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Viruses assigned to the genus Perhabdovirus comprise one of the three genera of rhabdoviruses that infect finfish, the others being spriviviruses and novirhabdoviruses. Perhabdoviruses form a distinct monophyletic group based on well-supported Maximum Likelihood trees inferred from complete L sequences and are most closely related to spriviviruses and vesiculoviruses. Viruses assigned to the genus have been isolated from a wide range of fish but they predominantly cause disease in farmed perciform fishes (order Perciformes, perch-like). The genus includes three species: Perch perhabdovirus (type species) has a single member, perch rhabdovirus (PRV); Anguillid perhabdovirus has been established for two viruses, eel virus European X (EVEX) and eel virus American (EVA); and Sea trout perhabdovirus has two members, lake trout rhabdovirus (LTRV) and Swedish sea trout virus (SSTV).
PRV particles have a compact bullet-shaped morphology and measure 115–130 nm in length and 85–95 nm in diameter (Betts et al., 2003).
The genome consists of a single molecule of negative-sense, single-stranded RNA of approximately 11 kb.
N, P, M, G and L share significant sequence homology with the homologous proteins of other rhabdoviruses.
The genomic RNA is approximately 11.4–11.8 kb with five genes in the order 3′-N-P-M-G-L-5′ encoding a nucleoprotein, polymerase-associated protein, matrix protein, glycoprotein and RNA-dependent RNA polymerase, respectively (Figure 1.Perhabdovirus). The genome contains a leader region of approximately 63–100 nt preceding the transcription initiation of the N gene, and a trailer of 40–67 nt following the transcription termination of the L gene. The transcriptional initiation and the termination/polyadenylation signal is conserved for all genes, 3′-UUGUC and 3′-AURC(U)7, respectively, but the non-transcribed intergenic regions are variable. There is inverse complementarity between the 3′-leader and 5′-trailer sequences.
Figure 1.Perhabdovirus. Schematic representation of perhabdovirus genome organisations. N, P, M, G and L represent ORFs encoding the structural proteins. Small alternative ORF occur in the P genes of some perhabdoviruses (orange) but it is not known if these are expressed as functional proteins.
PRV was first isolated from cultured perch (Perca fluviatilis) following a mortality event in France (Dorson et al., 1984) and subsequently from perch in Norway (Dannevig et al., 2001). Antigenically similar viruses have been recovered from diseased pike perch (Stizostedion stizostedion) (Nougayrède et al., 1992), grayling (Thymallus thymallus) and largemouth bass (Micropterus salmoides) in France (Betts et al., 2003) and from asymptomatic pike in Denmark (Jorgensen et al., 1989). These viruses are all assigned to the species Perch perhabdovirus. SSTV was discovered in brown trout (Salmo trutta m.lacustris) in Finland (Jorgensen et al., 1989, Koski et al., 1992, Björklund et al., 1994) and sea trout (Salmo trutta trutta) from Sweden (Johansson et al., 2002). More recently, viruses assigned to the species Sea trout perhabdovirus have been isolated from perch, eel and brown trout in Ireland (Ruane et al., 2014). Serologically related viruses have been isolated from eels in Japan (Sano et al., 1977, Kobayashi and Miyazaki 1996). EVA was isolated from American elvers (Anguilla nostrata) and EVEX was isolated from European eels (Anguilla anguilla); each is assigned to the species Anguillid perhabdovirus.
As for other fish rhabdoviruses, the replication temperature range for perhabdoviruses is lower than for mammalian rhabdoviruses, reflecting the aquatic poikilothermic nature of the hosts. Viruses are typically isolated on cultured fish cell lines at 15–25°C. The disease patterns are influenced by water temperature, age and condition of the fish, population density and stress factors. The immune status of the fish is also an important factor with both innate and adaptive immunity having important roles. Clinical disease is usually observed at water temperature between 5°–18°C and is most severe at temperatures below 10°C when it is believed the host immune response is suppressed or delayed.
Antisera to the reference isolate of PRV efficiently neutralise PRV isolates from fish of a range of species (Dorson et al., 1984) but there is no cross-neutralisation between PRV and SVCV (genus Sprivivirus) or VHSV (genus Novirhabdovirus) (Betts et al., 2003).
Perhabdovirus: from perch rhabdovirus, assigned as a member of the type species Perch perhabdovirus.
Viruses assigned to different species within the genus Perhabdovirus have the following characteristics: A) minimum nucleotide sequence divergence of 15% in L genes; and B) can be clearly distinguished antigenically in serological tests.
Viruses within species within the genus can be distinguished serologically but the species distinction is based primarily on nucleotide sequence divergence based on partial L gene sequences. Sequence divergence between viruses assigned to different species ranges from 18-19% between members of the species Anguillid perhabdovirus and Sea trout perhabdovirus, 19–23% between between members of the species Anguillid perhabdovirus and Perch perhabdovirus, and 17–21% between between members of the species Sea trout perhabdovirus and Perch perhabdovirus. Sequence divergence between viruses assigned to the same species range from 7.9% for Sea trout perhabdovirus to 9.3% for Anguillid perhabdovirus and 14.2% for Perch perhabdovirus. As occurs for many fish viruses, there is a broad range and overlapping host range for viruses in the genus Perhabdovirus.
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