Koi Herpes Virus (KHV) (Cyprinid herpesvirus 3(CyHv3))
The purpose of this discussion is to provide the reader with enough information to make an informed decision about what steps should be taken in order to
protect his/her existing collection from KHV.
There are approximately 100 herpes viruses of which eight are human herpesviruses. Most if not all of us have been infected by six of them. Only in extreme
cases are any of these fatal. In addition, herpesviruses have an ability to cause recurrent infections either by reactivation of a latent virus or by chronic low
grade infection. Their ability to “hide” in various parts of the body has been studied extensively. Once infected it is very difficult for the host to clear the virus
because of its’ ability to remain hidden and avoid the immune response.
While herpesviruses have been studied extensively and their hiding places have been identified in humans and some animals the latency site for KHV in koi is
unknown. While I could go on with this portion of the discussion it is probably best to leave it to the experts in that field. Suffice it to say that KHV is apparently
transmitted in several ways and there is no known cure. Unlike the human herpesviruses (for the most part) KHV is lethal once it is actively being shed by a
carrier. And that is probably the most important difference: KHV is lethal.
There are many theories about how to detect KHV, how to quarantine and “cycle” to prove that a given group is “KHV free”. There is no test or quarantine
method which can offer 100% assurance of this but there are methods which will greatly increase the odds that your collection will be kept safe from this
disease.
Quarantining all new arrivals is the first step in protecting your collection. A well-prepared quarantine tank as well as all associated equipment must be kept
safely at a distance from any existing collection until such time as it is believed safe to intermingle the koi. Serious effort must be made to keep all possible
types of contamination from occurring. This includes foods, nets, bowls, medications, filtration equipment, assorted tools and especially foot wear and
clothing. Hand and foot sanitation must be properly addressed. Each time something is done in the quarantine area, thorough sanitation of used materials
and equipment must be accomplished. Not an easy task but once a routine is established it is not really that difficult.
The PCR (polymerase Chain Reaction) test is most commonly used to prove whether a single or group of fish which are sick and dying are actually infected
with KHV. This test involves swabbing the affected fish internally as well as externally with a sterile swab, and having the sample tested by a well-certified
laboratory. This test is primarily used to confirm whether the virus is present and is actually causing the losses. PCR testing on apparently healthy and
vibrant stock is of no value. It is extremely unlikely to detect KHV unless the virus is being shed. At some point PCR testing may become sensitive enough to
detect the latent or carrier state of KHV but to date it is inadequate.
The ELISA and VN serological tests are both serological test methods. The ELISA (Enzyme Linked Immunosorbant Assay) and the VN (Virus Neutralization
Antibody Assay) are available with the VN being the more sensitive. The University of Minnesota is currently setting up ELISA testing while the University of
Georgia has been performing VN testing for quite some time. From a practical standpoint drawing blood samples and testing with VN makes more sense
simply because ELISA is not as sensitive and is more likely to give an incorrect test result especially when the test parameters are altered to show low levels of
titers. That obviously could change as methods to improve all test methods are constantly being studied.
For my part, I draw blood on all new arrivals shortly after they are brought in. The samples are submitted to the University of Georgia Infectious Diseases
Laboratory for VN testing. In the event of a positive test result a new sample is split and submitted as two separate individual samples for further testing and
subsequent analysis to determine whether the fish is indeed positive or if the original (first) blood sample was somehow contaminated.
As I said before: No test or quarantine procedure can guarantee 100% that a particular individual or group of koi are KHV free. Only that I make every effort
possible to increase the odds that koi coming from my establishment have a far less likelihood of carrying KHV.
To date I have been very fortunate. Of all the koi sampled and tested only one test was positive. That sample was later determined to be contaminated as
subsequent samples were all negative. I have kept that koi in my possession as it serves as a reminder to be thorough to a fault.
Karl Schoeler
Koi Valley