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. 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.   Dealers and hobbyists having eliminated all likely causes of high mortalities should only then consider Koi Herpes Virus (KHV) 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 if the incubatory criteria are all met.

Diagnosis
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. If the result of this is positive then usually a viral culture is carried out for conclusive diagnosis of KHV.  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 always possible in all outbreaks if insufficient tissue is available. In cases in which  healthy Koi  require testing to see if they have been exposed to the disease  the PCR and culture are not appropriate . There are various laboratory procedures that can be applied to the investigation which require the testing of 30 or more Koi.  In some cases hobbyists are unwilling to sacrifice Koi they feel have a chance of surviving the disease and in addition the speed of death can limit the possibilities for any examination.

The spread of KHV
 The mortality rate in most infected ponds 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 water temperatures , 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.

 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. Many outbreaks of what is assumed to be KHV have not been confirmed scientifically but the pattern of the outbreak and the mortality rate imply that the disease is now widespread in the U.K due to the continued import of Koi carrying the disease .It is now fact that Koi from various farms around the world cannot live together. 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.

Confusing terms  
 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.

Surviving KHV
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 as 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  of the  pond and filter and then sterilisation with a   disinfectant  or caustic soda is the best safeguard . After thorough rinsing and possibly even renewing the filter media the system can be restarted.

No Cure
There is no treatment for KHV at the present time. Claims that cold sore remedies or massive doses of chloramine T are actual 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 in some cases medications can be used to treat secondary health problems, which arise frequently to complicate both the diagnosis and the recovery phase.

Controlling KHV
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 is not always the culprit.   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 an outbreak was initiated and it is this fact that has perpetuated the sale of carrier Koi spreading KHV around the world.

If dealers or hobbyist have concerns about KHV
Aquatic Pathobiologist Dr Paula Reynolds can be contacted on 01205 723413