Anglers put on alert over killer carp disease - use dry nets!

Anglers and fishery owners are being warned to be on their guard following outbreaks of the Koi Herpes Virus - a killer carp disease for which there is no known cure.

The outbreak could lead fishery owners to introduce stricter controls over the use of landing nets and keepnets, with anglers having to use only dry nets or nets which have been disinfected for 15 minutes or more and then thoroughtly rinsed. Alternatively, some fisheries may insist that anglers use only the fishery's own nets.

There have been confirmed cases of KHV in the South and South East and reportedly suspected cases in the Midlands. At its worse KHV is 100 per cent fatal to koi and other species of carp.

Whilst the Environment Agency are controlling fish movements from known KHV sites, it has come under attack from some fishery owners because it is not prepared to say which fisheries have been affected.

The Environment Agency says this is to protect the long-term reputation and livelihood of the affected waters, whilst a growing band of fishery owners say the EA should release the details as anglers travel many miles to go fishing and the information could help to protect unaffected venues.

The Professional Coarse Fisheries Association, the UK's leading professional body for fishery owners, is contacting all its members and issuing them with advice and information on how best to combat and curtail the outbreak.

Disinfectant in net dips need to be kept fresh and it is advised that anglers and fishery owners immerse nets for a minimum of 15 minutes. Alternatively, some fisheries may insist that anglers use only nets supplied by the fishery itself.

As with all viruses and bacteria and most parasites, the KHV virus cannot survive on a dry host.

Koi herpes Virus, is a viral disease highly contagious to fish which may cause significant fish kills in koi and common carp. Historically, the first outbreak of KHV was reported in 1998 and confirmed in 1999 in Israel, where there is a highly successful international trade in breeding and rearing carp both for the table and for fisheries. Since then, other cases have been confirmed in the United States, Europe and Asia. In England there have been about 17 outbreaks in the last five years, an average of about three a year.

KHV is currently classified as a DNA-virus belonging to the herpes family. Although there have been some scientific discussion regarding the accuracy of this classification, more recent work shows strong evidence that KHV is indeed a herpes virus, based on morphology and genetics. Whilst KHV disease has been diagnosed in koi and common carp, other related carp species such as the common goldfish and grass carp seem to be unaffected.

As with other herpes viral infections, KHV is believed to remain in infected fish for life, and as a result exposed or recovered fish should be considered as potential carriers of the virus.

KHV disease is believed to kill 80 to 100 per cent of infected fish, and fish seem most susceptible at water temperatures of 72 to 81deg F (22-27deg C).

The disease affects fish of various ages, but studies show that fry have a greater susceptibility than mature fish. Clinical signs of KHV are often non-specific. Onset of death may occur very rapidly in affected populations, with deaths starting within 24 to 48 hours after the first signs. In experimental studies, 82 per cent of fish exposed to the virus at a water temperature of 22deg C died within 15 days.

KHV infection may produce severe gill lesions and high mortality rates. In some cases, secondary bacterial and parasitic infections may be the most obvious problem, masking the damage caused by the primary viral infection. Behaviorally, affected fish often remain near the surface, swim lethargically, and may exhibit respiratory distress and uncoordinated swimming.

External signs of KHV may include gill mottling with red and white patches, bleeding gills, sunken eyes, pale patches or blisters on the skin. Microscopic examination of gill biopsies often reveals high numbers of bacteria and various parasites whilst internal signs are inconsistent and non-specific, but may include adhesions in the body cavity and a mottled appearance of internal organs.

The herpes virus that is responsible for KHV seems to spread in the same ways as most herpes viruses, including by direct contact with infected fish, with fluids from infected fish, or with water or mud from infected systems.

Depending upon water temperatures, exposed and susceptible fish may become infected and either develop the disease and die or become carriers of the virus. Goldfish and other fish in the carp family are not susceptible to KHV disease and are said not to appear to act as carriers of the virus.

According to scientists, the virus appears to have an incubation period of 14 days, although this may be longer, indicating that appropriate temperature and possibly a second trigger may be necessary for outbreaks to occur. Mortality related to KHV disease typically occurs between 64deg F and 81deg F (18-27deg C). Almost no mortalities occur below 64deg F, and there has been no reported occurrence of the disease at or above 86deg F (30deg C).

There is no known treatment for KHV. Antiviral drugs are not currently available to treat KHV or any other viral diseases of cultured fish and currently there is no vaccine against KHV. However, preliminary experimental vaccine studies using intraperitoneal injection of a live attenuated virus demonstrated that fish developed high antibodies, were immune to the disease and survived a challenge.

Because KHV outbreaks have caused large losses at koi and common carp facilities, and because there is still some concern over the possibility that survivors are carriers, anyone with koi and common carp that have been diagnosed with KHV should consider eliminating the entire population. This should be followed by disinfection of all materials and systems that have contacted the infected fish.

Fishery owners are being advised that the best way to prevent KHV is to know their fish suppliers and have a good working relationship with them.

Anglers fishing abroad or within areas subject to fish disease outbreaks in England and Wales, can help to prevent the spread of fish disease by disinfecting fishing tackle after use. The Defra leaflet 'Keep Fish Diseases Out' highlights the importance of this and suggests suitable methods for disinfecting fishing tackle and clothing.

Advice for fishery owners

There are several disinfection methods available. The two commonly used methods are to ensure thorough drying of equipment for a minimum of 48 hours, preferentially in direct sunlight, and chemical disinfectants, primarily Iodine-based preparations (iodophors) or Virkon S.

Of these, drying in sunlight is clearly preferable. However, this is not always practical. In this case, chemical disinfectants can be applied.

Equipment should be cleaned of all mud and debris. The equipment should then be immersed or sprayed with the chosen disinfectant. An immersion/exposure time of at least 15 minutes is required, although this should ideally be as long as 30 minutes. The disinfectant should then be rinsed off with clean water and disinfectant washings must be disposed of in a way that does not harm the environment - they should never be tipped into water containing fish or other aquatic life. The EA has stated that if it soaks away into the ground this is not harmful.

Some disinfectants may contain hazardous chemicals therefore it is important that product labels and manufacturers/suppliers instructions are read and adhered to. Protective clothing and equipment should be worn when carrying out dilutions of disinfectants to prevent exposure to eyes and skin. If in doubt seek advice from the suppliers. Manufacturers guidance on disposal should also be adhered to.

What disinfectants can be used?

Iodophors include products such as Iosan CCT, Deosan Iodophor Udderwash and FAM 30. When active, iodophors are a dark brown solution, however they become colourless when inactivated by prolonged exposure to light. Dilution of iodophors varies between products, therefore manufacturer’s guidelines should always be adhered to.

Virkon S is a broad-spectrum virucidal disinfectant that is available in both powder and tablet form. It should be made up to a one per cent solution with water, and when used as directed it forms a pink solution that is stable for five days. However, any residue should be discarded daily and the solution kept out of direct sunlight. Any disinfectant used should be in accordance with the manufactrer's guidelines.

Most good agricultural suppliers will have appropriate disinfectants available.

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