Spatial and temporal heterogeneity of infectious hematopoietic necrosis virus in Pacific Northwest salmonids

Abstract

The aquatic rhaboviral pathogen infectious hematopoietic necrosis virus (IHNV) causes acute disease in juvenile fish of a number of populations of Pacific salmonid species. Heavily managed in both marine and freshwater environments, these fish species are cultured during the juvenile stage in freshwater conservation hatcheries, where IHNV is one of the top three infectious diseases that cause serious morbidity and mortality. Therefore, a comprehensive study of viral genetic surveillance data representing 2590 field isolates collected between 1958 and 2014 was conducted to determine the spatial and temporal patterns of IHNV in the Pacific Northwest of the contiguous United States. Prevalence of infection varied over time, fluctuating over a rough 5-7yearcycle. The genetic analysis revealed numerous subgroups of IHNV, each of which exhibited spatial heterogeneity. Within all subgroups, dominant genetic types were apparent, though the temporal patterns of emergence of these types varied among subgroups. Finally, the affinity or fidelity of subgroups to specific host species also varied, where UC subgroup viruses exhibited a more generalist profile and all other subgroups exhibited a specialist profile. These complex patterns are likely synergistically driven by numerous ecological, pathobiological, and anthropogenic factors. Since only a few anthropogenic factors are candidates for managed intervention aimed at improving the health of threatened or endangered salmonid fish populations, determining the relative impact of these factors is a high priority for future studies.

Transmission routes maintaining a viral pathogen of steelhead trout within a complex multi‐host assemblage

“…Among typed steelhead isolates, M group viruses were detected 1.8 times more frequently than U group viruses (176 M types, 96 U types), and among Chinook, U group viruses were detected 4.1 times more frequently than M group (39 M types, 158 U types). Consistent with previous reports, genotypes detected here did not occur at similar frequencies (Breyta et al., ; Breyta, Black, et al., ; Garver et al., ). The majority of genotypes (69 out of 90) were detected only at one site in a single year, while seven genotypes made up 77% of the genotyped records.…”

“…Over 3,000 virus isolates from fish sampled in Alaska, Washington, Oregon, Idaho, California, and Montana have been analyzed, and the data are publicly available as the MEAP‐IHNV database (Molecular Epidemiology of Aquatic Pathogens‐IHNV; http://gis.nacse.org/ihnv/). This typing program has detected 322 unique IHNV genotypes to date (Breyta, Black, et al., ; Kurath et al., ), falling into three major IHNV genogroups in North America, U, M, and L. Within our study region, both U and M group viruses co‐occur (Breyta, Black, et al., ; Garver, Troyer, & Kurath, ). Although virus isolates from each genogroup have been demonstrated to infect all salmonid host species tested to date, they differ in host‐specific fitness and virulence.…”

“…This definition assumes that homoplasy at the level of viral gene sequence is not occurring. There have been several well‐documented periods of emergence of IHNV, most of which were associated with significant mortality (Breyta et al., ; Breyta, Black, et al., ; Garver et al., ; Kurath et al., ; Troyer et al., ). On a landscape scale, then, a critical and poorly understood factor is the primary mode of transmission to juvenile fish.…”

“…This study is focused on IHNV epidemiology within the region consisting of the coastal watersheds of Oregon and Washington (excluding Puget Sound), and the Columbia River Basin, which represents a large watershed draining an area of 668,000 km 2 including most of inland Washington, Oregon, and Idaho. Within this study region, there have been several recorded high‐impact IHNV emergence events (Bootland & Leong, ; Breyta et al., ; Breyta, Black, et al., ; Groberg et al., ). There are extensive state, federal, and tribal hatchery culture programs that rear fish at freshwater sites in support of conservation goals or mitigation of habitat loss through activities such as hydroelectric power generation (Naish et al., ).…”

“…Phenotypic variation in host specificity of U and M virus types is an essential aspect of the complex ecology of IHNV in the study region. The majority of disease impacts in the study area are due to M genotype viruses in steelhead trout (Breyta, Black, et al., ). In coastal watersheds of Oregon and Washington, IHN disease has historically been all due to U genogroup viruses in sockeye salmon ( O. nerka ) (Emmenegger & Kurath, ) although disease outbreaks due to M genogroup types were detected during a major IHNV emergence in coastal steelhead during 2007–2011 (Breyta et al., ).…”