Koi Herpes Virus Information

Kathleen H. Hartman, Roy P.E. Yanong, Deborah B. Pouder, B. Denise Petty, Ruth Francis-Floyd and Allen C. Riggs

Introduction to KHV

Koi herpesvirus (KHV) is a highly contagious viral disease that may cause significant morbidity and mortality in common carp (Cyprinus carpio) (Hedrick et al. 2000; OATA 2001). Common carp is raised as a foodfish in many countries and has also been selectively bred for the ornamental fish industry where it is known as koi. The first recognized case of KHV occurred in England in 1996. Since then other cases have been confirmed in almost all countries that culture koi and/or common carp with the exception of Australia (Hedrick et al. 2000; OATA 2001; Anonymous 2003; Pokorova et al. 2005). This information sheet is intended to inform veterinarians, biologists, fish producers and hobbyists about KHV disease.

What is KHV?

Koi herpesvirus (also known as cyprinid herpesvirus-3 or CyHV-3) is classified as a DNA virus belonging to the virus family Herpesviridae (i.e., a herpes virus). Although there has been some scientific discussion regarding the accuracy of this classification (Ronen et al. 2003), recent work (Waltzek et al. 2005) shows strong evidence that KHV is indeed a herpes virus, based on morphology and genetics, and is closely related to carp pox virus (cyprinid herpesvirus-1 or CyHV-1) and hematopoietic necrosis herpesvirus of goldfish (cyprinid herpesvirus-2 or CyHV-2). Koi herpesvirus disease has been diagnosed in koi and common carp (Hedrick et al. 2000; OATA 2001). Other related cyprinid species such as the common goldfish (Carassius auratus) and grass carp (Ctenopharyngodon idella) appear to be clinically unaffected by KHV although KHV DNA has been identified in tissue of goldfish and other fish species using polymerase chain reaction (PCR) (Bergmann et al. 2006) testing methods. Hybrid goldfish (male goldfish C. auratus x female common carp C. carpio) were moderately resistant to mortality following experimental infection with KHV (Hedrick et al. 2006). As with other herpes viral infections, KHV is believed to remain in the infected fish for life; therefore, exposed or recovered fish should be considered as carriers of the virus (OATA 2001; Petty and Fraser 2005).

Koi herpesvirus disease may cause between 80-100% mortality in susceptible populations, with clinical signs of disease most commonly being expressed when water temperatures are between 72° and 81°F (22° and 27°C)(OATA 2001). This viral disease affects fish of various ages, but cohabitation studies show that fry have a greater susceptibility than mature fish (Perelberg et al. 2003). However, larger koi are commonly diagnosed with KHV disease in clinical cases submitted to laboratories.

What Are the Signs of KHV?

Clinical signs of KHV are often non-specific Mortality may begin very rapidly in infected populations, with deaths starting within 24 to 48 hours after the initial onset of clinical signs. In experimental studies, 82° of fish exposed to the virus at a water temperature of 72°F (22°C) died within the first 15 days (Ronen et al. 2003). KHV infection may produce severe gill lesions which exhibit as fill mottling with red and white patches (Figure 1)(may be similar to columnaris disease signs). The white patches are due to necrosis (death) of the gill tissue. Gill lesions caused by KHV disease are the common clinical signs in affected koi. Other external signs of KHV may include bleeding gills, sunken eyes, pale patches or blisters on the skin. Some KHV infected koi may have a notched nose (A. Goodwin, University of Arkansas at Pine Bluff, personal communication). In some cases, secondary bacterial and parasitic infections may be the most obvious problem, masking the damage caused by the primary viral infection. Microscopic examination of gill biopsies often reveals high numbers of bacteria and various parasites (Hedrick et al. 2000; OATA 2001; Goodwin 2003).

Internal signs of KHV are variable and non-specific but may include greater than normal adhesions in the body cavity and a mottled appearance of internal organs (Hedrick et al. 2000; Goodwin 2003).

Behaviorally, affected fish often remain near the surface, swim lethargically and may exhibit respiratory distress and uncoordinated swimming.

Koi with KHV disease

Figure 1. Koi with mottled gills and sunken eyes due to koi herpesvirus disease.

How Do Fish Get Infected With KHV?

Methods of spreading (transmission) of KHV include direct contact with infected fish, fluids from infected fish and water, mud or other fomites/vectores that have come into contact with contaminated systems. The infective virus enters susceptible fish through the gills and possibly through the gut (Dishon et al. 2005). Depending upon water temperature, susceptible fish that are exposed to KHV may either become infected, develop disease, and die or may survive the initial outbreak of the disease and become carriers of the virus (OATA 2001). It is believed that carriers of KHV disease will never again show clinical signs of KHV disease although they are still infected and spread disease to susceptible fish.

Bergan et al. (2006) detected KHV DNA in clinically healthy goldfish and other ornamental fish species, indicating that these species may also carry the virus and may possibly shed virus and cause KHV disease in susceptible carp. Goldfish carrying KHV do not show signs of KHV disease.

How Do I Know if My Fish Have KHV?

Positive diagnosis of KHV requires the assistance of a veterinarian or fish health specialist and a fish disease diagnostic laboratory. Diagnostic identification of KHV may be accomplished by several direct and indirect methods. Direct methods are procedures that detect actual virus or "pieces" of virus. Indirect methods are procedures that quantitate the immune response by measuring antibody levels (Hedrick et al. 2000; OATA 2001; Goodwin 2003).

Direct methods used to identify KHV include: 1. virus isolation and identification (i.e., growing the virus) using a susceptible cell line such as Koi Fin (KF) cell lines {optimal growth observed at temperatures between 59° and 77°F (15° and 25°C)} and 2. PCR techniques (i.e., testing for the presence of KHV DNA material). For these direct diagnostic tests, tissues are removed from fish that are collected alive then euthanized. Isolation and detection of the virus in tissues from fish dead longer than a few hours may be unreliable. Non-lethal direct diagnostic tests are available on samples such as blood, fecal matter, mucus and gill clips (i.e., biopsies), but these tests may yield less definitive or less accurate results. A positive cell culture test indicates an active, ongoing infection with KHV. Positive detection of KHV DNA using PCR indicates that the virus is present, so it identifies koi sick with KHV and may detect some KHV carriers.

Indirect tests for KHV include enzyme-linked immunosorbent assay (ELISA) and virus neutralization (VN) testing. These tests may be performed on a blood sample and, therefore, are non-lethal diagnostic tools. ELISA or VN can provide evidence that a fish has or at one time did have an immune response (i.e., production of antibodies) against KHV. A positive ELISA or VN test for KHV indicates that the fish has produced antibodies against KHV and is either experiencing an outbreak or is a carrier. However, antibody-producing immune cells take time to become activated, and over time, if a fish is no longer sick, KHV-specific-antibody production may slow down or stop. Therefore, ELISA or VN may not be able to detect antibodies to KHV if the infection occurred years before or if the fish has not yet had time to produce antibodies.

Negative results by either direct or indirect tests do not necessarily mean fish are not carriers. There is no test that definitively detects all carriers or survivors.

Is There Treatment for KHV?

There is no treatment for KHV. Antiviral drugs are not available to treat KHV or any other viral diseases of cultured fish. Studies have shown that koi may survive an outbreak of KHV if water temperatures are increased to 86°F (30°C) during the outbreak (Ronen et al. 2003). However, this technique only marginally increases survival rates, and artificially raising water temperatures above 80°F in holding facilities may result in an increased occurrence of other more common bacterial and parasitic diseases. High water temperatures are not generally recommended for routine husbandry and management of koi and common carp. In addition, and more importantly, koi which survive a KHV outbreak or those exposed at high water temperatures will become carriers of the virus. These carrier koi are a source of the disease to susceptible fish when conditions are appropriate for viral shedding and infection. Carrier fish will typically no succumb to KHV disease or show signs of clinical infection.

Currently there is no approved vaccine in the U.S. for KHV. However, preliminary experimental vaccine studies using a live attenuated virus for vaccine by intracoelomic injection demonstrated that fish developed in high antibody titers and were immune to the disease and survived a KHV disease challenge (Ronen et al. 2003). However, there is the potential that these fish may be carriers of the virus and may infect naive fish with which they come into contact, and indirect testing (for antibodies) may be confusing if vaccination status is uncertain.

Because KHV outbreaks have caused large losses at koi and common carp facilities and because there is concern that survivors are carriers, anyone with koi that have been diagnosed with KHV should consider depopulation (eliminating the entire population) as a logical option. All materials and systems that the infected fish have contacted should be cleaned and disinfected.

Viral particles in water may be active in water up to three days (Yoshimizu et al. 2006). However, common disinfection protocols (see below) may be used to eliminate the virus from water systems and equipment [see UF/IFAS fact sheet "Sanitation Practices for Aquaculture Facilities" (VM-87)]. Biofilters and biofilter media exposed to the virus should also be thoroughly cleaned and disinfected. Prior to disinfection, equipment should be cleaned of debris or organic build-up, as these may reduce the effectiveness of the disinfectant. Chlorine solutions (e.g., household bleach) may be used to disinfect large equipment or systems without fish. The recommended protocol for chlorine is 200 ppm (200 mg/L) for one hour (Noga 1996). Proper dosing of this active ingredient depends upon the types of chlorine used. For household bleach, which is 5.25% sodium hypochlorite per liter, 35 milliliters per gallon of water will give 200 mg/L final concentration.

Quaternary ammonium compounds (QACs) may also be used for systems and equipment. Quaternary ammonium compounds are gentler on nets than chlorine solutions. The recommended QAC concentration for disinfection is 500ppm (500mg/L) for one hour (Noga 1996). Proper dosing of QACs depends upon the type/concentration in the mixture used because concentrations will vary depending upon the product used. Different QAC products may range from 10% to 50% active ingredient. For example, Roccal-D Plus® (Pharmacia & Upjohn Company, Pfizer) is approximately 24% active ingredient; therefore, a final treatment concentration of 500 mg/L would require about 7.9 mL of Roccal-D Plus® per gallon of water. Rinse thoroughly after use of any type of disinfectant to eliminate residual disinfectant which may kill fish.

How Can KHV Be Prevented?

Prior to obtaining any fish, first ask the supplier if there have been any major unexplained losses in the population. Monitoring and testing for KHV may be done by laboratory tests, so ask suppliers if any testing for KHV has been done and request a copy of lab result documentation. The best way to prevent KHV is to know your fish suppliers and to have a good working relationship with them.

Quarantine is the most dependable method to avoid the introduction of pathogens into a pond or facility. To implement an effective quarantine procedure, all new fish must be kept in a separate system, ideally in a different building or area from the resident fish. Resident fish should be fed, handled and maintained before the new fish. The quarantined fish require dedicated equipment such as nets, buckets and siphon hoses that are used only for them. In addition, foot baths and hand washes should be used by anyone entering and leaving the quarantine area. Fish should be quarantined for a minimum for a minimum of 30 days. Specifically for KHV, new koi should be quarantined in water that is 75°F (24°C) for at least 30 days. At the conclusion of the quarantine period, any sick fish should be examined by a veterinarian and/or diagnostic lab to rule out KHV or other diseases. If all fish appear healthy, blood samples should be collected from these quarantined fish and submitted for antibody detection using either ELISA or VN methods.

Koi hobbyists are encouraged to promote the use of English-style koi shows, which keep different sources (owners) of koi separated during the show and judging. Additionally, separate nets and equipment should be used by all show participants for their own fish. The Japanese-style show, in which koi from different owners are placed together in the same tank, may result in spread of disease among susceptible fish. Regardless of the show style, fish returning from shows should be quarantined (that is, separated from other koi) for a minimum of 30 days and held at 75°F prior to being placed back into the general population. For added security, blood sampling may be helpful. Survivors, known as carriers, of fish previously exposed to KHV disease should never be used for show.

At the end of the quarantine period and before placing all fish together, place several new koi with several koi from the established population in a separate area away from the rest of the established population and watch them for signs of disease. This "test" can help determine with a smaller number of fish whether placing the two populations together following quarantine could cause problems. Unfortunately, there are no guarantees.

What Are the Regulatory Considerations Associated with KHV?

As of January 2007, KHV was added to the World Organization for Animal Health (OIE) notifiable disease list for fish. Because of this listing the United States Department of Agriculture Animal and Plant Health Inspection Service (USDA APHIS) asks that accredited veterinarians and diagnostic laboratories report positive cases of KHV to the area veterinarian-in-charge (AVIC) of the state where the fish originated. However, no client information (e.g., name, address) will be requested. This information will help USDA determine what the prevalence of KHV is in the United States. USDA APHIS has no health requirements specific for KHV for koi or other fish moving interstate or internationally. There is no mandatory depopulation for populations of koi infected with KHV. It is up to the owner of those animals to decide what course to take.

Can Humans get KHV?

There is no zoonotic concern with KHV. The herpes virus responsible for causing KHV disease in fish will not cause disease in humans.

How Does KHV Differ From Other Viral Diseases?

It is important to differentiate KHV from other viruses that may cause disease in carp and koi. Two other important viral diseases recognized in carp are spring viremia of carp (SVC) and carp pox (cyprinid herpesvirus-1 or CyHV-1). These diseases have significantly different management and regulatory implications. Both KHV and SVC are now listed as notifiable fish diseases by the OIE. However, within the U.S., SVC is considered a foreign or exotic (not present) fish disease of aquacultured susceptible species and, as such, the accredited veterinarian or laboratory is required to notify confirmed outbreaks to USDA APHIS Veterinary Services officials who will, in turn, notify the OIE. By comparison, KHV is considered to be endemic (continuously present) in the United States. USDA APHIS asks accredited veterinarians and laboratories to report cases of KHV to the area veterinarian-in-charge (AVIC) of the state where the outbreak is occurring. The information will help USDA determine what the true prevalence of KHV is in the U.S. However, every outbreak of KHV will not be notified to the OIE.

Spring viremia of carp disease is caused by an RNA virus, Rhabdovirus carpio, and has been reported in common carp and koi (C. carpio), grass carp (C. idella), bighead carp (Aristichthys nobilis), silver carp (Hypophthalmichthys molitrix), Crucian carp (Carassius carassius) and common goldfish (C. auratus).

Carp pox disease (cyprinid herpesvirus-1 or CyHV-1) is caused by a different herpesvirus (Hypophthalmichthys cyprini) that has wide geographic distribution and affects common carp and koi. Carp pox typically causes smooth raised growths ("wart-like masses") on skin and fins of older fish but may be associated with high mortality in fry less than two months of age (Hedrick et al. 2000). Carp pox is not a reportable or notifiable disease in the U.S. Water temperatures above 68°F (20°C) help reduce the skin fin growths on older fish, but does not eliminate the virus from the fish. In mature fish, carp pox is typically a non-lethal, self-limiting disease (i.e., most if not all growths will resolve at warmer temperatures on their own).

KHV Summary

Koi herpesvirus is a devastating disease of ornamental koi and common carp. There are several methods to detect various states of the infection, but at the current time there are no legally available vaccines in the United States. There is no effective treatment to rid the fish of the virus. Good management practices, including quarantine, testing, and appropriate husbandry, are vital components to preventing this disease for all koi and common carp producers, dealers and hobbyists.

View the original University of Florida Fact Sheet in its entirety.