recent study is the first to explore how infectious hematopoietic necrosis
virus (IHNV) spreads among juvenile hatchery-raised fish in the Pacific Northwest,
where high rates of infection and mortality can occur.
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.
However, ecosystem alterations and human activities have impacted its spread,
especially in cultured fish populations,” said Cary Institute disease ecologist
and study co-author Shannon LaDeau.
trout and chinook salmon, the most abundant IHNV-susceptible species in the
Columbia River Basin and adjacent coastal rivers, were the focus of the study http://onlinelibrary.wiley.com/doi/10.1002/ece3.3276/abstract.
are extensive state, federal, and tribal hatchery programs that rear these
species to help rebuild wild stock and mitigate loss of habitat due to
activities such as hydroelectric power generation.
Kurath, senior research microbiologist with the USGS Western Fisheries Research
Center and coauthor on the paper said, "IHNV disease costs Pacific
Northwest conservation efforts and the global aquaculture industry millions of
dollars annually. 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.”
viral transmission in hatchery-reared fish presents unique challenges. Author
Rachel Breyta, now at Oregon State University, led the study while a
Postdoctoral Fellow at the Cary Institute.
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," Breyta said.
trout and salmon are infected with IHNV, blood-producing tissues in the
kidneys, liver, and spleen deteriorate. In juveniles, this can lead to death.
Once infected, there is no effective treatment; prevention is the main
management strategy. Only juvenile Pacific salmon display symptoms of
infection; asymptomatic adults can maintain and transmit the virus.
is quick to evolve, allowing it to adapt to the host fish's immune system as
well as to new environments and host species. Infected fish shed the virus into
the water, where it can survive long enough to infect new fish hosts. Juvenile
fish can also contract the virus through transmission from parent to offspring
via infectious reproductive fluids.
prevent IHNV outbreaks, many hatcheries rely on water security -- using water
sources at low risk of being IHNV-contaminated, such as a spring or well water,
and biosecurity -- disinfecting eggs with iodine to kill the virus. Despite
these measures, IHNV persists, partly because many older conservation
hatcheries do not have access to secure water and operate with river or creek
water that contains fish that are potential IHNV hosts.
disease incidence throughout the Columbia River Basin was modeled using records
from the IHNV-VGS database, a repository for viral surveillance and genotyping
samples developed by the research team. Seven different state, federal, and
tribal agencies operate more than a hundred conservation fish hatcheries in the
region. The team analyzed thirteen years of IHNV testing data (2000-2012)
representing 6,766 unique records from 1,142 locations.
of both wild and hatchery-raised fish were included in this study; however,
there is not enough data on wild populations to determine whether infection of
wild fish is a result of isolated hatchery spillover events, or if wild
populations maintain IHNV throughout the study region.
65 percent of IHNV-positive samples were analyzed by viral genetic analysis,
with 90 distinct genotypes identified. This information was used to trace three
potential viral transmission pathways: from juveniles to juveniles in the same
hatchery site, from juveniles at one hatchery to juveniles at another hatchery,
and from infected adults returning to their 'home' hatchery site to juveniles
at that site.
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.
landscape-level analysis found that all three pathways were efficient at
spreading IHNV. Data suggest that returning adults were likely the most
frequent source of viral introduction to juveniles at hatcheries, followed by
within-hatchery juvenile-to-juvenile transmission, which accounts for the
highest disease incidence and is likely responsible for maintaining high
recurrence rates at specific hatcheries in the study region.
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."
research was supported by the U.S. Geological Survey Western Fisheries Research
Center and the Washington Cooperative Fish and Wildlife Research Unit at the
University of Washington. Funding was provided, in part, by the USDA as part of
the joint USDA-NSF-NIH Ecology and Evolution of Infectious Disease program.
Cary Institute of Ecosystem Studies is one of the world's leading independent
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