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Viral diseases

Viruses are extremely small infective agents that can only be seen with a very powerful electron microscope. Viruses are among the most successful organisms ever to have evolved and can infect all other living creatures, including bacteria. The structure consists essentially of a core of genetic material surrounded by a protein coat and can only live and reproduce by invading a living cell within a living creature. The virus literary injects its own genetic material into a single sell of the host and may proceed in one of two ways. The virus may incorporate itself into the host cells genetic material and have an infective stage, causing more virus particles to be produced. The virus then enters into a non-infectious stage until stress or other disease triggers it to become infective again. The other way a virus may proceed, is to cause the host cell to mass produce other virus particles that are released when the host cell ruptures. Viruses mutate easily and new strains are frequently created. An example is the common cold that humans encounter. A viral disease cannot be cured and the immune system of man or beast is the best defence. It is with the above in mind that one should view a viral infection in koi.

In several cases the disease is not fatal but merely disfiguring and will clear by itself over time. The best future option to combat viral disease is inoculation but most research is underway.

Carp pox

Carp pox has been known in the koi industry for many years. It is cased by a virus belonging to the herpes group and is regarded as cronic and not life threatening. Although carp pox is not frequently encountered in other countries, I have rarely seen a pond in South Africa without at least one infected fish. It suggests that it is a common disease but not easily transmittable. Infected fish develop small white raised areas resembling candle wax dripped onto the fish. It often develops during the early spring, so it is possible that frequent temperature changes may weaken the immune system, allowing the virus to be more effective. The symptoms sometimes disappeared with an increase in water temperature, but it does not mean that the koi is free from the disease. I have notice these waxy lumps every year on a fish during winter, only to disappear in summer.

Last Updated on Friday, 14 November 2008 08:39


Koi Herpes virus



Koi Herpes virus (KHV) is the most deadly disease threatening koi today. KHV was first describe in Israel in 1998, but apparently the disease was documented in Japan several years before that. KHV is suspected to be present in koi populations from many countries, and current evidence suggests that when purchasing a new koi, no county of origin should be considered free of the virus. Although called a “herpes” virus various views exist amongst scientists whether it is herpes virus or not, but for us mortals that are only interested in keeping our collections safe, it hardly matters. What is important is that when the virus strikes, it first of all undergoes a short period of incubation. The first mortalities occur after about seven days. However within two to four more days, mortalities rise sharply and can reach 90% or more. This may vary depending on water temperature. Disease caused by KHV is generally seen between 20 and 29 degrees Celsius. The virus infects the epidermis (skin) and gills of the koi, effectively nullifying the defence system of the fish against disease, and damaging the gills to such an extent that the osmotic integrity is compromised.

Last Updated on Tuesday, 07 September 2010 20:38



This is well – known viral disease in fish that effect cells and causes them to become enlarged (Megaloblastic), amounting to tens, and in some instances, hundreds of thousand-fold increase in cellular volume. For the cell, the infection is terminal, but otherwise it is not greatly significant for the host. It is not easily transmitted between fish and is not only associated with bad water quality and stress. Lymphocystis is not fatal but in some cases very disfiguring. I have noticed that in some cases its clears by itself when the water temperature starts to rise. It is said that the virus is transmitted between fish when the virus enters through an injury or wound on a fish. The virus may also be transmitted by fish lice, (Argulus) or leaches when the lesion rapture and viral particles are released into the water. Cannibalism of a diseased fish can also cause the virus to spread.

Last Updated on Thursday, 27 August 2009 19:30


Spring Viremia of Carp

Spring Viremia of Carp (SVC) is caused by the virus rhabdovirus. The disease has been reported in almost all carp species, including gold fish. It is contagious and mostly fatal when effecting fish. The virus is shed in the feces, urine and gill secretions of infected fish into the water where it infects other fish. The virus may be introduced into the pond by new additions of koi, contaminated equipment, clothing and also frogs and birds that migrate between bodies of water. The mortality rate associated with SVC can reach 90 percent depending of the age of the fish and water temperature. As the name implies, outbreaks of SVC are common in spring and autumn.

The rhabdovirus that causes SVC enters the fish through the gills. Other ways of contracting the disease is through blood sucking parasites like leeches and fish lice. Experimental transmission has been successful with introduction of infected fish, immersion into effected water and through intraperitonal injection. It is said that application of the virus to lesions on the skin did not result into transmission of the virus. It is also suggested that the virus can be passed on from female parent, to egg to juvenile.

Last Updated on Friday, 14 November 2008 08:40