Western Fisheries Science News, June 2015 | Issue 3.6

Release Date:

Genetic Analysis Finds that Erythrocytic Necrosis Virus (ENV) of Pacific Herring is an Iridovirus That May be Closely Related to Erythrocytic Viruses of Reptiles

Figure 1: Pacific herring infected with ENV. Figure 2: Nucleated red blood cells from an ENV-infected Pacific herring blood smea

Figure 1: Pacific herring infected with ENV displaying pale gills (top) due to anemia (loss of red blood cells) as compared to the gills from a healthy herring (bottom).  Figure 2: Nucleated red blood cells from an ENV-infected Pacific herring blood smear viewed with a light microscope display inclusions bodies within the cells.  Figure 3:  Icosahedral shaped virus particles, presumable iridoviruses, visualized in the blood of ENV-infected Pacific herring by electron microscopy.  Virus concentrations peaked 7-days after infection. Photos by USGS.

Viral erythrocytic necrosis (VEN) is a disease affecting the red blood cells (erythrocytes) of more than 20 species of marine and anadromous fishes in the North Atlantic and North Pacific oceans. Erythrocytic necrosis virus (ENV) is the agent that causes VEN. Fish infected with ENV may suffer no ill effects or the infection may become fatal due to extreme anemia (Figure 1).  Among populations of Pacific herring (Clupea pallasii) on the west coast of North America the disease causes elevated mortality in intermittent epidemics. Pacific herring are an important forage fish to marine ecosystems.  Thus if herring become sick (anemic) from an ENV infection and if large numbers of fish die (epidemics), the larger animals in the marine ecosystem lose an energy rich food source.  The loss of this energy rich food source severely impacts the larger animals in the marine ecosystem.

Traditionally ENV is presumptively diagnosed by observing inclusion bodies, assemblages inside the infected host cell that are typically areas of viral multiplication, when examining blood smears under a light microscope (Figure 2).  However this optical method is unable to confirm the specific virus type.  Further examinations of virus-infected blood smears with an electron microscope, a high-powered microscope that distinguishes objects at a very fine scale, suggested that it was possibly an iridovirus based on its size and icosahedral shape.  Viruses from the Iridoviridae family have DNA based genetic material, and can infect fish, amphibia, invertebrates, and insects.  The family name is derived from the iridescent sheen seen in insects heavily infected with an iridovirus.

Erythrocytic viruses, including ENV, frequently do not grow in standard cell culture systems.  Thus scientists were unable to obtain a sample with a sufficient number of virus particles to further characterize these viruses.  A 2012 manuscript by researchers of the U.S. Geological Survey, WFRC, looked at the progression of ENV replication in red blood cells by electron microscopy and determined that the peak concentration of virus occurs approximately 7 days after a fish is exposed to the virus (Figure 3). Based on this knowledge, WFRC scientists in a recent study  were able to generate a highly concentrated virus sample by performing six virus passages in herring at 7-10 day intervals.  The concentrated ENV blood sample had enough virus to be used in next generation sequencing methods that produced for the first time authentic ENV genetic sequence fragments.  These ENV sequence pieces were then compared to all virus sequences cataloged in the GenBank database (an open access online depository of annotated nucleotide sequences).  The ENV sequence fragments were found to be a close match to many iridovirus genes.  This was the first genetic confirmation that ENV belonged in the Iridoviridae family.  Interestingly, one of the ENV sequence fragments was most closely related to other viruses that also infect the red blood cells of lizards, snakes, and bearded dragons.  Scientists used four unique ENV gene sequence fragments to design rapid genetic tests (polymerase chain reactions [PCR]) that could confirm the presence of the virus in infected Pacific herring.

Summary highlights from the 2014 research study included:

  • First genetic evidence that erythrocytic necrosis virus (ENV) is an iridovirus.
  • Genetic analysis suggests that ENV is closely related to erythrocytic viruses of lizards, snakes, and bearded dragons.
  • Erythrocytic viruses likely belong to a new genus within the family Iridoviridae.
  • Conventional PCR assays were developed that can screen wild and cultured Pacific herring populations for the virus.

Newsletter Author – Debra Becker



USGS Participates in 7th National New Zealand Mudsnail Conference:  On June 16-17, WFRC scientist Jill Hardiman participated in the 7th National New Zealand Mudsnail (NZMS) conference, held at the WFRC (Seattle, WA).  The meeting brought together scientists and managers focused on NZMS and provided an overview of current policies, research, case studies, monitoring and tools, research gaps and priorities, and discussion of coordinated management.  Sessions gathered feedback from the NZMS community to determine the need to update the NZMS management plan and produce a guidance document which reflects the region’s management needs and priorities.

USGS at Western Fish Disease Workshop:  On June 2-4, USGS scientists Gael Kurath, Evi Emmenegger, Bill Batts, Ashley Mackenzie, and Rachel Breyta from the WFRC participated in the 56th Annual Western Fish Disease Workshop in Steamboat Springs, CO.  Scientists presented talks focused on current research on diseases of western fish species. The workshop is held annually and is part of the Fish Health Section of the American Fisheries Society.

USGS Scientist Participates on Shasta Juvenile Chinook Salmon Downstream Collector Team: On June 2-3, WFRC scientist John Beeman participated in a workshop to address juvenile Chinook salmon downstream collection at Shasta (CA).  The goal of the workshop was to identify conceptual level designs for juvenile Chinook salmon collectors that will be further evaluated for feasibility of use during a Pilot Implementation Program. The focus of the workshop was the McCloud River and the associated reservoir arm, but could also include future consideration of the Upper Sacramento River. Beeman is participating as part of the Shasta downstream passage team.

USGS Scientists Participate in Educational Lewis and Clark Journey Reenactment:  On May 23-27, USGS scientists participated in a reenactment of the voyage of Lewis and Clark from below Bonneville Dam to Astoria, OR. About 50 Stevenson High School students made the journey in kayaks over a 4-day period, accompanied by a mix of private volunteers as well as professionals from local, state, and federal agencies. USGS participated throughout the trip by providing information about the Columbia River, its ecosystem, and how it has changed since the time of Lewis and Clark’s journey. Scientists also discussed career opportunities with federal agencies and highlighted some of the work that USGS does to help inform the management of resources the river provides (hydropower, fisheries, irrigation, transportation, recreation, etc.).


Jezorek, I.G. and P.J. Connolly. 2015. Biotic and abiotic influences on abundance and distribution of nonnative Chinook salmon and native ESA-listed steelhead in the Wind River, Washington. Northwest Science, 89(1): 58-74.

Hayes, M.C., S.P. Rubin SP, R.R. Reisenbichler, and L.A. Wetzel. 2014. Migratory behavior of Chinook salmon microjacks reared in artificial and natural environments. Journal of Fish and Wildlife Management 6(1):176–186; e1944-687X.