Koi Herpes Virus - For the Average Hobbyist

The Koi Herpes Virus was first formally described in Israel around 1998 after they received some Japanese fish that were infected with this disease. The virus infects the epidermis [skin] of the Koi and causes these superficial cells to die and slough away. The serious damage is done to the skin, fins, and gills. As the skin dies away, bacterial infections, and a loss of osmotic integrity contribute to the loss of fish.

When the virus infects a group of fish, one could expect to lose upwards of 90% of the group. There is no cure for the viral infection. However, it appears that controlling bacterial infections, and providing considerable heat (86o F) ameliorates the symptoms in fish that are not so seriously affected as to die anyway. With heating, some confirmed cases have had losses reduced to as low as 16%

The virus is primarily spread directly from fish to fish. Of course, it is possible to spread the virus by way of fomites, such as nets, or splashed water.

The most common route of infection is via new fish, which are added to the resident population without adequate or lengthy quarantine. In other cases, old fish are returned with infection after a simple breeding loan. This has happened several times. Therefore, all fish being added to, or returned to a resident collection should have some interval of quarantine.

The KHV virus remains latent in populations of fish that are held below its ideal operating temperature, which happens to be above 64-68o F. Fish that are held in cool water, under 68o F can harbor the virus for lengthy periods and the virus will not activate until the fish is warmed into the low seventies. There is no need for “roller coaster” of water temperature. If the fish is above 70o F the virus can infect the fish and cause clinical disease.

KHV is only infectious to Koi. It will not infect Goldfish.


KHV is difficult to diagnose because growing the virus in cell culture is difficult. Worse, the virus appears at varying times in varying quantities in the infected tissues. If one were to biopsy the gill looking for the virus on the wrong day, the virus would not show up there and the diagnosis would be lost. In general, biopsy of liver and kidney tissue is the most accurate place to find virus to make the diagnosis.

There are three tests for KHV. One is the PCR test. The PCR test uses a DNA marker to find a piece of the virus DNA. It is very specific. The second test is “In Situ Hybridization” which is more accurate because it uses Formalin fixed tissues and it detects virus inside the nucleus of the cell, which proves not just presence of the virus, but in fact, infection of the virus. Finally, there is an agglutination test for the blood of Koi. The blood is “faked out” by introduced KHV proteins, and if the fish ever HAD an infection with KHV, antibodies will agglutinate the introduced protein. This indicates that the fish had an infection but somehow survived it.

Testing is desired to determine several things which are not clearly known at the present time.

Do we know for sure it’s a Herpes virus? If KHV is not a herpes virus, we can expect many different behaviors from this virus, including a “no carrier” state in the survivors, which would be a very good thing.

If the virus is in fact a herpes virus, then we would make an assumption that even if heated, the virus is present in the fish after successful recovery and could be considered infectious at least at some time in the future.

We would also like to determine once and for all if the survivors are in fact immune to the virus, or if they carry the virus. These are two different results and two different studies.

Finally, if this is a viral infection which is NOT eliminated by heating, and which DOES have a “carrier state” after successful recovery, then we must develop a vaccine to stop the virus. The vaccine would have to be very effective, and it would have to be deployed with such prejudice that it would intercept 100% of field infections before the virus had a chance to mutate into something else which would elude the vaccine.

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