Investigation and Disease Prevention of Spring Viremia of Carp Virus (SVCV)

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There has been long history of disease outbreaks and economic losses in wild and farmed carp species due to SVCV. Formerly thought to be restricted to Europe and Asia, SVCV was detected for the first time in North America from diseased koi at a North Carolina fish farm in 2002, and there were extensive eradication efforts with 135,000 fish euthanized in addition to the ~15,000 that died from the disease. Since then nine subsequent detections or outbreaks of SVCV have occurred in North America with the latest fish epidemic in 2010 occurring not only in carp species, but also was reported for the first time from centrachid fish species (bluegill and largemouth bass) and in 2015 the first isolation from an imported amphibian species (salamander) occurred. Thus this exotic virus is considered a threat to native fish populations and now also a potential risk to amphibian species in North America.

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Virus-exposed fathead minnow

Typical clinical signs of SVC disease, exopthalmia and abdominal distension with hemorrhaging seen in a virus-exposed fathead minnow. Credit: Eveline Emmenegger, USGS. (Public domain.)

SCV virus is listed by the World Organization of Animal Health (OIE) and is considered an exotic virus in United States. Susceptibility testing of various aquatic animal species to this exotic virus, persistence of the virus in its natural fish host species (koi), and development of molecular-based vaccines against SVCV strains are three focus areas of the SVC virus research project. Because of the exotic and invasive potential of SVC virus all live animal research is performed in our high containment aquatic biosafety level three (BSL-3) laboratory. A DNA vaccine based on the North Carolina strain of the SVC virus and recombinant virus construct based on a European SVCV strain were developed and are also being explored as possible therapeutic means of combatting SVC disease.

Three salmonid species (rainbow trout and steelhead trout, Chinook salmon, and sockeye salmon), one perch species, and two cyprinid species (koi and fathead minnows) have been tested with the North Carolina SVCV isolate. Virus challenged salmonid fish had cumulative percent mortalities ranging from 0 to 100%. Yellow perch and fathead minnows suffered moderate mortalities (29% to 33%) after exposure to SVCV. Young sockeye salmon were the most susceptible salmonid species tested, since they experienced the same high level of mortality as the primary host species (koi). This was the first report of SVCV infection in Percidae species and sockeye salmon. Our findings indicated that fish species type and host age were the primary risk factors for susceptibility to SVCV, but climate change and rearing practices may also contribute to infection risk and thus the potential invasiveness of this exotic virus to native North American aquatic animal populations is a real threat.

We continue to evaluate the susceptibility level of various fish species to the virus and have started susceptibility testing of novel aquatic species. Virulence differences between international and domestic SVC virus strains are also being assessed. The results from this research will provide resource managers with knowledge and tools that will enable them to better manage and protect our wild and cultured aquatic animal communities from SVC virus.

Investigation and Disease Prevention of SVCV Mortality Graph

Mean cumulative percent mortality of salmonids (rainbow trout, sockeye salmon, and Chinook salmon and koi exposed to SVCV or mock treatments. Mortalities in mock control groups were all less than or equal to 5% (symbols for mock treatments not shown in graph legends. Asterisks delineate mortality levels of SVCV-exposed fish species that were significantly different (p ≤ 0.02) than the corresponding mock-treated species. Sockeye salmon (1.5 months old [m]), Chinook salmon (3.0 m), rainbow trout (2.5 m), and koi (5.0 m) each received intraperitoneal injections of 6.5 X 105 plaque forming units of virus. Sockeye salmon fry suffered high mortalities, similar to the levels that the natural host (koi) experienced. Credit: Eveline Emmenegger, USGS. (Public domain.)

Fig. 5. Mean cumulative percent mortality of triplicate groups of koi

Mean cumulative percent mortality of triplicate groups of koi survivors exposed to SVCV (European) or mock infected by intraperitoneal injection for the challenge experiment. The original survivor treatment group in the virulence trial followed by the SVCV (1 × 105 PFU/fish) challenge of each group are listed to the right of graph. No deaths occurred in fish from the mock-challenged groups; mock/mock, IHNV/mock, rIHNV/mock, rIHNV-Gsvcv/mock, and high dose rIHNV-Gsvcv/mock (data not shown). Asterisks delineate mortality levels that were significantly different (p ≤ 0.05) than mortality in the mock/SVCV group. The relative percent survival (RPS) are listed by each group, relative to the mock/SVCV group.

Koi first exposed to a moderate and high dose of a non-virulent recombinant virus (rIHNV-Gsvcv) were then challenged with the wild-type SVC virus to see if they were protected. The original survivor treatment group in the exposure trial followed by the SVCV challenge with each fish being injected with 1 × 105 virus particles of each group are listed to the right of graph. No deaths occurred in fish from the mock-challenged groups (data not shown). Asterisks delineate mortality levels that were significantly different (p ≤ 0.05) than mortality in the mock/SVCV group. The relative percent survival (RPS) are listed by each group, relative to the mock/SVCV group. The high dose of the recombinant virus generated a protective immune response in infected koi, and therefore may be an attractive option for further vaccine development.  Credit: Eveline Emmenegger, USGS. (Public domain.)

Non-USGS Collaborators:

Dr. Sharon Clouthier and Tamara Schroeder, Fisheries and Oceans Canada, Freshwater Institute Science Laboratory, Winnipeg, MB, Canada

Dr. George Sanders, Department of Comparative Medicine, School of Medicine, University of Washington, Seattle, WA, USA

Michel Brémont, Stéphane Biacchesi, and Emilie Mérour, French National Institute for Agricultural Research (INRA), Jouy en Josas, France

Peng Jia and Hong Liu, Key Laboratory of Aquatic Animal Diseases, Shenzhen Exit & Entry Inspection and Quarantine Bureau, Shenzhen, China