Rainbow Trout Sleeping Disease Virus Is an Atypical Alphavirus

Abstract

Sleeping disease (SD) is currently a matter of concern for salmonid fish farmers in most parts of the world. A viral etiology of SD has recently been suspected, since virus-like particles have been observed in infected rainbow trout cells. In salmonid-derived cell lines, the maximal rate of virus production was observed at 10°C, while little virus was produced at 14°C. Through biochemical, physicochemical, and morphological studies, SD virus (SDV) was shown to be an enveloped virus of roughly 60 nm in diameter. The genome consists of 12 kb of RNA, with the appearance of a 26S subgenomic RNA during the time course of SDV replication. The screening of a random-primed cDNA library constructed from the genomic RNA of semipurified virions facilitated the identification of a specific SDV cDNA clone having an open reading frame related to the alphavirus E2 glycoproteins. To extend the comparison between SDV structural proteins and the alphavirus protein counterparts, the nucleotide sequence of the total 4.1-kb subgenomic RNA has been determined. The 26S RNA encodes a 1,324-amino-acid polyprotein exhibiting typical alphavirus structural protein organization. SDV structural proteins showed several remarkable features compared to other alphaviruses: (i) unusually large individual proteins, (ii) very low homology (ranging from 30 to 34%) (iii) an unglycosylated E3 protein, and (iv) and E1 fusion domain sharing mutations implicated in the pH threshold. Although phylogenetically related to the Semliki Forest virus group of alphaviruses, SDV should be considered an atypical member, able to naturally replicate in lower vertebrates.Sleeping disease (SD) syndrome of farmed freshwater rainbow trout has been observed in France for many years (4). The most characteristic sign of the disease is the unusual behavior of the fish, which stay on their side at the bottom of the tank. Histological observations of diseased fish showed a chronological appearance of lesions in the pancreas, in the heart, and in the muscle at the last stage of the disease (5, 6). Transmission of SD may occur through contact with contaminated tissue from fish that have SD (5). A viral etiology of SD was suspected, since virus-like particles were observed in purified homogenates from kidneys of diseased fish (3). However, all attempts to isolate a viral agent on commonly available fish cell lines by inoculating organ homogenates from diseased fish remained unsuccessful until recently (7). Isolation of SD virus (SDV) in cell culture was successfully achieved by direct inoculation of salmonid cell lines (CHSE-214 and RTG-2) with plasma from infected fish.The characterization of SDV was successfully achieved by optimizing viral production in tissue culture and by studying several physicochemical features of this virus. Data include the type of nucleic acid, size, and organization of the SDV genome. The viral genome has been shown to be an RNA molecule of roughly 12 kb. A cDNA library has been constructed, and the nucleotide sequencing of recombinant cDNA clones de...

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“…However, Hodneland et al (2005) recently showed that pancreas disease from Atlantic salmon and rainbow trout in Norway is in fact caused by a different and distinct virus subtype, and named it Norwegian salmonid alphavirus (NSAV). Together with the sleeping disease virus (SDV) (Boucher and Baudin Laurencin, 1994;Branson, 2002;Castric et al, 1997;Graham et al, 2003b;Villoing et al, 2000a) and salmon pancreas disease virus, the Norwegian salmonid alphavirus is included in the species Salmonid alphavirus in the genus Alphavirus of the family Togaviridae. All three salmonid alphavirus subtypes have a genomic organization characteristic to the Alphaviruses; with a positive-sense, single stranded genome of approximately 11.8 kb size.…”

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“…Consequently, alphaviruses can be cultured in many vertebrate and insect cell lines (11)(12)(13). In contrast, the distantly related fish alphaviruses, which are not known to have arthropod vectors, exhibit a narrow host range (fish cells only) that is at least partially a result of temperature sensitivity (14)(15)(16). However, the viral factor(s) that underlie the varying host range of alphaviruses are poorly understood.…”

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