Viral Infections

Gill damage caused by KHV - photo © Michael Emerton (huso huso)

by Terry White & Karen Paul

Koi Herpesvirus (KHV)

The Diseases of Fish (England and Wales) Order 2007 (SI 2007 No.864), which came into force on 6 April 2007, makes it a legal obligation for any person, including fish farmers or their staff, fish dealers and fish keepers to notify the Fish Health Inspectorate if they suspect that fish may be infected with KHV (Koi Herpesvirus) Disease.

What is KHV?

KHV (Koi Herpesvirus or CyHV-3) is a double stranded DNA virus belonging to the virus family Herpesviridae (herpes virus). It is a highly contagious disease and can cause 80-100% mortality in affected populations.

The virus is transmitted fish to fish very rapidly and usually occurs at temperatures between 15°C and 27°C. Studies by Ayana Perelberg and colleagues from the Hebrew University in Israel have shown that exposure of common carp for five minutes to 10-40 virus particles per ml of water is enough to cause approximately 70% mortality in fifteen days.

As with other herpes viral infections KHV is believed to remain in surviving infected fish for life. Any exposed fish should be therefore be considered as potential carriers of the virus.

How does the virus spread?

KHV can be transmitted by direct contact with infected fish, with fluids from infected fish or by water and material from infected systems.

What fish are affected?

KHV affects koi and common carp (Cyprinus carpio).

Other cyprinids such as common goldfish (Carrassius auratus) and crucian carp (Carrassius carassius) seem to be immune to the disease but can be infected with the virus and may become carriers.

Other fish that may become carriers include grass carp (Ctenopharynodon idellus) and tench (Tinca tinca).

What are the clinical signs of KHV?

KHV infection usually results in severe gill lesions and high mortality rates. Secondary bacterial and parasitic infections may also be present. Affected fish often remain near the surface, swim lethargically, and may exhibit respiratory distress.

Diagnosis

A carp that died of KHV - photo © Michael Emerton (huso huso)
On this carp that died of KHV there were no external signs but the gills were
severely damaged (see above) - photo © Michael Emerton (huso huso)

KHV can be very difficult to diagnose. Visual examination is unreliable because the typical signs are also seen in a number of other carp diseases. Some fish can carry the virus for long periods without showing any signs of disease.

The only reliable method to confirm the presence of KHV is by using state-of-the-art molecular biological techniques. Cefas, through its subsidiary company, Cefas Technology Limited, is offering tests for disease diagnosis to clients both within and outside the UK.

Three types of test are currently available:

KHV diagnostic PCR test: This polymerase chain reaction (PCR) test is performed on tissue samples from clinically affected fish, or fish showing signs of Koi Herpesvirus (KHV). The fish have to be killed for the test.

KHV screening PCR test: This polymerase chain reaction (PCR) test is designed to provide a disease status check to those dealing with large quantities of koi or common carp.

This test is for a large quantity of fish and requires either 30 or 150 fish to be supplied. The fish have to be killed for the test.

KHV ELISA test: ELISA stands for Enzyme Linked Immunosorbent Assay. This assay is a rapid, high throughput test for antibodies, and requires just a blood sample. The blood sample can easily be taken from live fish.

This test is for as minimum of 10 fish. The fish do not have to be killed for the test.

Full details and secure online ordering of the disease diagnosis services are available on the Cefas Technology Limited website.

Treatment

There is no cure or treatment for KHV. There are no antiviral drugs available to treat KHV or any other viral diseases of fish.

Studies by Ariel Ronen and colleagues from the Hebrew University in Israel have shown that fish may develop a natural resistance after exposure to KHV if water temperatures are increased to 30°C. However, this technique only slightly increases survival rates and artificially raising water temperatures above 30°C can result in an increase of other more common bacterial and parasitic diseases and is not generally recommended. More importantly the surviving fish may become carriers of the virus even though they do not develop any clinical signs. These carrier fish may then spread the disease to other fish that have not been exposed.

Vaccination

There are no KHV vaccines currently licensed for use in Europe.

KV3, an attenuated (reduced in strength) virus vaccine, has been developed by Prof. M. Kotler from the Hebrew University in Israel. The way the vaccine was developed was to attenuate it with many passages, identify a particular clone and than continue its attenuation by irradiation with UV light.

Carp exposed to the KV3 vaccine have been imported into the UK but Defra have warned fish farmers and fishery owners that there are risks associated with the import of vaccinated carp. It is not yet known if vaccinated carp could be a risk to indigenous fish in the UK environment.

Tests for KHV are not able to distinguish between previously infected fish and those that have been vaccinated. I would not buy vaccinated fish.

There are products on the market claiming to protect your fish from infection. There is no independant evidedence to back up such claims. I repeat; There is no cure or treatment for KHV. There are no antiviral drugs available to treat KHV or any other viral diseases of fish.

Quarantine

Quarantine is the best way to avoid infecting your fish with KHV and other infections. There are no guarantees though, once a fish has been infected with KHV it will most likely be a carrier and no period of quarantine will rid it of the virus or remove its ability to infect other fish in the future. Another sensible precaution is to only buy fish from trusted suppliers and take the time to find out about the general health of their stock. Have they had any unexplained losses recently?

For effective quarantine all new fish must be kept in a separate tank or system and away from your existing fish. Existing fish should be fed, handled, and maintained before the new fish. Use separate equipment such as nets, buckets, and hoses for the quarantined fish. New fish should be quarantined for a minimum of 8-9 weeks. Also remember that KHV normally only shows at temperatures between 15°C and 27°C.

During the quarantine period you should introduce one of your existing koi to check if the new fish are carriers.

Carp Pox

Carp pox (Cyprinid herpes virus 1 or CyHV-1) is closely related to KHV although not as infectious and, although unsightly, is not harmful to the fish. Once infected the koi will have it for life. Although carp pox is contagious, it will normally only spread to young fish with immature immune systems or older fish with weakened immune systems.

Carp pox is often brought on by stress or dropping water temperatures, often occurring during winter and spring when the immune system is at it's weakest. The condition tends to improve when temperature rises in spring but not in every case, some fish may be affected all year round. As the koi matures it's immune system will usually keep the virus under control but it may flare up again at times of stress when the immune system is weakened.

Carp pox shows itself as whitish, waxy, wart-like lumps anywhere on the skin, mouth and fins. It looks rather like blobs of candle wax, jelly-like in appearance but solid. The severity can vary hugely from a few tiny pimple-like spots to clusters of large 'warts' that will sometimes show a similar pigmentation to the surrounding skin.

Don't waste your money on so called treatments. There is no cure for carp pox although it will often be improved by raising the water temperature. Do not try to remove the lumps, it could spread the virus around the pond and damage the skin which will leave it open to more serious infections.

Spring Viremia of Carp (SVC)

SVC (Spring Viremia of Carp) is caused by Rhabdovirus carpio, a bullet-shaped RNA virus. SVC affects common carp and koi (Cyprinus carpio), grass carp (Ctenopharyngodon idella), crucian carp (Carassius carassius) and goldfish (Carrassius auratus). SVC is a notifiable disease. It is highly contagious and deadly.

Goldfish Herpes Virus (GHV)

Goldfish Herpes Virus (GHV, which is also known as Cyprinid Herpes Virus-2 or CyHV-2) is closely related to KHV (CyHV-3) and Carp Pox (CyHV-1). GHV is to goldfish what KHV is to carp. It is thought that GHV only affects goldfish but Cefas are currently carrying out research into the possible susceptibility of crucian carp(Carrassius carassius). GHV is not currently a notifiable disease.