Spring Viremia of Carp Virus in the United States: Findings, Potential Impacts, and Implications
Spring Viremia of Carp Virus (SVCV) is a rhabdovirus that under natural conditions infects five species of carp (common--including koi, bighead, grass, silver and crucian),4 goldfish,9 and sheatfish.6 Experimental infections have been established in pumpkinseed5 and European species such as roach,8 northern pike fry,2 and guppy.1
Clinical signs of SVCV and death occur most often between 11 and 17° C, and decrease above 17° C and cease at 22° C. The virus is shed in feces and possibly urine, gill mucous, and reproductive products. SVCV has been shown to remain viable in water for 14 days at 10° C, in mud for 42 days at 4° C, and under dry conditions for 21 days at 4-21° C. It is inactivated by lipid solvents, heating to 60° C for 15 minutes, exposure to pH below 4 and above 10, 3 percent formalin, 2 percent NaOH, or 500 ppm chlorine. It can be inactivated by a 10-minute exposure to UV light (254 nm).7
Until 2002, the virus was primarily associated with carp production in Europe, the Middle East and Asia. In 2002, SVCV (Asian genotype) was diagnosed as the cause of disease outbreaks in koi-carp on a North Carolina aquaculture facility and in feral common carp in Cedar Lake, Wisconsin. These marked the first diagnoses of SVCV in the United States.
Cedar Lake flows into the Apple River, which flows into the St. Croix River and then to the Mississippi River. Dead carp were observed in Cedar Lake in fall 2001, but fish were collected for diagnostics at that time. Dead and dying carp were also observed in pools of the Mississippi River in June 2002; however, the causes of these fish kills were also not determined. Follow-up monitoring done in September and October 2002 using viral isolation and a competitive serologic immunoassay,3 indicated a large percentage of carp in the St. Croix River and Pool 10 of the Mississippi River had detectable antibodies to SVCV, 34/40 and 128/164, respectively. No virus was isolated from these animals, indicating there may have been exposure to SVCV at some time.
No mortality has been documented in waters downstream of the North Carolina facilities; however, bluehead chub and common carp did test positive for antibodies to the virus using the competitive immunoassay. It should be noted that this serological test has not been validated for species other than carp and cross reactivity to other viruses has not been evaluated.
Carp kills have been investigated in Wisconsin since 1989. In-situ hybridization and RT-PCR were used in fall 2002 to retrospectively screen tissue blocks from historic carp kills in Wisconsin. Tissues from a carp kill in 1989 were considered positive for SVCV (European genotype) by both methods; suggesting that SVCV may have been present as early as 1989 in Wisconsin. This occurrence was likely a separate introduction of the virus based on genotypic results. The 1989 case was designated as the European genotype, and both 2002 cases (koi-carp from North Carolina and common carp from Cedar Lake, Wisconsin) were designated as the Asian genotype. While both new isolates are of the Asian genotype, DNA sequencing shows only 98 percent homology--potentially indicating they are the result of two separate introductions.
There are currently 115 species of threatened and endangered (T&E) fish in the United States, representing 14 families in 9 orders. SVCV has been shown to infect five orders of fish either naturally or experimentally. Each of these orders contains T&E species. These orders include the Cypriniformes, within which there are two families, Cyprinidae and Catastomidae, which contain 49 T&E species; the Siluriformes, which contains 6 T&E species, all within the family Ictaluridae; the Salmoniformes, which contains 14 T&E species with the families Salmonidae and Osmeridae; the Perciformes, which contains two families, Percidae and Gobidae, in which there are 21 T&E species; and the Atheriniformes, which contain 16 T&E species in the families Poecilidae, Cyprinodontidae, and Atherinidae. Therefore 106 of 115, or 93 percent, of T&E species are classified in families known to be susceptible to SVCV. Furthermore 41 species, or 36 percent of all T&E fish, are Cyprinids, the family known to be highly susceptible to SVCV. In addition to these potential direct impacts, there may be indirect impacts on other endangered species. There are currently 70 species of T&E freshwater mussels present in the U.S. Many of these animals require the presence of specific fish, which act as hosts for juvenile life stages, to complete their complex life cycles. Many of these fish hosts are in families that are susceptible to SVCV. Negative impacts to these host fish populations could prove even more damaging to these mussel populations. Also, this virus has been experimentally shown to infect fish in the family Centrarchidae. While no centrarchids are currently listed as T&E species, they are a major sport fish family, which includes large and small mouth bass, crappie, and all the sunfish. Negative impacts to these populations could significantly impact sport fisheries and result in reduced tax revenues, which would adversely impact state and federal agencies ability to implement natural resource programs.
There could also be serious economic impacts if this virus caused problems for the baitfish, catfish, and ornamental aquaculture industries. These impacts may not only be direct loss of fish due to infection and losses associated with disease management, but may result from nations imposing trade barriers that prevent the movement of U.S. products into their countries.
The finding of this virus in the U.S. has demonstrated the need for a national aquatic animal health plan. The U.S. Fish and Wildlife Service of the Department of the Interior, in conjunction with the National Marine Fisheries Service of the Department of Commerce, and the U.S. Department of Agriculture's Animal and Plant Health Inspection Service have formed the Joint Subcommittee on Aquaculture's Aquatic Animal Health Task Force to address this issue. This group is hoping to include all interested parties and to work toward the formation of a national strategy and plan that will help control the spread and importation of aquatic animal disease, thereby improving our abilities to prevent and respond to disease introductions and to minimize the impacts of any introduction on our natural and commercial resources.
The authors thank the individuals that assisted with sampling and processing the animals for these procedures. Unfortunately, their vast number makes it impossible to list them individually but their tireless efforts and contributions to this entire endeavor are very appreciated.
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