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Article - KOI Herpes Virus

What is Koi herpes virus?

Eighty representatives from all phases of pond care met recently at the University of Georgia's Koi Lab for a Koi Health Management Course.  We had veterinarians, hobbyists, dealers, fish farmers, and pond construction companies assembling to learn more about fish health.  One of the hottest topics was Koi Herpes Virus.  I thought you might like the latest information.

A few facts as known today:

  • KHV survivors- all post-infected fish are carriers.  They can never leave the pond and you can never safely add new fish. We do not know how long the fish will remain immune.  Virus can't replicate on its own and dies in 4 hours, but can easily be disinfected by weak bleach solution.
  • Frogs can transfer KHV from one pond to another via the slime on their backs but are not themselves carriers.  KHV is species-specific.
  • 74ºF activates the disease.  Indonesia is now a test zone for KHV vaccination (Magnoy products) due to their heavy outbreaks.  At 30ºC fish are "heat synchronized" (per Doc Johnson) which is what Magnoy is calling "Immunized."  There is no such thing as immunity until we have a vaccination that is proven successful.
  • Once the virus is "shed" from vaccinated koi into the environment, the EPA becomes involved to do non-target testing to rule-out impact on other species.
  • Stress does not bring on the virus.  Temperature does. Therefore, fish kept in a spring-fed pond with temperatures in the 60's can be KHV+ and never have the disease break until the fish leave the pond and go to a new home.

Vicki Vaughan says it takes a HUGE viral load to transmit the disease.  We've never seen a case where it was spread without active shedding.  Splashing water or sharing nets is doubtful as they have a difficult time infecting fish at the lab for testing.  She says, yes, exercise caution at shows, but probably the disease is not going to be transferred.  In the end, why take the chance?

Dr. Helen Roberts, DVM recommends sending samples (biopsy) of gill, spleen, liver and kidney of any suspected fish to the lab for analysis.  As with any other disease, the quicker you act, the better the prognosis for the fish (the rest of the fish).  

She also mentioned developing a "pathic pheumonic" in which anytime you see a symptom which resembles KHV (for example), really see it.  (I hope I described that right.)  It doesn't pay to put on the horse blinders today.  KHV isn't limited to imported koi.  Domestic koi are just as likely to be exposed, infected or diseased.  

Dr. Branson Ritchie asked us to consider certain aspects of working with a latent virus such as herpes which is contagious when it "sheds"-

            A few notes on ISOLATION/QUARANTINE:

  • This means no direct or indirect contact with other fish.
  • Sterilization of air, food, water, etcetera.
  • No visitors.
  • Not practical for companion animals.
  • Animal does not leave for any reason during quarantine period.

For fish dealers to do the quarantine and isolation, there are disadvantages.  

  • One is that the fish are generally sold quickly, not allowing a 14-30 day quarantine period.
  • They must test every fish that comes into their possession/store.
  • A negative population is highly suspect (parasites, at the very least, should be identified and eradicated before resale).
  • Tests are not absolute for KHV (there is a 2-3 week window period after infection before antibodies can be detected in the fish's blood)
  • What do you do with positive fish?
  • Do you also euthanize companion fish?
  • Would a test and eradication program be effective with a virus that causes a latent infection?  ( i.e. - chicken pox is a herpes virus and remains in your body until you die.  Does this make you a chicken pox carrier?  It hasn't been proven that you aren't.)
  • Virus incubation period is 5-14 days at 74ºF.
  • Any koi is susceptible to infection .  Very small koi are susceptible to disease.  Progression of disease: a) exposure…b) infection… c) disease.
  • IF this really is a herpes virus (KHV), then quarantine is of little value.  As yet we have no treatment for KHV.  Statistics at UGA's koi lab in the past year were 15 out of 162 tested positive for KHV.  

This means one tenth of the koi population is latently infected and we do not know how long the antibody titers last.  To put one infected koi into your pond can spell disaster for the entire population.  What quarantine will do, if done for 14 days at 74ºF, will remove any doubt that incoming fish are actively diseased, but won't tell if the fish is a carrier.  

Since the antibodies can be detected 2-3 weeks after the fish has been infected, maybe the only way to be sure a newly purchased fish is safe to take out of quarantine and introduce into a pond is to have the antibody test done after 2-3 weeks in quarantine.  The quarantine could be either in a dealer's or hobbyist facility. The likelihood of buying an infected or previously infected (carrier) fish is greater than you think.