Koi Health - Information on Koi Herpesvirus

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Koi Herpesvirus (KHV) represents a third cyprinid herpesvirus formally designated Cyprinid herpesvirus 3(CyHV-3). Other nomenclature (Carp Interstitial Nephritis and Gill Necrosis Virus (CNGV)) has been suggested. For purposes of this web site the causative agent of Koi Herpesvirus disease will be be abbreviated KHV and the semantics left to others.

We hope to provide enough practical information regarding KHV such that the reader will be able to make informed choices regarding the acquisition of koi and their subsequent quarantine and testing. Some of the material is admittedly oversimplified for clarity. A partial listing of references is included.

There are now approximately one hundred known herpesviruses isolated, with at least one herpesvirus for each species studied. There are eight known human herpesviruses, of which most of us have been infected by six. One of the characteristics of herpesvirus infection is the ability to cause recurrent infection by one of several mechanisms including reactivation of latent virus or persistent infections (chronic low grade virus multiplication). As a whole, the herpesviruses studied have evolved a number of mechanisms for "hiding" from the "immune system" during their latent or persistent infectious states. These places of hiding vary between herpesviruses but have been found to include nucleated nerve cells, lymphoid cells, salivary glands, kidney, etc. The ability of herpesviruses to avoid the immune response and remain latent as an intracellular virus makes it very difficult for the host animal to clear the virus. Once infected, many animals have the virus for life and may intermittently shed virus without showing signs of disease. The site of latency of KHV is unknown.

There are direct and indirect tests to demonstrate viral infections. Direct tests detect the presence of the virus or of a portion of a virus. Growing the virus in cell culture or a PCR test to a portion of the viral DNA would be examples of direct tests. Generally these are most useful when the virus is present in large numbers such as in a KHV disease outbreak. Indirect tests measure antibodies to a particular virus. They are most useful in detecting previous infection or the stage of infection.

Diagnosis of KHV disease outbreaks is usually made based on epdemiology, characteristic histopathology, and the presence of the virus in internal organs demonstrated by cell culture or PCR assay. Periodic KHV surveillance may be done in koi growing countries by periodically sampling fish from ponds and doing PCR testing. Some health certificates and certifications will state "KHV specific amplification was not detected when genomic DNA from specimens were used as template". Others may just say they are free of KHV. PCR technology has proven very useful for confirming diagnosis in acute disease outbreaks. Unfortunately, PCR has not been found sensitive enough to detect those KHV survivors which may carry the virus in a latent or persistent infective state (carriers). It currently takes measurement of anti KHV antibodies to determine if a koi has previously been infected with KHV and has the potential of periodic virus shedding, infecting other fish.

No KHV quarantine or testing procedure offers 100% assurance a koi previously infected with KHV will be detected. Through quarantine and serological testing we can greatly reduce the odds of infecting other koi. . Serological technology is available to measure antibodies to the KHV virus. Both Enzyme Linked Immunosorbant Assay (ELISA) and the Virus Neutralization Antibody Assay (VN) serologies are available at modest cost, the latter being the most sensitive. The KHV disease outbreak illustrated in the photos should have been prevented by testing the fish for the carrier state while in quarantine. Serology done on survivors strongly suggests some imports had survived a previous KHV infection as they carried low antibody titers. It is unclear how long a KHV survivor will maintain detectable antibodies but studies are in progress. The CEFAS group has demonstrated detectable antibodies a year or more following infection. Using serology on fish in quarantine is another tool we have available for protecting our collections. If a fish has detectable levels of antibodies to KHV, it has been previously infected and must be considered a carrier. Carriers have the potential to shed the virus at a later time, infecting other fish. Materials and procedures we find useful for drawing blood for anti KHV antibody testing are outlined.

PCR testing is useful when fish are sick and dying but is usually not of benefit when fish appear healthy. PCR testing is extremely helpful in confirming histopathology when fish are dying. Again, PCR testing is extremely unlikely to detect disease in healthy appearing fish. All PCR techniques are not equally sensitive. Current PCR technology is inadequate for detecting the latent or carrier state of Koi herpesvirus.

A safe and effective vaccine will be required to control KHV disease. Currently the alternative is quarantine and testing each and every koi for antibodies to KHV before allowing new fish to enter our ponds.