The unusually high temperatures experienced in the UK this summer has seen a number of outbreaks of Koi Herpes Virus (KHV) in hobbyist Koi ponds, as well as in natural bodies of water.  There have been reported outbreaks in several trade premises, believed to originate in fish imported from Israel, and in the past 12 months the USA has seen several outbreaks among Koi imported from Japan.

KHV has been with us for at least 20 years, having been confirmed in Israel and Germany in 1998, and in tissue samples which were taken in the UK in 1996.  The first recorded outbreak in Japan was in 2003 among food carp in Lake Biwa.  KHV has since been identified in at least 28 countries, a list of which includes all of the major Koi keeping countries of the world.

Gill necrosis and sunken eyes, 2 symptoms of KHV

Gill necrosis and sunken eyes, 2 symptoms of KHV

The 2003 outbreak in Japan sent shock waves through the Koi industry and resulted in some very significant changes.  The long-established practise of mixing Koi from different breeders for shipping from a central holding facility would cease as farms were required to health certify their Koi.  Koi shows would also change, the All Japan Koi Show in 2004 would see the introduction of English style shows, or the ‘my pool’ system, whereby Koi from different sources were no longer mixed together.

Before the end of decade well publicised KHV outbreaks were confirmed at both Momotaro Koi Farm and Sakai Fish Farm in Japan, and later Chogoro Koi Farm in Niigata also had an outbreak.

Sadly the 20 years that have passed haven’t provided us with a cure to KHV, it seemingly falls way down the importance chain for funding and investment in research.  Still the testing methods for detecting KHV are far from fool proof.  However, with what is known there are lots of things the hobbyist can do to minimise the risks they face of KHV getting into their personal Koi collection, and below we’ll look at some of those things.

The first and single most overriding things is NEVER introduce Koi to your pond from unknown sources.

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It is very evident that Koi can carry the virus for a number of years without encountering the conditions which will cause the virus to break out.  A significant factor in this is water temperature, research suggesting that outbreaks occur at water temperatures between 16-25°C, with 23-25°C being optimal.  According to Met Office data the average temperature in England in July and August is 20.4°C and 20.1°C respectively.  Scotland, Wales and Northern Ireland being lower.  Given this then it is clear to see that many ponds will not get warm enough, for long enough, to trigger an outbreak except in periods of unseasonably high, and prolonged temperatures, as has been experienced in 2018.

Whilst a fellow hobbyist’s collection may have always appeared healthy, or may not have had any new additions for a long time, be sure to consider the importance of time and temperature on the occurrence of KHV before acquiring a Koi from them and placing it directly into your own pond.

The more likely source of a new Koi is of course via a dealer.  It’s a very simple fact that not all dealers take the importance of quarantine equally!  Some quite ridiculously still operate as though it is the 1980’s when it was commonplace to collect Koi straight from the airport and sell them immediately, others thankfully operate a more belt and braces approach.  Hopefully you don’t need me to tell you which is the sensible dealer to visit?

By quarantining Koi prior to sale a dealer should be doing everything in their power to protect you and your collection from infection by parasites, bacteria or virus on new purchases.

In pre KHV days quarantining would have meant that the Koi were rested and parasite ‘free’.   Of course, now, as then, it’s quite normal for Koi to carry low levels of parasites which they can live with without concern.

Nowadays, if a dealer is really concerned with minimising the risk of them supplying you with KHV infected Koi then the quarantine period is costlier, takes longer and, for them, contains more risk.

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The Oxford Dictionary defines ‘biosecurity’ as – Procedures or measures designed to protect the population against harmful biological or biochemical substances.’

The World Organisation for Animal Health’s ‘Manual of Diagnostic Tests for Aquatic Animals’ states – ‘Biosecurity measures should include ensuring that new introductions of fish are from disease-free sources and installation of a quarantine system where new fish can be held with sentinel fish at permissive temperatures for KHVD. The fish are then quarantined for a minimum of 4 weeks to 2 months before transfer to the main site and mixing with naïve fish. Hygiene measures on site should be similar to those recommended for SVC and include disinfection of eggs, regular disinfection of ponds, chemical disinfection of farm equipment, careful handling of fish to avoid stress and safe disposal of dead fish.’

The reality of this for Koi dealers means that the Koi should be quarantined in a separate facility to the regular retail facility.  Nothing should be transferred between those facilities, for example nets, bowls, bags whilst the Koi are being quarantined.

Very significantly the quarantine period MUST include maintaining the Koi at a temperature range that is likely to cause the virus to break out if it is present in the Koi being quarantined, the practise of heat ramping.

It’s a simple proven fact that subjecting the Koi to cycles of heat ramping during the quarantine period increases the likelihood of KHV breaking out.

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Given the evidence, surely any dealer truly concerned with protecting their customers from the risk of KHV would heat ramp their Koi, wouldn’t they?  Whilst it is true that there is no guarantee that KHV will be exposed by heat ramping there are numerous cases where heat ramping has proven to be the key in doing so and had it not been undertaken seemingly healthy Koi would have been released into unsuspecting customer ponds like a time bomb just waiting for the right environmental conditions, conditions like this summer, to explode with catastrophic results.

Home Quarantine

If you really want to be as 100% sure of protecting yourself from the risks of introducing KHV to your own collection, the adopt a period of quarantine yourself.

Before the days of KHV then I generally considered home quarantine as a luxury.  Certainly many of the substandard quarantine systems I’ve seen used over the years did more harm than good when customers were buying from trusted dealers who had settled and treated newly imported Koi for parasites before release to the public.

As dealers have needed to take quarantine more seriously, and invest in setting up dedicated quarantine units, then perhaps for hobbyists they are also no longer a luxury.

There are excellent ready made fibreglass units available which sited in a suitable location can make ideal bio-secure quarantine systems.  Filtration systems such as the Evolution Aqua EazyPod are relatively small and compact, reasonably priced, and deliver high quality water and are perfect for quarantine unit.  It would be very simple to use a show pool and EazyPod as a temporary quarantine systems provided you have somewhere to locate it.  With a small electric heater the water could easily be raised to the required parameters.

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It’s not the end of the hobby

I remember the first outbreak of KHV being reported from Japan in 2003 extremely vividly, it was days before I was due to visit Niigata for the first time.  Fearing that we may not be able to access farms or purchase Koi in Niigata we amended the schedule and visited elsewhere.

It was easy to believe that this was the start of the end for the Koi hobby, 15 years on it clearly wasn’t, although the threat from KHV is perhaps greater than ever, certainly there is no progress in destroying the virus.  However, by adopting sensible practises, and some precautions, it is very easy for the hobbyist to almost completely negate any risk of them being affected by the virus.


Links to additional information

CEFAS Fish Health Inspectorate –

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European Union Reference Library for Fish and Crustacean Diseases –

Lincolnshire Fish Health –


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