Koi are a highly personable and hardy ornamental carp whose popularity for pond owners has exploded in the past several years. When a pond is confronted with Koi Herpes Virus (KHV), the results are devastating. The virus has a mortality rate of 80% and the few survivors are typically euthanized, resulting in depopulation of the entire group of fish.

Koi, carp, and goldfish are actually affected by three different types of herpes virus. The first is known as "carp pox" or Cyprinid Herpes Virus I (CyHV-1) and is characterized by the temporary appearance of soft raised patches on the fish’s skin which are also termed "pox". Cyprinid Herpes Virus 2 (CyHV-2) is a widespread and devastating disease of goldfish. Cyprinid Herpes Virus 3 (CyHV-3), also known as the Koi Herpes Virus (KHV) or carp gill necrosis virus, is a highly infectious and destructive virus of koi and the common carp.

Most ponds are infected with KHV through the introduction of a new fish which is a latent carrier or inapparent carrier of the virus. These carriers do not appear clinically sick but will harbor the virus for long time periods, thought to last for the life of the fish. When the carrier undergoes a stressful situation, there will be a reappearance of the virus.

KHV primarily occurs when water temperatures are in the 72 to 78°F range. Warming the water to 85°F prevents replication of the virus and will improve the functioning of the fish’s immune system. Unfortunately, this warm water recovery results in a latent carrier state. These fish may be resistant to further clinical disease but will be capable of spreading the virus to other fish when stressed. There is no complete recovery in which the virus is totally eliminated from the fish’s body.

The virus does not appear clinically when fish are held at cold water temperatures; however the fish does not mount an effective immune response at these cold temperatures to prevent further clinical appearance of the virus. A fish held at cold temperatures will develop clinical disease when warmer water temperatures are encountered.

The virus is spread from fish to fish through physical contact with contaminated fish, or contact with a contaminated environment, such as sharing the same water, nets, etc.

The disease is characterized by severe necrosis of the gill epithelium. Affected fish are lethargic and will show clinical signs of respiratory distress such as piping or lying near inflows of oxygenated water. Copious amounts of mucus secretions may be noted on the gills and skin.

There are four different types of testing used for detection of KHV: viral culture or neutralization, histopathology, PCR (polymerase chain reaction test) and ELISA (enzyme linked immuno-absorbent assay). PCR and ELISA tests work well on fish that are already sick. As antibody levels decline in recovering fish these survivors will produce negative results on serology tests. Histopathology of the gill necrosis will detect intranuclear inclusion characteristic of the virus in tissue sections when active lesions are present.

In order to detect fish exposed to the virus that have not yet developed clinical infection or to detect latent carriers, fish should be quarantined at 75°F for 10 to 14 days prior to introduction into an established population and tested for the presence of the virus. Simple quarantine procedures will decrease the possibility of exposure to KHV by 99% or more. Quarantine has the added advantage of preventing the introduction of fish parasites. In addition to quarantine it is important to know where your fish originate from. When you buy from a reputable breeder and purchase from few sources, your chances of contacting KHV will be lower than if you purchase fish from dealers with numerous wholesale or questionable sources.

A commercial vaccine for KHV is now available by KoVax Ltd. The vaccine is currently licensed in Israel with a high proportion of the koi and carp being produced in that country being vaccinated. Many countries are focusing their efforts toward eradication of the virus and the effect of vaccination on eradication efforts has not yet been determined. Because these effects have not been determined, vaccine usage is controversial at this time. To date, the vaccine has not yet been approved for use in the European Union or the United States.

Herpes viruses have lipid envelopes or coverings made of fat. This lipid envelop makes the KHV sensitive to disinfectants that may strip off the lipid layer. This fragile envelope also makes the virus sensitive to many environmental factors such as desiccation which will result in the death of the virus. Following an outbreak, the virus will not exist for long time periods outside of a host fish. When a pond is cleared of fish, the pond may be safely repopulated in a couple of weeks, especially when the tanks and equipment are thoroughly disinfected.


References:

Barthel, Tom. “Health 101. Education is Key to Keeping Pond Fish Belly-Down”. Pet Product News International. October 2007. P. 114.

Goodwin, Andrew. “Understanding Koi Herpes Virus”. Pet Age. May 2008. Pp. 52-58.

Kahn, Cynthia Editor. The Merck Veterinary Manual. 9th Edition. P 1511.

Watkins, Anne Culbreath. “Crazy for Koi”. Pet Age. February 2008. Pp. 48-52.

Watkins, Anne Culbreath. “Preventing Koi Herpes Virus”. Pet Age. February 2008. P. 50.