Veterinary Support - Koi Herpes Virus

What is a virus?

A virus is one of the smallest particles that we know of that is capable of reproducing itself. It does this by "hijacking" the machinery found in all cells that allows the cell to replicate itself. Viruses are so small that in one survey carried out to find the normal background level of a virus called IPN in one millilitre of seawater over 50 million IPN viruses were found. Viruses can only be seen by looking through an electron microscope.

What is KHV?

KHV is part of the Herpesvirudae or herpes virus family. It first came to international attention in 1998 although there had been probable clinical outbreaks in both the UK and Belgium during 1996. One case in the UK during 1996 has since been confirmed as KHV.

KHV can also be called Cyprinid herpesvirus 3 (CyHV-3) and would appear to be related to CyHV-1 the cause of carp pox and CyHV-2 which causes illness and mortalities in goldfish. Recently at a meeting of the European Association of Fish Pathologists it was decided that phylogenetically it should continue to be called KHV.

What does it cause?

A classic outbreak of KHV can occur at water temperatures between 18oC and 25oC and only affects Cyprinus carpio (i.e. common carp and koi carp). Commonly it occurs after recent introductions of fish and after rapid alterations in water temperature. However there are confirmed cases where there have been no introductions of fish for several years. Any survivors are likely to be carriers of the virus and infective for other fish. In an outbreak one would expect morbidity (number affected) of 100% and mortality (death) up to 80%. The incubation period (time from infection to clinical symptoms appearing) is fourteen days although this can be reduced to ten to twelve days at higher temperatures. Where naïve (never been exposed to the virus) fish are introduced to a pond with a concurrent infection (i.e. infection already present) then they can exhibit clinical symptoms in three days. It is also possible to produce the disease within four days by introducing fish into water containing the virus.

Typically one sees a severe gill necrosis and the skin of the fish will feel similar to sandpaper. Alternatively the mucus layer will be thickened with "blebs" of mucus apparent. There may be ulceration of the skin present with some fish showing a severe external parasite infestation or no parasites at all. Internal pathology is non specific but similar to what one would expect from a viral infection. It is possible to see on histology slides a "signet ring" appearance typical of herpes virus infection.

Additional symptoms can include anorexia for ten days prior to clinical symptoms, erratic uncoordinated swimming and acute death. Another clinical sign is that fish other than Cyprinus carpio appear clinically healthy in an outbreak.

At the time of writing it would appear that the temperature "window" at which clinical signs are seen has widened and that the percentage mortality has fallen. This would indicate either that the pathogenicity (ability of the virus to kill) has lessened or that the general population of koi has a greater immunity (resistance) to the virus than previously.

Where is it?

Currently the only continent that has not seen a clinical outbreak is Australasia and they do not allow koi importation. It can therefore be considered as having a worldwide spread and more importantly to the koi hobbyist at present no producer can be considered safe from the infection. To date there have been at least thirteen outbreaks in the wild within the UK and a very worrying development has been the detection of the virus after a fish kill in a canal connecting the Rhine and Danube rivers. This would indicate that the virus might be widespread across the whole of Europe. To date no infections have been recorded in Eastern Europe.

Is there a treatment available?

The simple answer is no. To date several antivirus agents have been tried with limited success and it needs to be remembered that they are not licensed for use in fish thus giving rise to concerns over safety. It is possible to control an outbreak by manipulating water temperatures either above or below the temperature "window". Although this may save some fish it still leaves the hobbyist with a dilemma of what to do with any survivors. Any saved fish are likely to be carriers and infectious to other fish so they must be kept in isolation.

There are three laboratories working on a vaccine in the USA, Israel and Japan as well as at least one commercial company. To date none have been successful although current indications are that there might be a vaccine developed by late next year. From experience I would not hold my breath.

How do I protect my fish from it?

How KHV is spread between ponds is as yet unclear. Obviously it can be transmitted by fish to fish contact and from infected water. There is some indication that it can survive for up to four hours on the hands if dipped in water containing the virus. It has been shown to survive in river water and mud for three days. It can be found in fish at least one year after exposure. Given the worldwide distribution of the virus and the fact it is found in the wild in both the UK and on the continent then there is no certainty that you can avoid contact with the virus. However there are several things that can minimise the chance of your fish being exposed to the virus:

1. Apply strict quarantine to your fish pond. This means that you should not introduce any new fish to the pond, share any equipment or allow any fish keeping friends from dipping their hands in the water.

2. If you wish to introduce a new fish then you should quarantine it for at least two weeks and preferably four weeks. During this time it should be kept at a permissive temperature (i.e. between 18-25oC) for at least 14 days. This should be done even if your supplier has already quarantined the fish.

3. Purchase fish from one supplier who only buys from one producer and quarantines the fish prior to sale. By minimising the possible number of suppliers in the chain you decrease your chances that the fish have been exposed to KHV.

4. Check whether your supplier is registered with the Koi Biosecurity Institute. This is a commercial agency whose aim is to ensure that fish sold have not been exposed to KHV. Although the institute has been around for a couple of years not many producers or retailers have registered or is it possible to state that the claims made by retailers or producers are correct. Further information is available from their website.

5. Buy only fish that have been exposed to KHV. Currently it cannot be said with certainty that these fish do not act as carriers of KHV and therefore are safe to mix with naïve fish.

Latest developments

With increasing political attention being given to KHV by European authorities it is likely that KHV will become a listed disease. There are two lists of concern, the first is an OIE listing (Office International des Epizooties or World Animal Health Organisation) and the second is an EU listing. The OIE listing would mean that government authorities would need to report any outbreaks to the OIE but would not have to institute a control policy unless they wished. Within the UK it is unlikely at this point that the authorities would wish to control KHV. An EU listing would have far more consequences for the trade and hobby as not only would the authorities have to report any outbreaks they would also need to have a control policy in place. This might mean that any fish with KHV or any fish in contact would be destroyed.

At the time of writing at a recent meeting of government scientists and other parties from around the world it was agreed that OIE listing should be supported. It is unclear as to whether EU listing will occur but KHV is listed in the latest draft of the EU Fish Health Regulations.

Work carried out in Germany has shown for the first time that goldfish can act as carriers although they appear immune from the disease. This has major implications but as yet has not been confirmed by other laboratories.

Although there are several tests available to confirm an outbreak of KHV such as PCR's and ELISA tests there is still concern over the accuracy of these tests, none of which are 100% accurate. It is still considered extremely difficult to isolate and grow the virus consistently so diagnosis needs to rely on clinical symptoms, histology (looking at samples under the microscope), PCR and post exposure ELISA. Not all of these tests need be positive for a clinician to diagnose or suspect KHV.

Work carried out in Japan concluded that: The virus was sensitive to disinfectants such as Iodophors, Sodium hypochlorite, Benzalkonium chloride within 30 seconds of exposure. As with all disinfectants beware that organic material in the water does not deactivate them. That exposure to heat in excess of 50oC for one minute killed the virus. UV irradiation at 4 mWs/cm2 was sufficient to kill the virus.