Viral erythrocytic necrosis (VEN) is a disease of marine and anadromous fishes, which is poorly understood, largely because its causative iridovirus, erythrocytic necrosis virus (ENV), is intractable to cell culture. Natural VEN epizootics and observations studies in wild populations suggest that temperature may be an important disease cofactor. Here, a controlled laboratory exposure study provides evidence for a direct relationship between temperature and the progression of viral erythrocytic necrosis (VEN) in Pacific herring. Waterborne exposure of Pacific herring to kidney homogenates containing ENV resulted in the establishment of infections, characterized by high infection prevalence (89%; 40/45) and mean viral loads (5.5 log10- gene copies / ug DNA) in kidney tissues at 44 d post exposure. Viral loads were higher in herring from the ambient (9.0 °C) and warm (13.5 °C) treatments (6.1 - 6.2 log10- gene copies / total DNA) than from the cool (6.9 °C) treatment (4.3 log10- gene copies / total DNA). Similarly, the peak proportion of diseased fish was directly related to temperature (P 0.001), with cytoplasmic inclusion bodies detected in 21% of herring from the cool, 52% from the ambient, and 60% from the warm treatments. The mean disease load in each fish (enumerated as the percent of erythrocytes with cytoplasmic inclusions), increased with temperature from 15% in the cool, 36% in the ambient, and 32% in the warm treatments at 44 days post exposure. Transcriptional analysis indicated that the number of differentially expressed genes among ENV-exposed herring increased with temperature, time post exposure, and viral load. Correlation network analysis of transcriptomic data showed robust activation of interferon and viral immune responses in hepatic tissue of infected individuals independent of other experimental variables.

Throughout a 20-year biosurveillance period, viral hemorrhagic septicemia virus was isolated in low titers from only 6 / 7,355 opportunistically sampled adult Pacific herring, reflecting the typical endemic phase of the disease when the virus persists covertly. However, more focused surveillance efforts identified the presence of disease hot spots occurring among juvenile life history stages from certain nearshore habitats. These outbreaks sometimes recurred annually in the same temporal and spatial patterns and were characterized by infection prevalence as high as 96%. Longitudinal sampling indicated that some epizootics were relatively transient, represented by positive samples on a single sampling date, and others were more protracted, with positive samples occurring throughout the first 10 weeks of the juvenile life history phase. Sampling and biological data associated with these surveillances are presented in this dataset.

Infectious hematopoietic necrosis virus (IHNV) represents one of the most critical challenges for salmonids in the Pacific Northwest. There are three genogroups of IHNV, designated U, M, and L; the U is further delineated into two subgroups, UC and UP, and the M is further delineated into four subgroups (MA – MD). The UP, UC and MD subgroups co-occur in the Columbia River Basin where the host species sockeye salmon, Chinook salmon, and steelhead trout spawn and rear. Field prevalence data shows that UC viruses exhibit a generalist strategy in Chinook, and steelhead, while two other virus lineages, MD and UP, are more consistent with being specialists in steelhead or sockeye salmon, respectively. The L is found in Northern California and is considered a specialist of Chinook salmon. This study sought to understand early entry and replication of the specialist and generalist IHNV strains in three salmonid hosts. Chinook, steelhead and sockeye were exposed by immersion to their specialist viruses (L, MD and UP, respectively) and to the generalist UC virus. As controls, these hosts were also exposed to buffer (mock control) and IHNV variants that were not specialist for their species (non-specialists). Mortality was monitored throughout the experiments. Fish were sampled at early timepoints post-infection (2, 5, and 8 days post-exposure). Kidney and fin tissues were taken to represent external and internal tissues, respectively. Viral load was assessed by reverse transcriptase quantitative PCR (RT-qPCR) targeting the nucleocapsid (N) gene of IHNV. Additional fish from each experimental group were sampled for histopathology but only a subset were processed and analyzed.

Infectious hematopoietic necrosis virus (IHNV) represents one of the most critical challenges for salmonids in the Pacific Northwest. There are three genogroups of IHNV, designated U, M, and L; the U is further delineated into two subgroups, UC and UP, and the M is further delineated into four subgroups (MA – MD). The UP, UC and MD subgroups co-occur in the Columbia River Basin where the host species sockeye salmon, Chinook salmon, and steelhead trout spawn and rear. Field prevalence data shows that UC viruses exhibit a generalist strategy in Chinook, and steelhead, while two other virus lineages, MD and UP, are more consistent with being specialists in steelhead or sockeye salmon, respectively. The L is found in Northern California and is considered a specialist of Chinook salmon. This study sought to understand early entry and replication of the specialist and generalist IHNV strains in three salmonid hosts. Chinook, steelhead and sockeye were exposed by immersion to their specialist viruses (L, MD and UP, respectively) and to the generalist UC virus. As controls, these hosts were also exposed to buffer (mock control) and IHNV variants that were not specialist for their species (non-specialists). Mortality was monitored throughout the experiments. Fish were sampled at early timepoints post-infection (2, 5, and 8 days post-exposure). Kidney and fin tissues were taken to represent external and internal tissues, respectively. Viral load was assessed by reverse transcriptase quantitative PCR (RT-qPCR) targeting the nucleocapsid (N) gene of IHNV. Additional fish from each experimental group were sampled for histopathology but only a subset were processed and analyzed.

This investigation sought to characterize the shedding of infectious hematopoietic necrosis virus (IHNV) in two populations of Columbia River Basin (CRB) Chinook salmon (Oncorhynchus tshawytscha). Juvenile spring- and fall-run Chinook salmon were exposed by immersion to each of three IHN virus strains from the UC, MD, and L subgroups, and then monitored for viral shedding from individual fish for 30 days. Detectable quantities of UC, MD and L IHN virus were shed by a subset of fish from each host population (1–9 out of 10 fish total in each treatment group). Viral shedding kinetics were consistent, with a rapid onset of shedding, peak shedding by 2–3 days, and then a rapid decline to below detectable levels by 7 days’ post-exposure to IHNV. Intraspecies variation was observed as spring Chinook salmon shed more UC virus than fall fish: spring Chinook salmon shed UC virus in greater numbers of fish, with 22-fold higher mean peak shedding magnitude, 33-fold higher mean total virus shed per fish, and 900-fold higher total virus shed per treatment group. The L and MD viruses had comparable shedding at intermediate levels in each host population. All viral shedding occurred well before host mortality began, and shedding magnitude did not correlate with virulence differences. Overall, the greater shedding of UC virus from spring Chinook salmon, combined with low virulence, indicates a uniquely high transmission potential that may explain the predominance of UC viruses in CRB Chinook salmon. This also suggests that spring-run fish may contribute more to the ecology of IHNV in the CRB than fall-run Chinook salmon.

Environmental variation has important effects on host-pathogen interactions, affecting large-scale ecological processes such as the severity and frequency of epidemics. However, less is known about how the environment modulates viral fitness traits and within host infection processes. Viral genetic variation, fish host immune response and environmental parameters such as temperature have been reported to strongly influence the replication and clearance of infectious hematopoietic necrosis virus (IHNV), a significant pathogen of salmon and trout. Here, we quantified the effect of water temperature on probability of infection across time following exposure of steelhead (Oncorhynchus mykiss) to a field isolate of IHNV. Warm water temperatures accelerated IHNV replication compared to colder water temperatures in cell lines, which have a more limited host immune response. In vivo challenge experiments also demonstrated a higher replication rate of IHNV at warmer water temperatures, but IHNV persisted for a shorter amount of time at these warmer temperatures and led to lower overall mortality compared to colder temperatures. Furthermore, fish were found to have higher prevalence of neutralizing antibodies at warmer water temperatures compared to colder temperatures. These results support the hypothesis that IHNV clearance or persistence is modulated by temperature, and this difference was influenced by temperature effects on the host immune responses. At later time points, the viral RNA that persisted was most commonly localized in the kidney and spleen pooled tissue; these tissues are composed of hematopoietic cells that are favored targets of the virus. By partitioning the effect of environmental variation into independent and common effects of host and pathogen responses, we can better understand the environmental regulation of host-pathogen interactions within hosts. Our results therefore provide insights into how different host-pathogen systems could react to environmental change.

Theory of the evolution of pathogen specialization suggests that a specialist pathogen gains high fitness in one host, but this comes with fitness loss in other hosts. By contrast, a generalist pathogen does not achieve high fitness in any host, but gains ecological fitness by exploiting different hosts, and has higher fitness than specialists in non-specialized hosts. As a result, specialist pathogens are predicted to have greater variation in fitness across hosts, and generalists would have lower fitness variation across hosts. We test these hypotheses by measuring pathogen replicative fitness as within-host viral loads from the onset of infection to the beginning of virus clearance, using the rhabdovirus infectious hematopoietic necrosis virus (IHNV) in salmonid fish. Based on field prevalence and virulence studies, IHNV subgroups UP, MD, and L are specialists, causing infection and mortality in sockeye salmon, steelhead, and Chinook salmon juveniles, respectively. The UC subgroup evolved naturally from a UP ancestor and is a generalist infecting all three host species but without causing severe disease. We show that specialist subgroups had highest peak and mean viral loads in the hosts in which they are specialized, and they had low viral loads in non-specialized hosts, resulting in large variation in viral load across hosts. Viral kinetics show that the mechanisms of specialization involve both the ability to maximize early virus replication and to avoid clearance at later times, with different mechanisms of specialization evident in different host-virus combinations. Additional nuances in the data included different fitness levels for non-specialist interactions, reflecting different trade-offs for specialist viruses in other hosts. The generalist UC subgroup reached intermediate viral loads in all hosts and showed the smallest variation in fitness across hosts. Evolution of the UC generalist from an ancestral UP sockeye specialist was associated with fitness increases in steelhead and Chinook salmon, but only slight decrease in fitness in sockeye salmon, consistent with low- or no-cost generalism. Our results support major elements of specialist-generalist theory, providing evidence of a specialist-generalist continuum in a vertebrate pathogen. These results also quantify within-host replicative fitness tradeoffs resulting from the natural evolution of specialist and generalist virus lineages in multi-host ecosystems.

Dataset for the publication 'Molecular testing of adult Pacific salmon and trout (Oncorhynchus spp.) for several RNA viruses demonstrates widespread distribution of piscine orthoreovirus (PRV) in Alaska and Washington'. This research was initiated in conjunction with a systematic, multi-agency surveillance effort in the United States (U.S.) in response to reported findings of infectious salmon anemia virus (ISAV) RNA in British Columbia, Canada. In the systematic surveillance study reported in a companion paper, tissues from various salmonids taken from Washington and Alaska were surveyed for ISAV RNA using the U.S. approved diagnostic method and samples were released for use in this present study only after testing negative. Here we tested a subset of these samples for ISAV RNA with three additional published molecular assays, as well as for RNA from salmonid alphavirus (SAV), piscine myocarditis virus (PMCV) and piscine orthoreovirus (PRV). All samples (n=2252; 121 stock cohorts) tested negative for RNA from ISAV, PMCV, and SAV. In contrast, there were 25 stock cohorts from Washington and Alaska that had one or more individuals test positive for PRV RNA. Findings of PRV RNA were most common in coho (Oncorhynchus kisutch Walbaum) and Chinook (O. tshawytscha Walbaum) salmon. The disease associated with PRV in Atlantic salmon, heart skeletal muscle inflammation (HSMI), has not been reported in Pacific salmon.

Piscine orthoreovirus genotype one (PRV-1) is the causative agent of heart and skeletal muscle inflammation (HSMI) in farmed Atlantic salmon (Salmo salar L.). The virus has also been found in Pacific salmonids in western North America, raising concerns about the risk to native salmon and trout. Here, we report the results of laboratory challenges using juvenile Chinook salmon, coho salmon, and rainbow trout injected with tissue homogenates from Atlantic salmon testing positive for PRV-1 or with control material. Fish were sampled at intervals to assess viral RNA transcript levels, hematocrit, erythrocytic inclusions, and histopathology. While PRV-1 replicated to high loads in all species, there was negligible mortality in any group. We observed a few erythrocytic inclusion bodies in fish from PRV-1 infected groups. At a few time points, hematocrits were significantly lower in the PRV-1 infected groups relative to controls but in no case was anemia noted. The most common histopathological finding was mild, focal myocarditis in both the non-infected controls and PRV-1 infected fish. All cardiac lesions were judged mild and none were consistent with those of HSMI. Together, these results suggest all three species are relatively susceptible to PRV-1 infection, but in no case did infection cause notable disease in these experiments.

This data set contains three spreadsheets with results of Influenza A Virus (IAV) screening in blood from Pacific, Red-throated, and Yellow-billed Loons at three locations in Alaska. Loons were captured along the Beaufort Sea Coast, Chukchi Sea Coast, and Yukon-Kuskokwim Delta from 2008-2017. Three different tests were used and the results are presented in separate spreadsheets. All serum samples were screened for IAV antibodies using a blocking enzyme-linked immunosorbent assay (bELISA). A subsample were screened using agar gel immunodiffusion (AGID), and hemagglutination inhibition assays (HI).