Koi Herpes Virus
article by Dr Paula Reynolds Aquatic Patho-biologist
Never jump to conclusions
Experienced dealers and hobbyists are well aware of the need to eliminate the obvious possibilities when Koi appear to die suddenly. Ammonia, nitrite and pollution caused by toxic substances leaching into the pond acid rain and even garden plants are all worth consideration whenever there are high mortalities in any pond or tank. In addition parasites such as whitespot have the potential to kill quickly and are an extremely common problem in newly purchased fish as Koi can harbour the whitespot organism whilst in perfect health and the new environment trigger an outbreak. Gill conditions associated with transportation often give rise to high mortalities. Only when all the likely causes have been eliminated should Koi Herpes Virus (KHV) be regarded as a possibility.
The signs of KHV
Koi Herpes Virus does not behave identically in all outbreaks and disease signs vary depending on how acute the outbreak is. Some Koi have little or no external indication of disease whilst in other cases the gills may haemorrhage, particles of gill tissue are shed, skin changes are observed and after death the gills are found to be necrotic. However the most common signs in nearly all affected Koi are lethargy, food refusal and excessive mucus. Water temperature is the main trigger factor in all outbreaks of KHV which occur between 18-27C/60-80F. Depending on the water temperature and the form in which the virus presents it can take 12-14 days or even longer for the infection to spread although it can break out sooner in situations in which all the incubatory criteria are met.
In order to diagnose Koi Herpes Virus it is necessary to sacrifice fish for post-mortem examination to eliminate all other possibilities and for samples to be tested. There are various testing methods that can be employed with a polymerase chain reaction test the most likely preliminary test. . This test requires a sample of genetic material from the fish containing DNA and this is replicated using polymerase material to produce sufficient DNA for analysis. The DNA sequences unique to a specific virus such as Koi Herpes virus are then revealed. In some cases if the PCR is positive for KHV a viral culture is then carried out. Despite the presence of inclusion bodies which strongly suggests a viral agent is the culprit, confirming KHV to be the actual causative pathogen is not possible in all outbreaks for a variety of reasons. In addition it is possible to test for the presence of the KHV antibody in healthy Koi but such tests can only reveal if the Koi have been exposed to KHV in the previous 6-9 months.
The spread of KHV
The mortality rate in most infected ponds or tanks is usually high although susceptibility to the virus depends very much on the genetic history and immune response in each exposed fish. Some Koi have a natural immunity to the virus and will appear totally unaffected and continue to feed whilst other fish around them are severely ill. Whilst the introduction of a new carrier Koi is the prime cause of an outbreak it is also possible for any Koi that has lived in a pond for years to be the carrier and disease did not break out until the incubatory requirements were all met particularly the appropriate water temperature, this is common in larger ponds and lakes. In such cases the disease is likely to cause mortalities in the new naive Koi so the disease progress is not identical in all cases.
It is no longer a safe practice to mix Koi from around the world together .Many outbreaks of what is assumed to be KHV are never confirmed scientifically but disease is now widespread in the U.K due to the continued import of Koi carrying the disease. Whilst KHV is the prime factor there are other heath problems that materialise when Koi from various sources are mixed in the same environment. It is safer to house valuable, prize winning or much loved pet Koi that are Japanese in origin apart from Koi from other sources as currently Japanese Koi are not deliberately exposed to it and are therefore, susceptible to the disease. KHV is highly infectious and sharing pond equipment, hospitalising fish for those without the facilities to care for sick Koi are a common factor in its spread.
Koi Herpes virus (KHV) as the name suggests is one of several herpes viruses which are the most common type of DNA virus causing disease in fish. The disease we commonly term carp pox, which causes the unsightly shiny white tumours in colder weather, should more correctly be called Cyprinid Herpes Virus (CHV). Only Spring Viraemia of Carp (SVC) which can affect carp including Koi is a notifiable disease in the U.K and CEFAS must be contacted if an outbreak of SVC is suspected. However, this disease is rare in a hobbyist’s pond so seek advice before contacting a busy government agency with what may be a common health problem. KHV is not a notifiable disease in the U.K.
Koi that survive KHV can pass disease on to any Koi from any source that has never been exposed to the disease previously. Therefore survivors of KHV should be housed apart and all equipment kept exclusive or they should be euthansed. The decision to euthanase every fish during a viral outbreak in case of a future incident must be a personal one for hobbyists but it is the only course of action in trade premises. Koi surviving KHV are not continuously infectious and only spread the virus during an outbreak of the disease. The time scale in between outbreaks varies and depends very much on the environment in which the Koi live and the fluctuations in water temperature. However Koi that have survived KHV rarely show signs of disease and as a consequence are not regarded as a likely source of disease. They appear very healthy but are in fact dangerous because they can fool both dealer and hobbyist.
Restocking the pond
It is possible to restock the pond with fish species that cannot contact KHV such as sturgeon and Orfe etc. In addition Koi from farms that subject their Koi to KHV before export can be housed with survivors and whilst KHV will break out the disease will be very minimal and the Koi likely to survive. The KHV virus dies very quickly in an empty pond devoid of all Koi however after a total mortality due to KHV if the Koi-keeper is anxious to restock the pond immediately power washing the pond and filter and then sterilisation with caustic soda or a strong disinfectant is advised. Possibly renewing the filter media is also a safeguard then after thorough rinsing the system can be restarted.
There is no treatment for KHV at the present time. Claims that cold sore remedies or massive doses of chloramine T actually cure are inappropriate as any resulting recovery from the virus is in fact a natural one that would have taken place anyway. Anti-viral pond treatments can be tried to reduce the severity of an outbreak and these can lower the mortality level. In some cases medications can be used to treat secondary health problems, which arise frequently to complicate both the diagnosis and the recovery phase. It can be beneficial for several reasons to raise the water temperature to approx 87F / 30C and hold it there for approx 3 weeks as soon as possible during an outbreak of KHV. This is not a cure but the disease will be over more quickly and as it is traumatic to observe the benefit is obvious although impractical for many hobbyists and irrespective the fish will still be carriers.
Our ability to control viral disease lies mainly in prevention rather than cure. Immunisation against some viral diseases is a reality in humans and animals but in fish, this is a fairly new field. Water creates an environment that changes the rules on disease transmission that apply to humans and land living animals increasing the potential for cross-infection. The susceptibility of each fish depends greatly on their genetic history and innate immunity. In addition, acquired immunity increases the capacity of the immune system to fight off pathogens including virus through exposure to disease as the fish matures. Although a generalisation the closer Koi are genetically to the common carp the more likely they are to survive a virulent viral disease, whereas we can predict that even inbred Koi will survive an outbreak of carp pox. Vaccines are currently being investigated along with other control measures in respect of Koi Herpes virus but this will take time to ensure they are legal, effective and safe.
How did my Koi contract KHV
High mortalities are a sad experience for any Koi–keeper and a financial burden for U.K. dealers and the world wide Koi industry. There is a tendency to blame the last Koi that was purchased whenever a health problem strikes when this fish is not always the culprit and in many cases it is the Koi that is least suspected that harbours disease . Newly imported fish pose the greater risk because of the variety of sources and the stress of environmental changes. That is why a period of quarantine is still a good safeguard and nowadays quarantining should be an accepted aspect of the hobby because of the increasing risk of all health problems not just KHV. Whilst quarantining lowers the risk posed by new Koi many outbreaks of KHV have been caused by carrier Koi already living in ponds and trade premises disease being triggered by increased water temperature .Whilst the carrier often survives it is the newer Koi naive to the virus that are killed. This scenario is confusing and makes it difficult to establish with certainty how any outbreak was initiated and it is this fact that has perpetuated the sale of carrier Koi preading KHV around the world.
Epidemiology - The science of disease transmission
DNA - Deoxyribonucleic acid-Genetic material found in most organisms
CHV - Carp Herpes Virus or Carp Pox
KHV - Koi Herpes Virus
SVC - Spring Viraemia of Carp
CEEFAS - Government Laboratory -Weymouth Dorset
Notifiable Disease - Classification of disease implying government must be advised of outbreaks but not applicable to KHV