Abstract
AbstactInfectious salmon anaemia (ISA) is a serious disease of marine-farmed Atlantic salmon (Salmo salar) caused by ISA virus (ISAV), which belongs to the genus Isavirus, family Orthomyxoviridae. ISA is caused by virulent ISAV strains with deletions in a highly polymorphic region (HPR) of the hemagglutinin-esterase (HE) protein (designated virulent ISAV-HPR∆). This study shows the historic dynamics of ISAV-HPR∆ and ISAV-HPR0 in Chile, the genetic relationship among ISAV-HPR0 reported worldwide and between ISAV-HPR0 and ISAV-HPR∆ in Chile, and reports the 2013 ISA outbreak in Chile. The first ISA outbreak in Chile occurred from mid-June 2007 to 2010 and involved the virulent ISAV-HPR7b, which was then replaced by a low pathogenic ISAV-HPR0 variant. We analyzed this variant in 66 laboratory-confirmed ISAV-HPR0 cases in Chile in comparison to virulent ISAV-HPR∆ that caused two new ISA outbreaks in April 2013. Multiple alignment and phylogenetic analysis of HE sequences from all ISAV-HPR0 viruses allowed us to identify three genomic clusters, which correlated with three residue patterns of ISAV-HPR0 (360PST362, 360PAN362 and 360PAT362) in HPR. The virus responsible for the 2013 ISAV-HPR∆ cases in Chile belonged to ISAV-HPR3 and ISAV-HPR14, and in phylogenetic analyses, both clustered with the ISAV-HPR0 found in Chile. The ISAV-HPR14 had the ISAV-HPR0 residue pattern 360PAT362, which is the only type of ISAV-HPR0 variant found in Chile. This suggested to us that the 2013 ISAV-HPR∆ re-emerged from ISAV-HPR0 that is enzootic in Chilean salmon aquaculture and were not new introductions of virulent ISAV-HPR∆ to Chile. The clinical presentations and diagnostic evidence of the 2013 ISA cases indicated a mixed infection of ISAV with the ectoparasite Caligus rogercresseyi and the bacterium Piscirickettsia salmonis, which underscores the need for active ISAV surveillance in areas where ISAV-HPR0 is enzootic, to ensure early detection and control of new ISA outbreaks, as it is considered a risk factor. This is the first report of ISA linked directly to the presence of ISAV-HPR0, and provides strong evidence supporting the contention that ISAV-HPR0 shows a strong relationship to virulent ISAV-HPR∆ viruses and the possibility that it could mutate to virulent ISAV-HPR∆.
Highlights
Infectious salmon anaemia (ISA) is a serious viral disease of marine-farmed Atlantic salmon (Salmo salar) caused by ISA virus (ISAV), which belongs to the genus Isavirus, family Orthomyxoviridae
It suggests that the ancestral relationship between ISAV-HPR0 and ISAV-HPRΔ is a complex one since the origin of ISAV-HPR0 in Chile seems to confirm the “insertion hypothesis” that the virulent ISAV-HPRΔ undergoes insertion mutations resulting in ISAV-HPR0 and attenuation [28]
The ISA outbreaks in Chile were related to the sanitary condition in the salmon aquaculture industry [27], and the virulent ISAV-HPRΔ was replaced by low virulent ISAV-HPR0, the virus remains a risk factor for the industry
Summary
Infectious salmon anaemia (ISA) is a serious viral disease of marine-farmed Atlantic salmon (Salmo salar) caused by ISA virus (ISAV), which belongs to the genus Isavirus, family Orthomyxoviridae. The virus can be present in a net-cage for up to 6 months before significant mortality is noted [2] This is arguably the most important viral disease of marine-farmed Atlantic salmon (S, salar) because of the associated socialeconomic losses, and ISAV remains an emerging fish pathogen because of the asymptomatic infections in wild and farmed fish and the potential for emergence of new epizootic strains. The systemic disease ISA is caused by virulent ISAV strains with deletions in a highly polymorphic region (HPR) spanning residues 337V to M372 in the stem of the HE protein (designated virulent ISAV-HPRΔ) that have either an insertion or the 266Q → 266L mutation in the F gene [8]. Direct evidence for how the insertion or deletion in HPR affects virulence will only be possible when a reverse genetics system for ISAV is developed
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