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

Rachel Breyta,Andrew R. Wargo,Shannon LaDeau,Kerry A. Naish,Gael Kurath,Ilana Brito,Paige Ferguson,Maureen K. Purcell

doi:10.1002/ece3.3276

Abstract

This is the first comprehensive region wide, spatially explicit epidemiologic analysis of surveillance data of the aquatic viral pathogen infectious hematopoietic necrosis virus (IHNV) infecting native salmonid fish. The pathogen has been documented in the freshwater ecosystem of the Pacific Northwest of North America since the 1950s, and the current report describes the disease ecology of IHNV during 2000–2012. Prevalence of IHNV infection in monitored salmonid host cohorts ranged from 8% to 30%, with the highest levels observed in juvenile steelhead trout. The spatial distribution of all IHNV‐infected cohorts was concentrated in two sub‐regions of the study area, where historic burden of the viral disease has been high. During the study period, prevalence levels fluctuated with a temporal peak in 2002. Virologic and genetic surveillance data were analyzed for evidence of three separate but not mutually exclusive transmission routes hypothesized to be maintaining IHNV in the freshwater ecosystem. Transmission between year classes of juvenile fish at individual sites (route 1) was supported at varying levels of certainty in 10%–55% of candidate cases, transmission between neighboring juvenile cohorts (route 2) was supported in 31%–78% of candidate cases, and transmission from adult fish returning to the same site as an infected juvenile cohort was supported in 26%–74% of candidate cases. The results of this study indicate that multiple specific transmission routes are acting to maintain IHNV in juvenile fish, providing concrete evidence that can be used to improve resource management. Furthermore, these results demonstrate that more sophisticated analysis of available spatio‐temporal and genetic data is likely to yield greater insight in future studies.

Highlights

Steelhead trout (Oncorhynchus mykiss) are a culturally and ecologically important salmonid fish in the Pacific Northwest
In the 1980s, infectious hematopoietic necrosis virus (IHNV) in the Columbia River Basin adapted to increase prevalence in Chinook salmon (Oncorhynchus tshawytscha) (Arkush, Mendonca, McBride, & Hedrick, 2004; Black, Breyta, Bedford, & Kurath, 2016), which are often reared with steelhead trout and share similar spawning run timing
We describe the prevalence of IHNV in steelhead and other sympatric Pacific salmonids (Oncorhynchus spp.), across the Columbia River Basin and adjacent coastal rivers during the period from 2000 to 2012, and evaluate a suite of predictor variables for explaining juvenile infection rates and epidemiologic patterns across the landscape

Summary

| INTRODUCTION

Steelhead trout (Oncorhynchus mykiss) are a culturally and ecologically important salmonid fish in the Pacific Northwest. 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, 2016; Kurath et al, 2003), falling into three major IHNV genogroups in North America, U, M, and L. The IHNV-VGS database was used here to (1) analyze steelhead and sympatric salmonids testing effort and infection rates, (2) evaluate spatial and temporal patterns of IHNV prevalence, and (3) evaluate support for three hypothesized transmission pathways that may be responsible for IHNV infection in juvenile hatchery fish

| METHODS

Findings

| DISCUSSION