Salmon gill poxvirus disease in Atlantic salmon fry as recognized by improved immunohistochemistry also demonstrates infected cells in non-respiratory epithelial cells.

Gill diseases cause serious losses in farming of Atlantic salmon and the number of agents involved increases. Salmon gill poxvirus (SGPV) and the gill disease in causes where SGPV apparently was the only disease-causing agent were initially characterized. Recently, it was further shown that SGPV can be a common denominator in widely different multifactorial gill diseases. Here, we present the challenge of diagnosing gill disease with SGPV in salmon fry of 0,3-5 grams. Apoptosis of gill lamellar epithelial cells and hemophagocytosis was also observed in fry similar to findings in smolts and grow-out fish. Using our newly developed immunohistochemistry method, we further demonstrate that some of the apoptotic epithelial cells covering the oral cavity were positive for SGPV. Thus, SGPV is not restricted to respiratory epithelium alone and may infect the fish at very early life stages. Furthermore, as the cases examined here are from Norway, Faroe Island and Scotland, we show that SGPV is more widespread than previously reported.

A survey of salmon gill poxvirus (SGPV) in wild salmonids in Norway.

In 2016, the Norwegian health monitoring programme for wild salmonids conducted a real-time PCR-based screening for salmon gill poxvirus (SGPV) in anadromous Arctic char (Salvelinus alpinus L.), anadromous and non-anadromous Atlantic salmon (Salmo salar L.) and trout (Salmo trutta L.). SGPV was widely distributed in wild Atlantic salmon returning from marine migration. In addition, characteristic gill lesions, including apoptosis, were detected in this species. A low amount of SGPV DNA, as indicated by high Ct-values, was detected in anadromous trout, but only in fish cohabiting with SGPV-positive salmon. SGPV was not detected in trout and salmon from non-anadromous water courses, and thus seems to be primarily linked to the marine environment. This could indicate that trout are not a natural host for the virus. SGPV was not detected in Arctic char but, due to a low sample size, these results are inconclusive. The use of freshwater from anadromous water sources may constitute a risk of introducing SGPV to aquaculture facilities. Moreover, SGPV-infected Atlantic salmon farms will hold considerable potential for virus propagation and spillback to wild populations. This interaction should therefore be further investigated.

Emergence of carp edema virus (CEV) and its significance to European common carp and koi Cyprinus carpio.

Carp edema virus disease (CEVD), also known as koi sleepy disease, is caused by a poxvirus associated with outbreaks of clinical disease in koi and common carp Cyprinus carpio. Originally characterised in Japan in the 1970s, international trade in koi has led to the spread of CEV, although the first recognised outbreak of the disease outside of Japan was not reported until 1996 in the USA. In Europe, the disease was first recognised in 2009 and, as detection and diagnosis have improved, more EU member states have reported CEV associated with disease outbreaks. Although the structure of the CEV genome is not yet elucidated, molecular epidemiology studies have suggested distinct geographical populations of CEV infecting both koi and common carp. Detection and identification of cases of CEVD in common carp were unreliable using the original PCR primers. New primers for conventional and quantitative PCR (qPCR) have been designed that improve detection, and their sequences are provided in this paper. The qPCR primers have successfully detected CEV DNA in archive material from investigations of unexplained carp mortalities conducted >15 yr ago. Improvement in disease management and control is possible, and the principles of biosecurity, good health management and disease surveillance, applied to koi herpesvirus disease, can be equally applied to CEVD. However, further research studies are needed to fill the knowledge gaps in the disease pathogenesis and epidemiology that, currently, prevent an accurate assessment of the likely impact of CEVD on European koi and common carp aquaculture and on wild carp stocks.

Ca. Branchiomonas cysticola, Ca. Piscichlamydia salmonis and Salmon Gill Pox Virus transmit horizontally in Atlantic salmon held in fresh water.

Elucidation of the role of infectious agents putatively involved in gill disease is commonly hampered by the lack of culture systems for these organisms. In this study, a farmed population of Atlantic salmon pre-smolts, displaying proliferative gill disease with associated Candidatus Branchiomonas cysticola, Ca. Piscichlamydia salmonis and Atlantic salmon gill pox virus (SGPV) infections, was identified. A subpopulation of the diseased fish was used as a source of waterborne infection towards a population of naïve Atlantic salmon pre-smolts. Ca. B. cysticola infection became established in exposed naïve fish at high prevalence within the first month of exposure and the bacterial load increased over the study period. Ca. P. salmonis and SGPV infections were identified only at low prevalence in exposed fish during the trial. Although clinically healthy, at termination of the trial the exposed, naïve fish displayed histologically visible pathological changes typified by epithelial hyperplasia and subepithelial inflammation with associated bacterial inclusions, confirmed by fluorescent in situ hybridization to contain Ca. B. cysticola. The results strongly suggest that Ca. B. cysticola infections transmit directly from fish to fish and that the bacterium is directly associated with the pathological changes observed in the exposed, previously naïve fish.

Salmon gill poxvirus, a recently characterized infectious agent of multifactorial gill disease in freshwater- and seawater-reared Atlantic salmon.

Gill diseases cause considerable losses in Norwegian salmon farming. In 2015, we characterized salmon gill poxvirus (SGPV) and associated gill disease. Using newly developed diagnostic tools, we show here that SGPV infection is more widely distributed than previously assumed. We present seven cases of complex gill disease in Atlantic salmon farmed in seawater and freshwater from different parts of Norway. Apoptosis, the hallmark of acute SGPV infection, was not easily observed in these cases, and qPCR analysis was critical for identification of the presence of SGPV. Several other agents including Costia-like parasites, gill amoebas, Saprolegnia spp. and bacteria were observed. The studied populations experienced significant mortalities, which increased to extreme levels when severe SGPV infections coincided with smoltification. SGPV infection appears to affect the smoltification process directly by affecting the gills and chloride cells in particular. SGPV may be considered a primary pathogen as it was often found prior to identification of complex gill disease. It is hypothesized that SGPV-induced gill damage may impair innate immunity and allow invasion of secondary invaders. The distinct possibility that SGPV has been widely overlooked as a primary pathogen calls for extended use of SGPV qPCR in Atlantic salmon gill health management.

Presence and interaction of inflammatory cells in the spleen of Atlantic cod, Gadus morhua L., infected with Francisella noatunensis.

Serious infectious diseases, accompanied by macrophage-dominated chronic inflammation, are common in farmed Atlantic cod. To increase knowledge relating to morphological aspects of such inflammatory responses, cod were challenged with Francisella noatunensis, an important bacterial pathogen of this fish species. Tissue and cell dynamics in the spleen were examined sequentially over 60 days. Small clusters of mainly macrophage-like cells (MLCs) staining for non-specific esterase and acid phosphatase developed with time. These foci were transiently infiltrated by pleomorphic proliferating cells of unknown nature and by granulocyte-like cells (GCLCs) staining for peroxidase and lysozyme. The latter cell type, which appeared to be resident in the red pulp of control fish, migrated into the inflammatory foci of infected fish. Cells expressing genes encoding IFN-γ and IL-8 increased in number during the study period. Bacteria were detected only in the MLCs and their number increased despite the extensive inflammation. Our results demonstrate an intimate spatial relationship in inflammatory foci between at least three cell types. The presence of GCLCs, together with MLCs, suggests pyogranulomatous inflammation as a more appropriate descriptive term than granulomatous inflammation.

Nodavirus provokes subclinical encephalitis and retinochoroiditis in adult farmed Atlantic cod, Gadus morhua L.

Viral nervous necrosis (VNN) caused by beta-nodavirus affects many species of farmed marine fish, in particular juveniles. Apparently healthy, normally feeding, adult farmed Atlantic cod, Gadus morhua, were sampled in a farm 14 months after an outbreak of VNN with clinical signs. Following necropsy, brain and eye tissues were examined by histology, immunohistochemistry and polymerase chain reaction (PCR). Nodavirus-provoked cell death and inflammation was detected in eye and brain, particularly in the retina and cerebellum and differed from that previously described in Atlantic cod during clinical stages of VNN. Virus was detected both by PCR and immunohistochemistry. This is, to the best of our knowledge, the first description of pathological changes associated with chronic subclinical nodavirus infection in Atlantic cod. Our observations suggest that severe infection and pathological changes may go undetected if investigations are restricted to clinical examination and macroscopic evaluation at necropsy.