Fish Viral Disease

Information on fish viral diseases such as koi herpes virus (KHV), fish pox, lymphocystis, and dwarf gourami iridovirus (DGIV).

The koi herpes virus is a highly infectious virus that only affects the common carp. Via cbransto/Flickr
The koi herpes virus is a highly infectious virus that only affects the common carp. Via cbransto/Flickr

Fish viral diseases are impossible to treat directly; the best you can do is avoid buying infected fish and quarantine all new fish prior to putting them in your fish pond or aquarium.

Identification and Transmission

Although fish with viral diseases may display symptoms the aquarist can recognize, positive identification requires biomedical tests beyond the abilities of the average fish hobbyist.

Viruses can be transmitted between fish by direct contact, or by water or wet objects being moved from one fish aquarium to another. In situations where the virus is known to be extremely contagious, as with the koi herpes virus (KHV) and dwarf gourami iridovirus (DGIV), strict quarantining procedures are essential.

Triggering Factors

At least some fish viral diseases appear to be prompted by poor aquarium conditions. The most common disease in aquarium fish, Lymphocystis, is usually associated with heavy metals including copper, poisons such as insecticides, and consistently poor aquarium water quality.

There is also some evidence that inbreeding makes fish more vulnerable to fish viral infections. This may be particularly relevant where “fancy” varieties of ornamental fish are concerned.


There are no cures for fish viral diseases in ornamental fish, though research into a KHV vaccine is ongoing.

Koi Herpes Virus (KHV)

The koi herpes virus is a highly infectious virus that only affects the common carp (Cyprinus carpio), including such ornamental fish species as koi and mirror carp. Goldfish are not affected by KHV, and they do not appear to be carriers of the virus, either.

Typical symptoms of the koi herpes virus include bloody or discolored gills; sunken eyes, pale or bloody patches on the skin and damage to the internal organs. Confirmation of a KHV infection is only possible using biomedical techniques, such as testing the fish’s blood serum for antibodies that the fish produces in response to the KHV infection. Because KHV is highly contagious, quarantining new fish and isolating any suspect fish is extremely important.

Symptoms of the koi herpes virus usually only manifest during warm weather when the water temperature is between the mid-60s and low-80s Fahrenheit. KHV has a very high mortality rate – at least 80 percent. But some aquarists have said that increasing the water temperature into the mid-80s can help, presumably because the immune system works quickly enough to develop the antibodies that fight the fish virus before too much harm is done. But even if a fish becomes asymptomatic, it remains a carrier and can infect other carp. Such fish will need to be kept in permanent isolation or with species that are not susceptible to the disease, such as goldfish.

Fish Pox

Fish pox (also known as carp pox) is a fish herpes virus that normally only affects cyprinid fishes, including koi and goldfish. It is not particularly infectious and is not life-threatening.

The symptoms of fish pox include white, gray or pink growths on the skin and fins. These growths are often likened to the color and texture of molten white candle wax. Fish pox infections appear to be chronic but cyclical; infected fish periodically display the symptoms and then recover within a few months.

There is no cure for fish pox, though raising the water temperature will speed up the progress of the infection to the point where the fish becomes asymptomatic. There is no guarantee that the infection will not flare up again sometime in the future.


Lymphocystis is the most common viral disease observed among saltwater fish, though advanced freshwater fish (perciform fish, such as cichlids and gouramis) may suffer from lymphocystis, as well. It is not particularly infectious, but instead seems to develop in response to poor environmental conditions. Among wild fish, heavy metals, poisons and consistently poor aquarium water quality have all been identified as possible triggering factors of lymphocystis.

Lymphocystis causes the body or fins of the fish to develop wartlike growths. These growths often have a characteristic light brown coloration and a rough, cauliflowerlike texture. The disease is not life-threatening, unless the growths obstruct a major body opening of the fish, such as the anus.

Provided aquarium conditions improve, Lymphocystis infections usually go away by themselves, though this may take several months.

Dwarf Gourami Iridovirus (DGIV)

The quality of the dwarf gouramis in the trade has steadily declined for years, with batches of fish showing significantly higher levels of mortality than 10 years ago. Historically, retailers and aquarists have blamed bacterial infections, such as fish tuberculosis (Mycobacterium marinum). In recent years, though, attention has focused on a virus known as dwarf gourami iridovirus or DGIV.

Dwarf gourami iridovirus is apparently specific to the dwarf gourami (Colisa lalia), including the various fancy varieties of the species, such as neon gouramis and sunset gouramis. Infected fish develop a variety of symptoms, including loss of color, decrease in activity and appetite, the appearance of sores and lesions on the body, abdominal swelling and finally death. This fish disease is highly contagious, completely untreatable and invariably fatal.

Dwarf gourami iridovirus is apparently very common. One recent study of fish exported from Singapore found that 22 percent of all dwarf gouramis carried the virus. Aquarists should never purchase dwarf gouramis from fish aquariums containing fish exhibiting symptoms consistent with the dwarf gourami iridovirus, and all new fish should be quarantined for at least six weeks prior to being placed in the main fish aquarium.

For most aquarists, my best advice is to keep the hardier alternatives to dwarf gouramis. The thick-lipped gourami (Colisa labiosa) and the banded gourami (Colisa fasciatus) are both similar in size, temperament and coloration and make excellent alternatives.

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Fish · Health and Care

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