) and spring viraemia of carp virus (SVCV) declines as infection generally only establishes when water temperatures are less than 14°C for VHSV and IHNV and 17°C for SCVC. The risk of establishment of other exotic pathogens (epizootic haematopoietic necrosis and epizootic ulcerative syndrome) increases. The spread of Lactococcus garvieae northwards in Europe is likely to continue, and thus is more likely to be both introduced and become established. Measures to reduce the threat of exotic pathogens need to be revised to account for the changing exotic diseases threat. Increasing water temperatures and the negative effects of extreme weather events (e.g. storms) are likely to alter the freshwater environment adversely for both wild and farmed salmonid populations, increasing their susceptibility to disease and the likelihood of disease emergence. For wild populations, surveillance and risk mitigation need to be focused on locations where disease emergence, as a result of climate change, is most likely.