Frequently asked questions on KHV


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Matt Clarke answers some of the most common questions on the deadly Koi Herpes Virus - KHV.

What is KHV?

Koi Herpes Virus, or KHV as it is more commonly known, is a member of the herpes virus family, Herpesviridae. As the name suggests, it primarily affects Common carp, Cyprinus carpio and it was first discovered in domestic Koi, hence the common name, Koi Herpes Virus. It is related to the viruses (or virii) that cause Carp pox and Haematopoietic Necrosis Herpesvirus.

It's been known to both the aquatic trade and scientists as KHV for several years, however, it has only recently been formerly classified and confirmed as a herpes virus. (See KHV gets new formal name in study, News, May 26, 2005)

So what is it correctly known as now?

Technically, it's no longer called KHV. The official name is Cyprinid Herpesvirus 3 (or CyHV-3 for short), however, the KHV name has been in use for such a long time that both scientists and fish enthusiasts will probably continue to call it by its old name for the forseeable future.

In the interim period before KHV was renamed as CyHV-3, scientists referred to the disease as Carp Interstitial Nephritis and Gill Necrosis Virus (CNGV) - a name which essentially sums up the symptoms of the virus.

Israeli scientists, who have been working on the virus since it caused massive commercial problems there at the end of the 1990s, studied the genetics of the virus in 2006 and said that it was genetically unusual among the herpesviridae. (See KHV is oddball of the family, News, August 8, 2006.)

What is a virus?

Virus is latin for poison, but viruses aren't poisons in the usual chemical sense. They are submicroscopic particles containing genetic material, such as DNA or RNA, enclosed in a protein case called a capsid and they have basically evolved to infect cells. KHV is a member of the Herpesviridae family, which are DNA viruses. Other members of the herpes virus family infect humans and cause coldsores and genital warts. Unpleasant, but arguably not as serious as KHV is to carp.

What medications can I use to treat it?

That's the snag. You can't. While parasites, fungi and bacterial infections can respond to chemotherapeutic medications and antibiotics, viruses cannot be treated in this way.

As a result, there is no cure for KHV. Once your fish get it, they either die or they survive and become potentially dangerous carriers that can pass the disease on to other fish. Vaccines that prevent fish from developing the virus are the only possible defence outside quarantine and biosecurity.

Has KHV had a big impact on the trade?

It's had a devastating effect, both on the ornamental fish trade and the recreational angling industry.

There have been some very serious outbreaks of the virus around the world, which have had a serious economic effect on the carp farming world and have reduced consumer confidence in purchasing fish from some countries, including the two main Koi producing countries: Israel and Japan.

In 2003, over 1000 tons of farm-bred carp were killed by the virus in a widespread outbreak in Japan. (See Koi Herpes Virus epidemic hits Japan, News, November 10, 2003).

In 2005, a KHV outbreak hit New York's Chatauqua Lake, resulting in the deaths of 20,000 carp. (See 20,000 dead in Chautauqua Lake KHV outbreak News, June 29, 2005).

By 2006, the disease was causing widespread problems in UK stillwater fisheries and angling lakes. Nine cases of the virus were reported in one month at the height of the outbreak. (See UK's biggest ever KHV outbreak gets worse, News, July 20, 2006.)

So it spreads easily then?

Yes, it appears to spread quite readily, especially when fish are moved from place to place. Many outbreaks are genetically linked, suggesting that the same strains have spread from one outbreak to another.

In September 2005 Practical Fishkeeping exclusively reported that scientists from the Gunma Prefectural Institute of Public Health and Environmental Sciences in Japan were to publish a paper in the journal Veterinary Microbiology stating that the viral strains isolated from carp in the 2004 Gunma Prefecture outbreak were closely related to those seen in other outbreaks of the disease. (See Gunma KHV outbreak caused by same virus, News, September 6, 2005).

In the UK and elsewhere in Europe, illegal imports of carp for angling lakes are believed to be one possible cause for the spread of the virus through fisheries. The authorities are particularly keen to clamp down this. (See Customs seize illegal carp, News, November 30, 2006).

What fish are susceptible to KHV?

Initially, scientists believed that only common carp and Koi - both Cyprinus carpio - were susceptible to the virus. However, recent research has shown that it can affect other species too, at least experimentally.

American veterinary researchers showed in 2006 that the virus was capable of infecting Fathead minnow cells in experimental culture (See KHV replicated in Fathead minnow cells, News, 13 November, 2006).

More recently, experts from the Hebrew University in Israeli have managed to experimentally propagate the virus in cells from Silver carp and goldfish, which they believe places these species at risk of infection or act as carriers. (See Other fish vulnerable to KHV says study, News, 15 June, 2007).

Are fish carrying the virus hard to spot?

Yes, the virus only shows its symptoms (or becomes symptomatic) at high water temperatures, so if fish are kept cool they may carry the virus without showing any signs of disease.

This means that you could purchase perfectly healthy-looking fish carrying the KHV virus, add them to your pond, and then suffer from the virus at a much later date when your water warms up to a specific temperature range for an extended period.

More frighteningly, shops can also purchase fish carrying the virus, which appear completely healthy, and then keep them alongside other fish to which the virus is then passed.

Unfortunately, biosecurity measures in the aquatic trade are rather poor and new fish are often kept on the same centralised filtration systems as old stock, sharing the same water. Some shops even mix newly imported fish with existing ones, often during a quarantine period.

What should wholesalers and retailers be doing to ensure that newly imported fish with KHV aren't sold on?

The single most-important thing to consider when quarantining pond fish (and any other fish for that matter) is biosecurity.

In my opinion, quarantine procedures for fish in the aquatic trade are still in the dark ages. An unbelievable number of retailers "quarantine" new fish in tanks alongside existing stock.

The best approach is to treat all incoming fish as potentially infected and keep them well away from existing, pre-quarantined stocks. You should also keep them on a separate system, and avoid potential contamination by using separate nets and other equipment in your quarantine area. KHV can spread through the water, so always keep new fish well away from stock on sale.

The other really important thing in quarantining pond fish in the KHV-era is temperature. KHV doesn't begin to show itself until the water is between 23 and 28C (73-82F). The recommendation is to place new fish in a separate pond system, gradually warm them up to 25 or more for a two-week period, then allow them to cool back down to ambient temperature.

According to the Ornamental Aquatic Trade Association (OATA), anyone quarantining in this recommended manner would be "very unlucky" to have infected fish pass through the quarantine period unnoticed. In essence, the KHV quarantine period is really there to see if you can trigger the symptoms of the virus. If you don't do this, you've got little chance of telling whether the fish are infected with the virus.

OATA produced a superb biosecurity document for its members to assess their existing quarantine and import procedures to determine the level of risk. This is well worth reading if you import Koi.

OATA's KHV Suggested Actions document explains what you need to do to best ensure that carp are correctly quarantined to get KHV to show up if it's being carried.

I'm a retailer and my wholesaler says he quarantines all of his fish for KHV, so I don't need to do it. Presumably this means I'm safe. Right?

Not necessarily. If his biosecurity measures are good, and he's heat-treating newly imported Koi without mixing them with existing new stock, it's likely to make a difference.

However, it's certainly advisable to quarantine your fish, even if your wholesaler already claims to have done so already. If you insist on not doing this, you're placing an incredible amount of trust on your supplier. If he isn't quarantining correctly, or at all, and you purchase infected fish and add them to your systems and sell them on, the disease is likely to spread all over the place.

I would personally go for the belt-and-braces approach of quarantining everything again, and keeping new stock well away from existing fish, even if your supplier already claims to have done so. It might cost a lot more to quarantine fish in this way, but it's likely to save you money (and your reputation) in the long-run.

So if a shop gets KHV in its systems that's it?

No. Many businesses have recovered from KHV outbreaks after a rigorous disinfection procedure.

There's no reason why you shouldn't continue to purchase fish from a dealer who has had a KHV outbreak in the past. The experience is likely to have forced the supplier to improve their biosecurity and ensure that the quarantine procedures they use are better than ever before.

Is there anything I should do before I add the fish to my pond?

If you're a consumer and you're purchasing new Koi for your pond you'd certainly be advised to quarantine them at high temperature before adding them to your pond.

Are any vaccines under development?

Yes, there are several vaccines under development. Scientists from Israel, which suffered one of the first major outbreaks of the virus, have been working on a KHV vaccine for some time; there's also vaccine under development with Henderson Morley PLC in the UK. (See British company to start KHV vaccine trials, October 3, 2006.)

In April 2005, Practical Fishkeeping reported that virologists at the Hebrew University Hadassah Medical School in Jerusalem and the Ministry of Agriculture and Rural Development in Israel, had announced the production of a new attenuated vaccine for KHV, which they claim can be used as a prophylactic to protect fish against contracting the virus. (See KHV vaccine is safe, says study, News, 25 April, 2005).

Are the vaccines legal in the UK?

Not yet. No vaccine has been licenced for use in the UK, but it isn't illegal to import fish that have been vaccinated with the unapproved vaccines, so there are vaccinated fish on sale.

The Institute of Fisheries Management (IFM), an organisation covering fisheries management rather than the aquarium trade, currently opposes the use of vaccines on fish for stocking in angling lakes.

It said: "The technology used to develop this vaccine is unproven, and there is a risk that the live viruses in the vaccine will revert back to a full pathogenic Koi Herpesvirus.

"The vaccine is also unlikely to confer protection on the fish longer than six months, making it inadequate long term protection." (See IFM warns against KHV vaccine, 12 December, 2006.)

What's an attenuated vaccine and how does it prevent KHV?

An attenuated virus is, put simply, a weakened or less vigorous virus that stimulates the immune system to create antibodies to fight off the virus, but lacks the strength to trigger a full-blown infection. This forms the basis of the vaccine used on Israeli Koi.

How else can scientists test whether fish are carriers of the virus?

In late 2005, scientists from the Department of Pathology at the Hebrew University-Hadassah Medical School in Jerusalem, Israel, used the Polymerase Chain Reaction (PCR) molecular technique to analyse fish droppings and found that they contained viral DNA that could be used in diagnoses.

The study, which was published in the journal Applied Environmental Microbiology, was first reported on the Practical Fishkeeping website. It showed that carp faeces could be used to detect the presence of the virus in carriers and the presence of antibodies in fish that had survived a previous KHV infection.

The scientists also showed that infected carp faeces were capable of passing on the virus to naiive fish, and suggested that it may be possible for the virus to remain active in faeces during the non-permissive cold seasons.

What are naiive fish?

Naiive fish are fish that have previously had no exposure to the virus and have no antibodies to the virus.

It sounds frightening. Is there a legal obligation to inform the authorities if I suspect my fish have the virus?

As KHV is currently not a notifiable disease in the UK, there is no legal obligation to inform the authorities if you suspect that your fish are infected with the virus. Other diseases, such as Spring Viraemia of Carp (SVC), which can be similarly virulent are notifiable and you must alert Defra or your vet if you suspect your fish are suffering from the disease.

Many have been highly critical of the lack of notifiable disease status for KHV and have suggested that this has allowed many cases to go unreported.