Introduction; Koi herpesvirus (KHV) is a highly contagious virus that causes severe morbidity and mortality in common carp (Cyprinus carpio) varieties (Hedrick et al. 2000, Haenen et al. 2004). Besides being raised as a foodfish in many countries, carps have also been selectively bred for the ornamental fish industry where they are known as koi. The first case of KHV was detected in 1996 (Haenen et al. 2004). Since then, other cases have been confirmed in almost all countries that culture Koi or common carp, with the exception of Australia (Hedrick et al. 2000; Haenen et al. 2004, Pokorova et al. 2005). Veterinary professionals, biologists, fish producers and hobbyists can use this information sheet to learn more about KHV disease. 

KHV - What Is It? Koi herpesvirus, or Cyprinid herpesvirus 3, belongs to the family Alloherpesviridae (which includes fish herpesviruses). KHV is indeed a pathogen, as Waltzek and colleagues have shown in their work (Waltzek et al. 2005, 2009).  

A herpesvirus based on virus morphology and genetics, closely related to carp pox virus (Cyprinid herpesvirus 1; CyHV1) and goldfish hematopoietic necrosis virus (Cyprinid herpesvirus 2; CyHV2). Koi and common carp have been diagnosed with Koi herpesvirus disease (Hedrick et al. 2000; OATA 2001). KHV DNA has been detected in goldfish and other fish species exposed to the virus using polymerase chain reaction (PCR) testing methods (El-Matbouli et al. 2007, Sadler et al. 2008, Bergmann et al. 2009, Kempter et al. 2009). However, non-cyprinid species such as grass carp (Ctenopharyngodon idella) and other cyprinids (e.g., hybrid goldfish (male goldfish C. auratus x female common carp C. carpio) were moderately resistant to death following experimental infection with KHV (Hedrick et al. 2006). Hybrid common carp, other cyprinids, or non-cyprinid species can harbor KHV, which later transmits the disease to nave common carp varieties. The koi herpesvirus disease (KHVD) affects fish of various ages, often resulting in 80–100% mortality in susceptible populations when water temperatures are between 60° and 77°F (16° and 25°C) (Haenen et al. 2004). Although, as with other herpesviruses, KHV may remain in the infected fish for life; therefore, fish that recover from a KHV outbreak must also be considered carriers (Petty and Fraser 2005, St. Hilaire et al. 2005, Eide et al. 2011).

KHV is often characterized by a lack of specific symptoms. Mortality may begin within 24 to 48 hours after the onset of clinical symptoms in infected populations. In experimental studies, 82% of fish exposed to the virus at 72°F (22°C) died within 15 days (Ronen et al. 2003). KHV infection may cause severe gill lesions characterized by gill mottling with red and white patches (Figure 1) (these may be similar to columnaris disease signs; SRAC publication #479b on Columnaris Disease). Necrosis (death) of the gill tissue is responsible for the white patches. Affected Koi usually have gill lesions caused by KHV disease. Other external symptoms of KHV include bleeding gills, sunken eyes, and pale patches on the skin. Koi infected with KHV may have notched noses (Goodwin 2012). Secondary bacterial and parasitic infections can mask the damage caused by a primary viral infection in some cases. Gill biopsies often show a high number of bacteria and various parasites (Hedrick et al. 2000; Haenen et al. 2004). Symptoms of KHVD are variable and nonspecific but include greater than normal adhesions in the body cavity and enlargement or mottled appearance of internal organs (Hedrick et al. 2000; Haenen et al 2004). Behaviorally, affected fish tend to remain near the surface, swim lethargically and exhibit respiratory distress and uncoordinated swimming.

When do fish get infected with KHV? Direct contact with infected fish, contact with fluid from infected fish, and contact with water, mud, or other fomites/vectors that come into contact with contaminated systems are some of the ways KHV can spread. (Costes et al. 2009) explain that the infectious virus is primarily transmitted via the skin, including gill tissue. As a result of exposure to K HV, susceptible fish may either become infected, develop disease, and die, or survive the initial outbreak of the disease and become carriers of the virus (Petty and Fraser 2005, St. Hilaire et al. 2005, Eide et al. 2011). Fish that survive exposure to KHV or receive the KHV vaccine may become immunized and develop a level of protection against the virus, though the length of protection remains unknown (Ronen et al. 2003, Perelberg et al. 2008). The virus may be carried by fish that survive outbreaks of KHV and may be able to spread the disease to others. While they may carry and shed the KHV virus, these carrier fish may not show any signs of infection.

What effect does water temperature have on KHV disease? Typically, KHV disease (KHVD) outbreaks occur in spring and autumn when water temperatures are between 60° and 77°F (16° and 25°C), with an incubation period ranging from 7–21 days depending on temperature (Haenen et al. 2004). KHVD has been reported at temperatures as low as 60°F (15.516°C) in common carp outbreaks in Japan, and experimental trials have shown that it is capable of causing mortality at temperatures up to about 82°F (28°C). Fish kept at lower temperatures are less susceptible to clinical disease than those kept at warmer temperatures. Infish can be infected and not show signs of disease at lower temperatures (e.g., 55°F [13°C]), but when permissive water temperatures are repeated, the fish show typical KHV signs and may die (Gilad et al. 2003, St. Hilaire et al. 2005). 

Where can I find out if my fish have KHV?

 A veterinarian or fish health specialist and a fish disease lab are needed for a positive KHV diagnosis. KHV can be diagnosed by several direct and indirect methods. Detecting actual viruses or pieces of viruses is called a direct method. A fish's immunity to a KHV infection can be determined by measuring antibodies (Adkison et al. 2005, St-Hilaire et al. 2005, 2009). KHV is identified two ways: 1) isolation and identification (i.e., growing the virus in a susceptible cell line such as the Koi Fin (KF-1) [optimum growth observed between 59° and 77°F (15° and 25°C)] and 2) PCR techniques (i.e., testing for KHV DNA). Direct diagnostic tests are done on fish that are collected alive, then euthanized. It's hard to isolate the virus in tissues from fish that have been dead for more than a few hours. Non-lethal direct diagnostic tests are available on blood, fecal material, mucus, and gill clips (i.e., biopsies), but these tests may not be as conclusive or accurate. The results of a positive cell culture test indicate that KHV is active and ongoing.ection of KHV DNA using PCR indicates that the virus is present, so it can be used to confirm KHV disease in clinically sick fish and perhaps even to identify carriers (Eide et al. 2011). 

The enzyme-linked immunosorbent assay (ELISA) is one of the most widely used indirect testing methods for KHV. KHV ELISA requires a blood sample, so it is a non-lethal diagnostic method. Fish ELISA results can reveal whether they are mounting, or have previously mounted, an immune response (i.e., antibodies) against KHV. ELISA test results for KHV indicate that the fish has produced antibodies against the virus after a previous exposure. Antibody producing immune cells take time to activate, so if a fish is no longer ill, the production of anti-KHV antibodies may slow down or stop over time. Therefore, ELISA may not detect antibodies to KHV if the infection occurred years ago or if the fish has not yet developed antibodies.  Test results that are negative, whether direct or indirect, do not necessarily indicate that fish are carriers of the disease. No test can definitively determine whether fish are carriers or survivors.

KHV can be treated? Currently, there is no treatment for KHV. KHV or any other viral diseases of cultured fish cannot be treated with antiviral drugs. If water temperatures are raised to 86°F (30°C) during an outbreak of KHV, Koi may survive (Ronen et al. 2003). It is important to note, however, that this technique only marginally increases survival rates, and artificially raising water temperatures above 80°F in holding facilities may result in an increased incidence of other bacterial and parasitic diseases. For routine husbandry and management of koi and common carp, high water temperatures are generally not recommended. The Koi that survive a KHV outbreak or those exposed to high water temperatures may also become carriers of the virus. When conditions are suitable for viral shedding and infection, these carrier koi can be a source of the disease to susceptible fish. Typically, carrier fish do not succumb to KHV disease or show clinical signs of infection because of previous exposure to the virus.  In a recent announcement, USDA APHIS approved the use of an attenuated live vaccine for the prevention of KHVD in common carp and Koi over 100 grams (the vaccine is not approved for use in brood stock). A vaccine developed by an Israeli research group (Ronen et al. 2003) generates high anti-KHV antibody levels and protects common carp or Koi against a subsequent viral challenge (Ronen et al. 2003, Perelberg et al. 2008). Concerns have been raised regarding the effectiveness of the vaccine and the length of protection provided to vaccinated fish. It is currently unknown whether vaccinated fish become carriers of the vaccine strain or if they are protected against infection by unvaccinated common carp. With regards to the vaccine, there are currently no commercially available diagnostic tests that can differentiate between vaccinated fish and naturally infected or exposed fish.   

KHV outbreaks have caused large losses at koi and common carp facilities, and it is believed that survivors are carriers, so anyone with Koi that have been diagnosed with the virus should consider depopulation (eliminating the entire population). Cleaning and disinfecting all materials and systems with which the infected fish have come into contact is essential.

What can be done to prevent KHV? If there have been any major losses in the population, ask the supplier before purchasing any fish. 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 (that is, separation from other Koi) 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 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 ELISA. (Atkinson et al. 2005; St. Hilaire et al. 2009)  

Is KHV transmissible to humans? It is not zoonotic. Humans will not become ill from the herpesvirus that causes KHV disease in fish.