Deadly Fish Virus Spreading in Pacific Northwest

Author Rachel Breyta inspects a one-year-old steelhead trout for signs of IHNV exposure. Samples of gill and fin tissue are taken for later analysis. Together with pathologists from the Northwest Indian Fisheries Commission and the U.S. Fish and Wildlife Service, the sampling was conducted in partnership with the Quinault Nation near Tahola, Washington.
(Craig Olsen, Northwest Indian Fisheries Commission)

(CN) – A highly contagious virus that can be lethal to juvenile fish represents a major threat to local conservation efforts and the fishing industry in the Pacific Northwest, according to new research that analyzes the pathogen’s spread.

Known as infectious hematopoietic necrosis virus, or IHNV, the pathogen affects Pacific salmon, trout and other species and can spread among juvenile hatchery-raised fish.

The new study, published Wednesday in the journal Ecology and Evolution, explores how the virus is transmitted and its impact on fish populations in the Pacific Northwest. The research focuses on steelhead trout and Chinook salmon – the most abundant IHNV-susceptible species in the Columbia River Basin and nearby coastal rivers.

“IHNV is the most significant viral pathogen of Pacific salmon and trout in North America and has likely been present in Pacific salmon for thousands of years,” said co-author Shannon LaDeau, a disease ecologist at the Cary Institute in New York. “However, ecosystem alterations and human activities have impacted its spread, especially in cultured fish populations.”

Federal, state and tribal hatchery programs rear trout and salmon to help increase wild stock and minimize habitat loss stemming from activities like hydroelectric power generation. IHNV makes such efforts more costly and challenging, according to co-author Gael Kurath, a senior research microbiologist with the U.S. Geological Service’s Western Fisheries Research Center.

“IHNV disease costs Pacific Northwest conservation efforts and the global aquaculture industry millions of dollars annually,” she said. “These expenses include direct losses due to fish mortality or culling of infected fish and eggs, costs of routine surveillance and testing, and program losses due to restrictions of movement of infected fish.”

Analyzing viral transmission in hatchery-reared fish presents a variety of challenges.

A single hatchery pond of juvenile steelhead trout undergoing an IHNV epidemic. (Dworshak National Fish Hatchery, Orofino, Idaho)

“While hatcheries can be disinfected and controlled, juveniles migrate to the Pacific Ocean as part of their life cycle. When they return to hatcheries to spawn as adults, complicating variables – such as swimming through IHNV-infested waters – come into play,” said Oregon State University’s Rachel Breyta, who led the study as a postdoctoral fellow at the Cary Institute.

IHNV infections can damage blood-producing tissues in the spleen, liver and kidneys of trout and salmon. The pathogen can be fatal to juveniles. Prevention is the primary management strategy for the virus, since no effective post-infection treatment exists. While only juvenile Pacific salmon display symptoms of infection, asymptomatic adults can carry and transmit the virus.

The pathogen evolves quickly, enabling it to adapt to a fish’s immune system, as well as new environments and host species. Infected fish release IHNV into the water, where it can survive long enough to infect new hosts. Infectious reproductive fluids can also lead to parent-to-offspring transmission of the virus.

To avoid IHNV outbreaks, hatcheries often rely on water security – using water sources that pose a low risk of being contaminated with the virus – and biosecurity, which includes disinfecting eggs with iodine to kill the pathogen. These efforts fail to eliminate IHNV occurrence, however, as many older conservation hatcheries do not have access to secure water and operate instead with creek or river water containing potential IHNV hosts.

The team used records from the IHNV-VGS database, which stores data on viral surveillance and the researchers’ genotyping samples, to model disease incidence throughout the Columbia River Basin. There are more than 100 conservation fish hatcheries in the region, operated by seven different federal, state and tribal agencies. The team reviewed 13 years of IHNV testing data from 2000 to 2012, which included 6,766 unique records from 1,142 locations.

Though samples of both hatchery-reared and wild fish were included in the study, there is not enough data on wild populations to determine how such fish are infected.

About 65 percent of IHNV-positive samples were examined using viral genetic analysis, which identified 90 distinct genotypes. These data were used to map three possible viral transmission pathways – from juveniles at one hatchery to juveniles at another, from juveniles to juveniles within the same hatchery site and from infected adults returning to their “home” hatchery to juveniles at the same site.

“By identifying the strain of the virus infecting a fish and cross-referencing this information with known locations of different IHNV strains across the landscape, we can map where a fish became infected, the potential transmission pathway, and other locations at risk of IHNV outbreaks,” Breyta said.

The team found that all three pathways can spread the virus efficiently. The data suggest that returning adults were the most common source of viral introduction for juveniles at hatcheries, while within-hatchery juvenile-to-juvenile transmission – which accounts for the highest disease incidence – is likely responsible for high recurrence rates at certain hatcheries in the study region.

“Infected juvenile fish and infected migrating adult fish are likely to play important roles in spreading IHNV,” LaDeau said. “We expect that our efforts to better understand viral transmission will lead to more targeted management strategies. Disrupting transmission is key to stopping the expansion of this virus, which is a conservation threat and an economic liability.”


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