Koi Herpes Virus (KHV)

Why suspect KHV?

Whenever there is a problem in the Koi pond causing high mortalities, it is vital to explore all avenues before jumping to the conclusion a serious disease such as a virus, is the culprit. Ammonia, nitrite, pollution by various toxins and parasites all have the potential to kill many fish quickly. Whitespot for example is an extremely common health problem in newly purchased Koi. A new environment can trigger an outbreak of Whitespot even if the Koi were in perfect health when purchased. Bacterial gill disease also accounts for high mortalities. Only when all these possibilities are eliminated should KHV even be considered.

What are the signs of KHV?

KHV does not behave identically in all cases. The external signs can vary depending on the form the disease takes and how acute the outbreak is. In some cases infected Koi show no external indication of illness at all whilst others suffer skin changes including dry patches and severe gill necrosis. Commonly, nearly all infected Koi become lethargic, refuse food and exude mucus. However there are many reasons for Koi being off colour, listless and refusing food without there being anything serious wrong. Outbreaks of KHV commonly occur between 18-27C/60-80F and depending on water temperature and the manner in which the virus presents, the incubation period is 12-14 days. Mortality levels are usually high but this is not the case in attenuated outbreaks.

How did my Koi contract KHV?

There is a tendency to blame the last Koi purchased for introducing any serious disease but new Koi are not always the culprit. KHV can be carried by an asymptomatic Koi that has been in the pond for many years. This type of outbreak is triggered as and when conditions become conducive to the virus. Introducing unquarantined Koi is a large factor in outbreaks as their health is unknown. Dealer quarantine is excellent when carried out to a high standard by biosecure dealers. However, not all dealers can meet the criteria and the cross - infective risks in some trade premises are such that a further period of home quarantine is advisable. Nowadays the potential for all health problems not just KHV is higher and even gifts of fish from a friend should be quarantined, as their sources are unknown and have to be regarded as suspect.

Can it be cured?

Claims have been made for a variety of “cures” for KHV but cure is not the correct word. As with most viruses, chemicals have little effect and recovery is natural in fish that are able to mount a sufficient immune response. However, in some outbreaks it is possible to treat the secondary health problems that can complicate both diagnosis and recovery. Such treatment can appear to those inexperienced with viral disease to bring about a cure when in fact the Koi were in recovery anyway. Antiviral drugs can help to alleviate the disease but will not cure KHV. Antimicrobials and antibiotics may help with any bacterial secondary infections but are not actual cures for a virus.

How soon can I restock?

Hobbyists are keen to move on after KHV and ask how soon the pond can be restocked. Situations vary but in the case of a total wipeout, drain the pond and power wash every surface and the filter thoroughly. Use a disinfectant/virocidal product when cleaning. The brand is of less relevance than the thoroughness of the cleaning process but caustic soda or chlorine-based products are easy to obtain. After thorough rinsing and possibly renewing the filter media, if it is a type that cannot be cleaned properly, the system can be restarted. Include all pond equipment nets and bowls in the cleaning process. When possible leaving the pond to dry completely after cleaning in sunlight is also effective.

Survivors of KHV

In cases where Koi survive KHV, they must be regarded as carriers and a decision made about their future. Koi that survive KHV cannot live in the same pond as Koi naive to KHV. However, survivors can be housed apart as long as they have totally separate equipment and the two ponds cannot ever share the splash factor when Koi are up to their usual antics. The decision to euthanise is a personal one for hobbyists that cannot house survivors and want to buy new Koi. Whilst there is no requirement for any hobbyist to have to destroy pet fish in all trade premises stock surviving KHV must be humanely destroyed. Although a generalisation, the closer Koi are genetically to the common carp the more likely they are to survive a viral disease such as KHV. Susceptibility to KHV depends on DNA and Koi, mirror carp, common carp, and ghost Koi are all susceptible. However, a virus is a highly complex pathogen and subject to mutation and there are possibilities for viruses to survive in other host species even if they cannot contract the disease. Such factors add to the difficulties for understanding let alone curing viral diseases.

How can I know for sure my Koi have KHV?

A Polymerase Chain Reaction (PCR) is used when Koi are in an advanced stage of disease and there are different types of PCR available. The drawback of this test is that it will be negative in healthy fish, as the test does not establish the presence of virus unless disease is actually in progress. This makes the test unreliable as a tool in preventing the spread of KHV, none the less, it is extensively used to screen healthy Koi. The ELISA test is used to screen for previous exposure to KHV as opposed to confirming the actual disease state and this is achieved by looking at antibody titres. All tests pose slight drawbacks and other herpes viruses have been found to create a false positive for KHV. Such results are rare but can make difficulties for dealers regarding the health status of certain Koi. The PKDA Consultant has carried out trials of new testing techniques. This is extremely costly and she reports that private funding has to be raised for such ventures. Research is painstaking and time consuming, but cost effective yet reliable alternative testing may be available in the near future.