Koi Herpesvirus Update 2004
Thomas B. Waltzek M.S. and Ronald P. Hedrick Ph.D.

The following article appears in the July-August 2004 issue of California Veterinarian.

In 1997, an infectious disease resulting in massive mortalities of koi (Cyprinus carpio koi), a colorful strain of the common carp (Cyprinus carpio carpio), was reported in Germany (1). In the following years losses continued in Germany and new outbreaks in koi and common carp occurred throughout Europe (Great Britain, France, Belgium, Netherlands, Luxemburg, Denmark, Switzerland, Austria, Poland), Asia (Indonesia, China, Taiwan, Japan), Israel, U.S.A., and South Africa (1-6). The agent responsible for these epidemics, koi herpesvirus (KHV), was first isolated by our laboratory in 1998 from koi experiencing heavy mortality in both the U.S.A. and Israel (3) and has been repeatedly isolated since (6-8). The rapid global spread of KHV is believed to be linked to the largely unregulated international koi trade.

A presumptive diagnosis of the disease includes observation of high mortality (often 80-100%) among koi or common carp but not other fish species at water temperatures ranging from 18 – 26 degrees C despite treatments to control bacterial infections and or ectoparasite infestations. External pathological signs exhibited by moribund fish include excessive mucus production on the body and gills, swollen and necrotic gill filaments, discolored patches on the gills and skin, and enophthalmos (2,3). Internally, the kidney and spleen may be enlarged. A definitive diagnosis requires the isolation of the virus from infected fish tissues using cell lines of koi and carp origin, followed by polymerase chain reaction (PCR) testing of the isolates. As it is often difficult to isolate KHV via cell culture methods, particularly from decomposed or frozen samples, detection then relies solely on the PCR results obtained from fish tissues. Published PCR assays include single round (9,10) and quantitative tests (11).

Based on the observation that KHV disease is restricted to a temperature range between 18-26 degrees C, a protocol has been developed to produce "naturally resistant" koi (8). Healthy fish are cohabitated with fish dying from the disease for 5 days at temperatures favorable for viral replication and then prior to disease onset are transferred to a non-permissive temperature (30 degrees C) for 30 or more days. These water temperature treated fish develop antibodies against the virus and show improved survival upon re-exposure to the virus. The potential to develop fish that continue to harbor the virus by this method has been a major concern. An additional control method has been pursued by researchers in Israel using an attenuated live virus vaccine (8). Experimental trials demonstrating the irreversibility of the attenuation however, are needed prior to any formal applications of this vaccine. Now that the full sequence of the KHV genome is known, several additional vaccine strategies should be forthcoming.

With the onset of summer and elevated water temperatures our laboratory has recently confirmed the first KHV outbreaks of the year in koi from Northern California. Yearly epidemics in the state (10), serve as poignant reminders of the constant global threat this viral agent poses to both the ornamental koi and common carp industries. Israeli aquaculturists have lost an estimated $3 million annually since 1998 due to KHV epidemics (6). Damages to the Indonesian fishery have reached $5.5 million since the disease hit in 2002 (4). KHV outbreaks in 2003 from two lakes in Japan killed nearly 1,124 metric tons of common carp equaling a net loss of approximately $2.55 million (12). British agencies are concerned that recent fish kills could be the first record of KHV in their wild carp populations potentially threatening Britain’s 3.5 billion pound-a-year angling industry (13). The koi industry has experienced enforced quarantines of KHV positive facilities, restrictions on international koi commerce, cancellation of important koi shows, substantial lawsuits between hobbyists and their suppliers, and untold losses of fish by koi breeders, retailers, and hobbyists around the world.

Literature Cited

1. Bretzinger A, Fischer-Scherl T, Oumouma M, Hoffmann R, Truyen U (1999) Mass mortalities in koi, Cyprinus carpio, associated with gill and skin disease. Bull Eur Assoc Fish Pathol 19:182-185

2. Walster C. (1999) Clinical observations of severe mortalities in koi carp, Cyprinus carpio with gill disease. Fish Vet J 3:54-58

3. Hedrick RP, Gilad O, Yun S, Spangenberg JV, Marty GD, Nordhausen RW, Kebus MJ, Bercovier H, and Eldar A (2000) A Herpesvirus associate with mass mortality of juvenile and adult koi, a strain of a common carp. J Aquat An Health 12:44-57

4. Focus on Koi Herpes Virus (KHV). Fish Health Section/Asian Fisheries Society Electronic Newsletter (2004) Jan./Feb. 3:1-3, accessed at: http://afsfhs.seafdec.org.ph

5. Engelsma MY, Haenan OLM (2004) Global distribution of KHV with particular reference to Europe. International Workshop on Koi Herpesvirus. Accessed at: http://www.frmltd.com/Workshop_KHV.htm

6. Perelberg A, Smirnov, M, Hutoran, M, Diamant, Bejerano Y, Kotler M (2003) Epidemiological description of a new viral disease afflicting cultured Cyprinus carpio in Israel. Is J Aqua Bamidgeh 55:5-12

7. Neukirch M, Kunz U (2001) Isolation and preliminary characterization of several viruses from koi (Cyprinus carpio) suffering gill necrosis and mortality. Bull Eur Assoc Fish Pathol 21:125-1358

8. Ronen A, Perelberg A, Abramowitz J, Hutoran M, Tinman S, Bejerano I, Steinitz M, Kotler M (2003) Efficient vaccine against the virus causing a lethal disease in cultured Cyprinus carpio. Vaccine 21:4677-46849

9. Gilad O, Yun S, Andree KB, Adkison MA, Zlotkin A, Bercovier H, Eldar A, Hedrick RP (2002) Initial characteristics of koi herpesvirus and development of a polymerase chain reaction assay to detect the virus in koi, Cyprinus carpio koi. Dis Aquat Org 48:101-108

10. Gray WL, Mullis L, LaPatra SE, Groff JM, Goodwin A (2002) Detection of koi herpesvirus DNA in tissues of infected fish. J Fish Dis 25:171-178

11. Gilad O, Yun S, Zagmutt F, Leutenegger CM, Bercovier H, Hedrick RP (2004) Concentrations of a herpes-like virus (KHV) in tissues of experimentally-infected Cyprinus carpio koi as assessed by real-time TaqMan PCR. Dis Aquat Org. In press

12. Hall, K (2003) Herpes virus outbreaks that’s killing carp spreads. Associated press Nov. 7, accessed at: http://www.enn.com/2003-11-07/s_10215.asp

13. Meikle, J (2004). Killer disease threatens angling industry. Guardian Unlimited Jan. 4, accessed at: http://www.guardian.co.uk/uk_news/story/0,3604,1119897,00.html

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For more information, please contact:
Professor Ronald P. Hedrick, Department of Medicine and Epidemiology
School of Veterinary Medicine
University of California, Davis
Davis, CA 95616