Koi Herpes Virus

How do fish get infected with KHV?

Methods of spreading (transmission) of KHV include direct contact with infected fish, contact with fluids from infected fish; and contact with water, mud, or other fomites/vectors that have come into contact with contaminated systems. The infectious virus primarily enters susceptible fish through the skin, including gill tissue (Costes et al. 2009). Depending upon water temperature, susceptible fish that are exposed to K HV may either become infected, develop disease, and die; or they may 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, although the length of protection remains unknown (Ronen et al. 2003, Perelberg et al. 2008). Survivors of KHV outbreaks may become carriers of the virus and may be capable of spreading the disease to susceptible fish. These carrier fish may not show signs of KHV infection although they may be carrying and/or shedding the virus.

 

How does water temperature affect KHV disease?

KHV disease (KHVD) outbreaks typically occur in spring and autumn when water temperatures are between 60° and 77°F (16° and 25 °C) with an incubation period of 7–21 days depending upon water temperature (Haenen et al. 2004). KHVD at temperatures as low as 60°F (15.516°C) has been reported in common carp outbreaks in Japan, and experimental trials have shown that the virus can induce mortality up to about 82°F (28°C). Warmer water temperatures more rapidly induce clinical disease as compared to fish held at lower temperatures. At lower water temperatures (e.g., 55°F [13°C]), the virus can infect fish without inducing clinical signs of disease, but when permissive water temperatures are again experienced, the fish show typical KHV signs and may die (Gilad et al. 2003, St. Hilaire et al. 2005).

 

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 determine whether a fish has mounted an immune response against KHV following exposure to virus by measuring anti-KHV antibody levels in the blood (Adkison et al. 2005, St-Hilaire et al. 2005, 2009). Direct methods used to identify KHV include: 1) virus isolation and identification (i.e., growing the virus) using a susceptible cell line such as the Koi Fin (KF-1) cell line {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). 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 material, 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 can be used to confirm KHV disease in clinically sick fish and perhaps even to identify carriers (Eide et al. 2011).

 

One indirect testing method for KHV that has been widely used is the enzyme-linked immunosorbent assay (ELISA). The KHV ELISA uses a blood sample and, therefore, is a non-lethal diagnostic tool. ELISA data can provide evidence that a fish is currently mounting, or has previously mounted, an immune response (i.e., production of antibodies) against KHV. A positive ELISA test for KHV indicates that the fish has produced antibodies against KHV after a previous exposure to the virus. However, antibody producing immune cells take time to become activated, and over time, if a fish is no longer sick, anti-KHV antibody production may slow down or stop. Therefore, ELISA 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 may 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. Due to previous exposure and immunization to the virus, carrier fish will typically not succumb to KHV disease or show signs of clinical infection. Recently, USDA APHIS approved the use of an attenuated live vaccine for prevention of KHVD, to be administered to common carp or Koi heavier than 100 grams (the vaccine is not approved for use in brood stock). The vaccine, originally developed by an Israeli research group (Ronen et al. 2003), generates high anti-KHV antibody titers and protects vaccinated common carp or Koi upon subsequent viral challenge (Ronen et al. 2003, Perelberg et al. 2008). However, concerns have been raised regarding the efficacy of the vaccine and the length of protection afforded vaccinated fish. Currently, it is unknown whether vaccinated fish become carriers of the vaccine strain or if they are protected against infection by the wild-type strain which would pose a threat to unvaccinated common carp. Another challenge with regards to the vaccine is that no diagnostic tests are commercially available that can differentiate vaccinated versus naturally infected/exposed fish.

 

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.

 

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 (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)

 

Can humans get KHV?

There is no zoonotic concern with KHV. The herpesvirus 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; CyHV1). These diseases have significantly different management and regulatory implications (Table 1). 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 aqua cultured 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 widespread 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 a single stranded 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). For more information on SVC see Table 1 Carp pox disease (Cyprinid herpesvirus 1; CyHV1) is caused by a different herpesvirus from KHV (Herpesvirus cyprini) that has a 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 (Sano et al. 1990). Carp pox is not a reportable or notifiable disease in the U.S. Water temperatures above 68° F (20°C) help reduce the skin and fin growths on older fish, but does not eliminate the virus from the fish (Table 1). 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). Summary Koi herpesvirus disease is a devastating disease of ornamental koi and common carp. There are several methods to detect various states of the infection. There is no effective treatment to rid the fish of the virus.