Infectious haemolytic anaemia causes jaundice outbreaks in seawater-cultured coho salmon, Oncorhynchus kisutch (Walbaum), in Chile

The same strain of Piscine orthoreovirus (PRV-1) is involved in the development of different, but related, diseases in Atlantic and Pacific Salmon in British Columbia

Piscine orthoreovirus Strain PRV-1 is the causative agent of heart and skeletal muscle inflammation (HSMI) in Atlantic salmon ( Salmo salar Linnaeus, 1758). Given its high prevalence in net pen salmon, debate has arisen on whether PRV poses a risk to migratory salmon, especially in British Columbia (BC) where commercially important wild Pacific salmon are in decline. Various strains of PRV have been associated with diseases in Pacific salmon, including erythrocytic inclusion body syndrome (EIBS), HSMI-like disease, and jaundice/anemia in Japan, Norway, Chile and Canada. We examined the developmental pathway of HSMI and jaundice/anemia associated with PRV-1 in farmed Atlantic and chinook ( Oncorhynchus tshawytscha (Walbaum, 1792)) salmon in BC, respectively. In situ hybridization localized PRV-1 within developing lesions in both diseases. The two diseases showed dissimilar pathological pathways, with inflammatory lesions in heart and skeletal muscle in Atlantic salmon and degenerative-necrotic lesions in kidney and liver in chinook salmon, plausibly explained by differences in PRV load tolerance in red blood cells. Viral genome sequencing revealed no consistent differences in PRV-1 variants intimately involved in the development of both diseases suggesting that migratory chinook salmon may be at more than a minimal risk of disease from exposure to the high levels of PRV occurring in salmon farms.

First Complete Genome Sequence of Piscine Orthoreovirus Variant 3 Infecting Coho Salmon (Oncorhynchus kisutch) Farmed in Southern Chile

We report here the complete genome of an isolate of piscine orthoreovirus variant 3 sequenced from a moribund coho salmon with jaundice that was reared in a seawater farm in southern Chile. The genome consists of 23,627 bp, including 10 segments that range from 1,052 bp (segment S4) to 4,014 bp (segment L1).

First description of clinical presentation of piscine orthoreovirus (PRV) infections in salmonid aquaculture in Chile and identification of a second genotype (Genotype II) of PRV

BACKGROUND

Heart and skeletal muscle inflammation (HSMI) is an emerging disease of marine-farmed Atlantic salmon Salmo salar, first recognized in 1999 in Norway, and recently associated with piscine orthoreovirus (PRV) infection. To date, HSMI lesions with presence of PRV have only been described in marine-farmed Atlantic salmon in Norway. A new HSMI-like disease in rainbow trout Oncorhynchus mykiss associated with a PRV-related virus has also been reported in Norway.

METHODS

Sampling of Atlantic salmon and coho salmon was done during potential disease outbreaks, targeting lethargic/moribund fish. Fish were necropsied and tissues were taken for histopathologic analysis and testing for PRV by RT-qPCR assay for segment L1 and conventional RT-PCR for PRV segment S1. The PCR products were sequenced and their relationship to PRV strains in GenBank was determined using phylogenetic analysis and nucleotide and amino acid homology comparisons.

RESULTS

The Atlantic salmon manifested the classical presentation of HSMI with high PRV virus loads (low Ct values) as described in Norway. The coho salmon with low Ct values had myocarditis but only in the spongy layer, the myositis of red muscle in general was mild, and the hepatic necrosis was severe. Upon phylogenetic analysis of PRV segment S1 sequences, all the Chilean PRV strains from Atlantic salmon grouped as sub-genotype Ib, whereas the Chilean PRV strains from coho salmon were more diversified, grouping in both sub-genotypes Ia and Ib and others forming a distinct new phylogenetic cluster, designated Genotype II that included the Norwegian PRV-related virus.

CONCLUSIONS

To our knowledge the present work constitutes the first published report of HSMI lesions with presence of PRV in farmed Atlantic salmon outside of Europe, and the first report of HSMI-like lesions with presence of PRV in coho salmon in Chile. The Chilean PRV strains from coho salmon are more genetically diversified than those from Atlantic salmon, and some form a distinct new phylogenetic cluster, designated Genotype II.

Piscine reovirus, but not Jaundice Syndrome, was transmissible to Chinook Salmon, Oncorhynchus tshawytscha (Walbaum), Sockeye Salmon, Oncorhynchus nerka (Walbaum), and Atlantic Salmon, Salmo salar L

A Jaundice Syndrome occurs sporadically among sea-pen-farmed Chinook Salmon in British Columbia, the westernmost province of Canada. Affected salmon are easily identified by a distinctive yellow discolouration of the abdominal and periorbital regions. Through traditional diagnostics, no bacterial or viral agents were cultured from tissues of jaundiced Chinook Salmon; however, piscine reovirus (PRV) was identified via RT-rPCR in all 10 affected fish sampled. By histopathology, Jaundice Syndrome is an acute to peracute systemic disease, and the time from first clinical signs to death is likely <48 h; renal tubular epithelial cell necrosis is the most consistent lesion. In an infectivity trial, Chinook Salmon, Sockeye Salmon and Atlantic Salmon, intraperitoneally inoculated with a PRV-positive organ homogenate from jaundiced Chinook Salmon, developed no gross or microscopic evidence of jaundice despite persistence of PRV for the 5-month holding period. The results from this study demonstrate that the Jaundice Syndrome was not transmissible by injection of material from infected fish and that PRV was not the sole aetiological factor for the condition. Additionally, these findings showed the Pacific coast strain of PRV, while transmissible, was of low pathogenicity for Atlantic Salmon, Chinook Salmon and Sockeye Salmon.

Orthomyxoviruses of Fish

The family Orthomyxoviridae is well known for containing influenza viruses with a segmented RNA genome that is prone to gene reassortment in mixed infections (known as antigenic shift) resulting in new virus subtypes that cause pandemics, and cumulative mutations (known as antigenic drift), resulting in new virus strains that cause epidemics. This family also contains infectious salmon anemia virus (ISAV) and tilapia lake virus (TiLV), which are a unique orthomyxoviruses that infect fish and is unable to replicate above room temperature (24°C). This chapter describes the comparative virology of members in the family Orthomyxoviridae in general, helping to understand the emergent teleost orthomyxoviruses, ISAV and TiLV. The most current information on virus–host interactions of the fish orthomyxoviruses, particularly ISAV, as they relate to variations in virus structure, virulence, persistence, host range and immunological aspects is presented in detail.

Haemato-biochemical disorders associated with nodavirus like-agent in adult leaping mullet Liza saliens (Risso, 1810) in the Caspian Sea

Betanoda virus is an emerging problem in several marine fish species in various geographic areas all over the world. In recent years, mullets stock of the Caspian Sea decreased dramatically and a betanoda virus like-agent was introduced as the cause of mullet's mortality. The main objective of the present study is to compare hemato-biochemical parameters in healthy and infected sharpnose mullets (Liza saliens) to betanoda virus like-agent. The adult sharpnose mullets (34 clinically affected fish + 34 apparently healthy fish) were captured from the southeast the Caspian Sea from 2012 to 2013. All of the captured fish were 4 or 5 years old. The main clinical sings of infected fish were lethargy, severe abdominal distention, abnormal swimming and hyperinflation of swim bladder. The results showed that the weight and total length of infected fish were significantly lower than the healthy fish. The results of histopatological evaluation and indirect florescent antibody test were confirmed the presence of a betanoda virus-like agent in infected fish. The diseased fish were severely anemic (hypochromic macrocytic anemia) and had a serious haemopoietic disorders. The anemia associated with a sever leukopenia, a significant rise of neutrophils and immature neutrophils and a significant decries of lymphocytes percentage. Total protein, albumin and total immunoglobulin levels were significantly reduced in the serum of infected fish, while the activities of aspartate aminotransferase and alanine aminotransferase significantly increased when compared to the healthy fish. These results suggested that the feeding of the infected fish to betanoda virus like-agent were disturbed and it could be cause haemato-biochemical disorder and mortality of the fish in Iranian water of the Caspian Sea.

Host tropism of infectious salmon anaemia virus in marine and freshwater fish species

The aquatic orthomyxovirus infectious salmon anaemia virus (ISAV) causes a severe disease in farmed Atlantic salmon, Salmo salar L. Although some ISA outbreaks are caused by horizontal transmission of virus between farms, the source and reservoir of the virus is largely unknown and a wild host has been hypothesized. Atlantic salmon are farmed in open net-pens, allowing transmission of pathogens from wild fish and the surrounding environment to the farmed fish. In this study, a large number of fish species were investigated for ISAV host potential. For orthomyxoviruses, a specific receptor binding is the first requirement for infection; thus, the fish species were investigated for the presence of the ISAV receptor. The receptor was found to be widely distributed across the fish species. All salmonids expressed the receptor. However, only some of the cod-like and perch-like fish did, and all flat fish were negative. In the majority of the positive species, the receptor was found on endothelial cells and/or on red blood cells. The study forms a basis for further investigations and opens up the possibility for screening species to determine whether a wild host of ISAV exists.

Infectious salmon anaemia - pathogenesis and tropism

Infectious salmon anaemia (ISA) is a serious disease of farmed Atlantic salmon caused by the aquatic orthomyxovirus infectious salmon anaemia virus (ISAV). ISA was first detected in Norway in 1984 and was characterized by severe anaemia and circulatory disturbances. This review elucidates factors related to the pathogenesis of ISA in Atlantic salmon, the dissemination of the virus in the host and the general distribution of the 4-O-acetylated sialic acids ISAV receptor. The knowledge contributes to the understanding of this disease, and why, almost 30 years after the first detection, it is still causing problems for the aquaculture industry.

First detection, isolation and molecular characterization of infectious salmon anaemia virus associated with clinical disease in farmed Atlantic salmon (Salmo salar) in Chile

Background

Infectious salmon anaemia (ISA) is a viral disease of marine-farmed Atlantic salmon (Salmo salar) caused by ISA virus (ISAV), which belongs to the genus Isavirus, family Orthomyxoviridae. The virus is considered to be carried by marine wild fish and for over 25 years has caused major disease outbreaks in marine-farmed Atlantic salmon in the Northern hemisphere. In the Southern hemisphere, ISAV was first detected in Chile in 1999 in marine-farmed Coho salmon (Oncorhynchus kisutch). In contrast to the classical presentation of ISA in Atlantic salmon, the presence of ISAV in Chile until now has only been associated with a clinical condition called Icterus Syndrome in Coho salmon and virus isolation has not always been possible. During the winter of 2007, unexplained mortalities were registered in market-size Atlantic salmon in a grow-out site located in Chiloé in Region X of Chile. We report here the diagnostic findings of the first significant clinical outbreak of ISA in marine-farmed Atlantic salmon in Chile and the first characterization of the ISAV isolated from the affected fish.

Results

In mid-June 2007, an Atlantic salmon marine farm site located in central Chiloé Island in Region X of Chile registered a sudden increase in mortality following recovery from an outbreak of Pisciricketsiosis, which rose to a cumulative mortality of 13.6% by harvest time. Based on the clinical signs and lesions in the affected fish, and laboratory tests performed on the fish tissues, a confirmatory diagnosis of ISA was made; the first time ISA in its classical presentation and for the first time affecting farmed Atlantic salmon in Chile. Rapid sequencing of the virus-specific RT-PCR products amplified from the fish tissues identified the virus to belong to the European genotype (Genotype I) of the highly polymorphic region (HPR) group HPR 7b, but with an 11-amino acid insert in the fusion glycoprotein, and ability to cause cytopathic effects (CPE) in CHSE-214 cell line, characteristics which make it distinct from common European Genotype ISAV isolates from Europe and North America.

Conclusion

In conclusion, the present work constitutes the first report of a case of ISA in farmed Atlantic salmon in Chile. The clinical signs and lesions are consistent with the classical descriptions of the disease in marine-farmed Atlantic salmon in the Northern hemisphere. The outbreak was caused by ISAV of European genotype (or Genotype I) of HPR 7b but distinct from common European Genotype ISAV isolates.