Abstract
Infectious haematopoietic necrosis virus (IHNV) and viral haemorrhagic septicaemia virus (VHSV) are the most important viral pathogens impacting rainbow trout farming. These viruses are persistent in Italy, where they are responsible for severe disease outbreaks (epizootics) that affect the profitability of the trout industry. Despite the importance of IHNV and VHSV, little is known about their evolution at a local scale, although this is likely to be important for virus eradication and control. To address this issue we performed a detailed molecular evolutionary and epidemiological analysis of IHNV and VHSV in trout farms from northern Italy. Full-length glycoprotein gene sequences of a selection of VHSV (n = 108) and IHNV (n = 89) strains were obtained. This revealed that Italian VHSV strains belong to sublineages Ia1 and Ia2 of genotype Ia and are distributed into 7 genetic clusters. In contrast, all Italian IHNV isolates fell within genogroup E, for which only a single genetic cluster was identified. More striking was that IHNV has evolved more rapidly than VHSV (mean rates of 11 and 7.3 × 10−4 nucleotide substitutions per site, per year, respectively), indicating that these viruses exhibit fundamentally different evolutionary dynamics. The time to the most recent common ancestor of both IHNV and VHSV was consistent with the first reports of these pathogens in Italy. By combining sequence data with epidemiological information it was possible to identify different patterns of virus spread among trout farms, in which adjacent facilities can be infected by either genetically similar or different viruses, and farms located in different water catchments can be infected by identical strains. Overall, these findings highlight the importance of combining molecular and epidemiological information to identify the determinants of IHN and VHS spread, and to provide data that is central to future surveillance strategies and possibly control.
Highlights
Infectious haematopoietic necrosis virus (IHNV) and viral haemorrhagic septicaemia virus (VHSV) are the causative agents of infectious haematopoietic necrosis (IHN) and viral haemorrhagic septicaemia (VHS), respectively, two OIE (World Organisation for Animal Health), listed viral diseases that severely affect trout farming (OIE, 2016)
A maximum likelihood phylogenetic analysis reveals that all the Italian IHNV strains fall within the European genogroup E (Figure 2)
The Italian strains belonging to genogroup E are related to IHNV strains from Croatia, France, Germany and Switzerland
Summary
IHNV and VHSV are the causative agents of infectious haematopoietic necrosis (IHN) and viral haemorrhagic septicaemia (VHS), respectively, two OIE (World Organisation for Animal Health), listed viral diseases that severely affect trout farming (OIE, 2016). IHNV and VHSV both belong to the genus Novirhabdovirus within the family Rhabdoviridae (ICTV, 2014). Sublineage Ib circulates in marine fish of the Baltic and North Sea and the English Channel, while clade Ic comprises Danish rainbow trout strains from the 1980s. Sublineages Id and Ie include viral isolates from the Scandinavian peninsula and from the Black Sea, respectively, while viral strains within genotypes II and III appear to be restricted to the Baltic Sea and the North Atlantic Ocean. The designation U, M, and L reflects the geographical distribution of IHNV in North America (Kurath et al, 2003; Kuzmin et al, 2009; Enzmann et al, 2010), while genogroup E is related to the M genogroup and comprises European isolates from France, Italy and Germany (Enzmann et al, 2005). Genogroup J includes rainbow trout isolates from Japan and South Korea (Nishizawa et al, 2006; He et al, 2013)
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