Pilchard orthomyxovirus (POMV). II. Causative agent of salmon orthomyxoviral necrosis, a new disease of farmed Atlantic salmon Salmo salar

Abstract

Since 2012, an orthomyxo-like virus has been consistently linked to epizootics in marine farmed Atlantic salmon in Tasmania, Australia. Here we describe the properties of the virus, designated the pilchard orthomyxovirus (POMV), in cell culture and present data verifying its direct role in a disease of Atlantic salmon. In infected cells, viral RNA was detectable in both the nucleus and cytoplasm, consistent with the replication cycle of an orthomyxovirus. Viral replication in vitro was temperature-dependent (within a range of 10-20°C), and yields of virus were typically in excess of 107 TCID50 ml-1. In controlled infection trials, cell culture-derived POMV produced significant morbidity in Atlantic salmon fry, pre-smolt and post-smolt. In all cases, the development of disease was rapid, with moribund fish detected within 5 d of direct exposure to POMV, and maximum cumulative morbidity occurring within 4 wk. The experimentally infected fish developed a characteristic suite of gross and microscopic pathological changes, which were consistent with those observed in Atlantic salmon overtly affected by POMV-associated disease on sea farms. These included necrotic lesions across multiple organs that were directly associated with the presence of the virus. Together, our observations indicate that POMV is an endemic virus likely transmitted from wild fish to farmed Atlantic salmon in Tasmania. The virus is pathogenic to Atlantic salmon in freshwater and marine environments and causes a disease that we have named salmon orthomyxoviral necrosis.

Transcriptome Response of Atlantic Salmon (Salmo salar) to a New Piscine Orthomyxovirus

“…Morbidity in the cohabitant fish commenced at 6 dpi and peaked at 18 dpi, with cumulative morbidity of 51.5%. All moribund fish (POMV-positives and POMV-suspects) presented clinical signs consistent with POMV pathology, including loss of equilibrium, lethargy, lack of response to stimuli, and dark pigmentation of the skin [ 3 ]. At the necropsy, moribund fish displayed generalized congestion, petechial hemorrhaging of visceral fat and internal organs, splenomegaly, and yellow fluid and mucus inside the gastrointestinal tract.…”

“…Fish used in this study were all-female diploid Atlantic salmon smolts (mean ± SE weight: 119 ± 1 g) obtained from the Florentine hatchery (Salmon Enterprises of Tasmania, SALTAS). POMV has not been detected in freshwater fish in Tasmania, and therefore, the fish used for the challenge trials were considered specific (POMV) pathogen-free stock [ 3 ]. Fish had been vaccinated against yersiniosis by bath as fry (at 2.5 g and 8 g) and intraperitoneally (IP) at the parr stage (50 g).…”

“…Samples for this study were optained from the POMV challenge experiment described in detail in Samsing et al [ 10 ]. In brief, Atlantic salmon post-smolts were challenged in seawater with POMV via cohabitation with trojan fish intraperitoneally injected (IP) with 200 µL of cell culture supernatant containing POMV at a titer of 10 8.8 TCID 50 mL −1 (tissue culture infective dose 50% per millilitre) [ 3 ]. The adipose fin of trojan fish was clipped to distinguish them from non-injected fish.…”

“…POMV belongs to the Orthomyxoviridae family, which includes serious human and animal pathogens such as influenza A viruses (IAV) and infectious salmon anaemia virus (ISAV), which is a notifiable finfish disease under the regulations of the World Organization for Animal Health, Office International des Epizooties (OIE) [ 2 ]. The clinical disease caused by POMV, recently named salmon orthomyxoviral necrosis (SON) [ 3 ], causes lethargy, dark coloration of the skin, and petechial hemorrhages on the ventral areas of the body. Unlike infectious salmon anaemia (ISA), the disease caused by ISAV, POMV does not cause anemia on its host.…”

“…Unlike infectious salmon anaemia (ISA), the disease caused by ISAV, POMV does not cause anemia on its host. In addition, the in vivo manifestation of SON in farmed Atlantic salmon appears to occur more rapidly than ISA [ 3 ]. Histopathological findings in clinically affected fish include cellular necrosis in different organs, such as the liver, spleen, kidneys, and heart.…”