KOI HERPESVIRUS (KHV)
It is natural for every Koi keeper to be concerned if their Koi become listless and are not feeding normally. Taking panic measures is a common reaction particularly if mortalities have occurred. However, keeping calm and assessing the possible causes for a change in Koi behaviour is the best way to help them. Avoid well intentioned advice as it can result in too many suggestions. Often a worried Koi keeper uses a succession of pond treatments and luck is needed for any medication to work if the problem is unknown, and the condition of the fish can be made worse by unnecessary chemical exposure.
It is impossible to diagnose KHV by comparing signs in Koi during a suspected outbreak of the virus with pictures of a confirmed case as, the classic signs of the virus are not observed in every outbreak. Koi can appear normal or exhibit only a slight change in their behaviour or may be obviously ill. This is due to the fact there are now several strains of KHV and cases can differ considerably. What are regarded as essential signs that must be present in Koi during a KHV outbreak are misleading and are not based on knowledge or experience in fish virology. Lethargy, loss of appetite, excess mucus, and sunken eyes can be observed although such signs are also seen during diseases other than KHV. In some outbreaks, the gills may haemorrhage or develop grey patches that can indicate necrosis and changes to the skin can also occur. The water temperature is the primary trigger for an outbreak between 18-27°C (64-80°F) although the virus can take 12-14 days to spread due to influence of the pond conditions. In large ponds and lakes, the incubatory requirements for KHV such as the water temperature might never be met and any carriers of the virus in residence might never trigger an outbreak.
Assess the Koi and their living conditions
- Test the pond water for ammonia, nitrite, a swing in pH or KH, and a drop in the dissolved oxygen level.
- If a recent water change was carried out, have chlorine and chloramine been eliminated?
- Check the pond for anything unusual for example, leaves, berries, blossom, many plants and garden products, can be toxic to fish.
- Has there been heavy rain recently or has the pond water suddenly been discoloured?
- Consider if the odd behaviour in the Koi began before or after a new item was installed in the pond.
- Has the pond or filter had recent maintenance?
- Have pond treatments been used recently including a blanket weed product or buffering agent?
- Have new Koi been introduced and are they also off colour or is it only the original Koi that are not behaving normally?
- Do any Koi have excessive mucus on their skin as this can be caused both by nitrite and also by certain parasites?
- Are the Koi irritated and flicking as this can indicate either a water quality issue or parasite infection?
- Take mucus smears if a microscope is available or ring a dealer to see if he offers this service or a company that offers a pond side service. Many dealers will not want sick fish in their premises unless they have separate facilities.
- Only when all the triggers of abnormal behaviour are eliminated, should disease be regarded as a possibility. If KHV is suspected panic measures will not help and there is nothing to be gained by moving the fish from their pond.
A post-mortem examination on Koi that are not going to survive provides the most reliable diagnosis as it is a fact that other diseases and conditions can mimic some of the signs of KHV. A polymerase chain reaction test or PCR is the primary diagnostic tool used for suspected cases of KHV. It is important to note that, with several strains of KHV now infecting Koi, there are cases that can prove difficult or even impossible to test. Viruses have the capacity to mutate and change in order to enter new hosts and it is this capacity that makes their study extremely complicated. Samples from fish, in which the signs suggest KHV, are amplified so sufficient material is available to compare with DNA sequences unique to specific viruses. This test confirms positive outbreak and if the result is negative a new investigation is required as KHV has been eliminated as the cause of the disease.
Dealing with disease
There are many claims made that cold sore remedies, large doses of Chloramine T, and lowering or raising the water temperature, are cures for KHV and these are all untrue. However, it can be beneficial to increase the water temperature to around 30°C (86°F) and hold it for approximately 3 weeks. This is not a cure; however, the use of heat speeds up the disease process so it is over sooner. This helps Koi keepers as it is not pleasant to observe an outbreak and there are also some benefits for any surviving Koi that have been noted. The use of anti-viral and bactericidal products in the pond are not a cure although they might lower the cross infection level a little or ease secondary health problems. In addition, small frequent water changes will improve the water conditions as they can deteriorate during outbreaks of KHV.
The Koi immune response
The immune system is naturally stronger in some Koi at fighting any disease and many will have the capacity to survive KHV depending on the severity of the illness. During a KHV outbreak, certain Koi can appear seriously ill while others will cope better although this pattern does not always indicate which fish will recover. If a newly introduced Koi is in the early stages of KHV it may well become a victim of the disease it is spreading to other Koi. However, a carrier of the virus can live in a pond for years and not shed the virus until all the criteria are met and an outbreak is triggered. There are variable situations in which KHV outbreaks occur that reinforce the fact that in many ponds the source of KHV might never be known.
Survivors of KHV carry the virus for life and this means they could infect naive Koi not previously exposed to the virus when the criteria are appropriate. From research carried out in the LFH facilities over many years there are changes in survivors of KHV and this includes their capacity to cross infect other Koi. However, it is impossible to know when such changes will take place in any Koi and a virus is an extremely complex and unpredictable pathogen. Therefore, housing KHV survivors apart from naive Koi is the only safe preventative measure. Many fish diseases are host specific with some susceptibility existing in closely related species. In respect of KHV, ghost Koi as well as common and mirror carp can also contract KHV.
KHV is a notifiable disease in the UK and Cefas an arm of Defra need information that helps prevent the spread of the virus and their main concern are the outlets that sell Koi not private ponds. In trade premises, after KHV, all Koi that pose any risk have to be humanely euthanased. Koi keepers can be concerned that this also applies to their Koi if they contracted KHV when this is not the case. It is acknowledged that all fish in private ponds are pets and no Koi keeper has to euthanase their surviving Koi. However, it is important that all the survivors remain in their pond for life after a KHV outbreak along with other pond fish species that live with them even though they cannot contract KHV. If a new pond is built for the survivors so the main pond can be sterilised and restocked, all nets and other equipment must be kept exclusive to the survivors’ pond. Hygiene precautions between ponds must be adhered to as water droplets could spread the virus and hands should be washed after working in the survivors’ pond. This situation sounds difficult although many Koi keepers have made it work.
Cleaning after KHV
When there are no survivors or they have been euthanased and the pond is to be restocked, cleaning is vital. The KHV virus dies quickly in an empty pond, however, due to the nature of viruses, certain elements can be left behind. Thorough cleaning of the entire pond and filter system should include either the use of caustic soda, or an anti-viral disinfectant product. Renewing the filter media if possible and allowing the pond to dry in the sun can also help. It is important to include pond equipment in the cleaning such as pumps, nets, bowls etc. The pond must be rinsed thoroughly before refilling with water and both chlorine and chloramine removed before restocking with new Koi.
It is possible to test healthy Koi to see if they carry the KHV antibody as that potentially could trigger a future outbreak. However, this type of test differs to that used for live active KHV. Many factors have to be considered and the living conditions of the Koi must meet the criteria before antibody tests can be carried out or they will not be conclusive. A major drawback for both Koi dealers and hobbyists is that testing individual Koi cannot reflect the health status of other fish in the same pond. The only way to know if a whole pond is totally KHV free is to test every Koi and that is impractical. This means that antibody tests are usually carried out only in very specific situations.
It is a sad fact that KHV is in farms in most Koi producing countries including Japan. It is impossible to control a virus once it is in a farm and this means that, no matter how assiduously eradication is attempted, there will be KHV carriers in some fish stocks for some time to come. After importation into the UK, a period of biosecure quarantine reduces such risks and lowers the opportunity for other health problems to develop after the Koi are sold. However, quarantine has numerous drawbacks for the Koi trade and not all dealers are able to carry out a full quarantine period. This means there are no standard practices in use in this country and only a few dealers are able to follow a recognised quarantine protocol. Whilst Koi dealers are more aware of health issues, Koi are also sold from non specialist outlets. There have been fewer outbreaks of KHV in newly imported Koi in recent years. However, there has been an increase in the cases occurring due to the sale of second hand Koi. Swapping fish with other Koi keepers and housing a friend's Koi while their pond is upgraded are not safe practices. Even the gift of a Koi, unless it has been purchased wisely from a biosecure Koi dealer, may not be safe.
Information on fish diseases often contains inaccuracies or is not up to date and this is not helpful to Koi keepers. It makes sense not to focus on any specific disease such as KHV and to accept that it is a proven fact that health problems of any kind are rare in a well-managed pond. Koi need to live in optimum conditions and then the only risk is the introduction of disease in new Koi which can be minimised by a wise choice of Koi dealer.