Koi Herpes Virus
KHV Disease was first recorded in Israel in 1998 following large-scale mortalities in farmed fish. Further losses attributed to the virus were later found in Europe and the USA and have subsequently been reported throughout the world including Japan, Malaysia and South Africa. The virus was first identified in the UK in 2000 and has subsequently proven to be widely distributed in ornamental dealers and hobbyists fish stocks.
Koi herpes virus is a relatively new disease that has been causing great anguish and consternation around the world. It appears to be specific for Koi and other carp (Cyprinus carpio) varieties. Even closely related cyprinids such as goldfish (Carassius auratus) do not appear to become clinically ill when exposed to the virus. This disease has been detected worldwide.
The best that one can say at present is that KHV is a herpes-like virus. It appears to be very similar to herpes viruses in some respects, but it is sufficiently different enough to have an alternative name suggested. This is a carp nephritis and gill necrosis virus (CNGV), a name that reflects the damage this virus causes.
It seems highly likely that infection occurs through the gills. If the water temperature is between 68° to 79°F, then the virus causes extensive damage to the gills. Infection of Koi with KHV at a water temperature of 73°F ended with a total mortality of 95%. The disease progressed rapidly at 82°F, but there was a marginally lower mortality of around 89% to 95%. At 62°F the death rate was 85%, but no deaths occurred at 55°F. However, moving these virus-exposed fish from 55° to 72°F triggered a rapid onset of the disease and subsequent deaths.
KHV-infected fish show obvious gill discoloration and necrosis (dying off) of infected gill tissue. This may appear in distinct patches with loss of tissue down to the cartilage of the primary lamellae. Secondary bacterial and fungal infections readily establish themselves. The fish appear to lose much of their mucous coat, they appear thin, and the eyes are often sunken in appearance. This may be due to a combination of oxygen starvation from the gill damage and the damage to the kidneys seen in many individuals.
In a full-blown outbreak, many Koi and other carp will be seen dead and dying. The water temperature will be in the permissive range of 68° to 76°F. Infected fish will appear lethargic and will swim close to the water surface.
Fish affected with Koi Herpesvirus can become anorectic, lethargic, float at an angle or swim erratically and have erythema of the skin and increased mucus production. Larger fish were affected earliest.
Severe gill damage will lead them to gather in areas with high oxygen levels, such as water inlets or waterfalls. There will be obvious respiratory distress (increased breathing rate, gasping at the surface). Incoordination (fishes bumping into objects) such as that resulting from a swim bladder inflammation may also result, especially in severe cases. The skin and gills will exhibit a decreased mucus production, and furthermore, there may be hemorrhages in fins and in the body. Examination under the opercula (gill covers) will reveal severe gill necrosis. This appears as white, gray, or black, and damage and erosion to the gills is present, often with the cartilage exposed. There is often secondary gill disease present, indicated by high numbers of bacteria or gill flukes. Lastly, the eyes are often sunken.
The KHV appears most deadly at temperatures between 68-85°F (20-29°C). The pathogen is highly contagious, transmitted even with brief contact, and can be spread via equipment such as nets and hoses. It causes a decreased immune response (immunosuppression) thereby allowing a countless number of secondary bacterial infections to occur, which can confuse the diagnostic process.
KHV destroys epithelial cells of the gills, and on the skin causes excess mucus production. Death occurs from hypoxia due to gill damage. Since there is no treatment for herpesvirus perfect water quality, high oxygen concentration, and treatment of secondary infections are all important in keeping your pond KHV free.
Treatment options at present are limited. Covering antibiotics and surfactants such as Chloramine-T may be of benefit. Those fish that survive to go on and recover appear to be solidly immune to further infections. It may be that some of these become carriers of the virus, although experimentally recovered Koi failed to pass on KHV to other, non-exposed Koi.
Main Predisposing Factors to an Outbreak of KHV
Temperature: Mortalities up to 95% can occur within the permissive temperature range of 64° to 77°F.
Stress: Overcrowding and particularly the movement of fishes from one aquarium or pond to another are likely to trigger outbreaks.
The symptoms of Koi Herpes Virus are:
These Tips are from the Associated Koi Clubs of America:
Investigation of Koi herpesvirus latency (or dormant state) in Koi.
Koi herpesvirus (KHV) has recently been classified as a member of the family of Alloherpesviridae within the order of Herpesvirales. One of the unique features of Herpesviridae is dormant infection following a primary infection. However, KHV latency has not been recognized. To determine if latency occurs in clinically normal fish from facilities with a history of KHV infection or exposure, the presence of the KHV genome was investigated in healthy koi by PCR and Southern blotting. KHV DNA, but not infectious virus or mRNAs from lytic infection, was detected in white blood cells from investigated koi. Virus shedding was examined via tissue culture and reverse transcription-PCR (RT-PCR) testing of gill mucus and feces from six koi every other day for 1 month. No infectious virus or KHV DNA was detected in fecal secretion or gill swabs, suggesting that neither acute nor persistent infection was present. To determine if KHV while dormant can be reactivated, six koi were subjected to a temperature stress regime. KHV DNA and infectious virus were detected in both gill and fecal swabs by day 8 following temperature stress. KHV DNA was also detectable in brain, spleen, gills, heart, eye, intestine, kidney, liver, and pancreas in euthanized koi 1 month post-temperature stress. Our study suggests that KHV may become latent in white blood cells and other tissues, that it can be reactivated from latency by temperature stress, and that it may be more widespread in the koi population than previously suspected.
Refrences:Eide KE, Miller-Morgan T, Heidel JR, Kent ML, Bildfell RJ, Lapatra S, Watson G, Jin L.
Source: Department of Biomedical Sciences, College of Veterinary Medicine, Oregon State University, Corvallis, OR 97331, USA.
J Virol. 2011 May;85(10):4954-62. Epub 2011 Mar 9.
As of January 2007, KHV was added to the World Organization for Animal Health (OIE) notifiable disease list for fish. Because of this listing the United States Department of Agriculture Animal and Plant Health Inspection Service (USDA APHIS) asks that accredited veterinarians and diagnostic laboratories report positive cases of KHV to the area veterinarian-in-charge (AVIC) of the state where the fish originated. However, no client information (e.g., name, address) will be requested.
This information will help USDA determine what the prevalence of KHV is in the United States. USDA APHIS has no health requirements specific for KHV for Koi or other fish moving interstate or internationally. There is no mandatory depopulation for populations of Koi infected with KHV. It is up to the owner of those animals to decide what course to take.
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SVC Spring Viraemia Carp kills 5-10% of the population of affected fish and seems endemic. KHV kills 90% of affected fish if untreated and is rare.
PCR, or polymerase chain reaction, which is used to confirm the presence of both KHV and SVC Spring Viraemia Carp is an analytical technique that is widely used in genetic fingerprinting. In the test, an identifiable part of the DNA of an organism or viral particle is amplified using the DNA's replication process. The PCR test is extremely sensitive, so very little material is needed for positive identification. Samples should be fixed in 75 percent ethanol and stored in a freezer until they can be sent on to a laboratory that specializes in these tests. Lately, improved analytical techniques allow dry swabs of gill tissue to be used for viral detection. Your local Koi club will be able to advise on where PCR tests can be done.
July 2011 Koi Herpes Virus Outbreak in Sweden
Sweden reported an outbreak of Koi herpesvirus to the World Organization for Animal Health (OIE) in May.
Twelve Koi died in Lilla Edet, Sweden, and the Swedish National Veterinary Institute confirmed the presence of Koi herpesvirus via real-time PCR testing. An additional 25 susceptible fish were destroyed.
Sweden last reported the disease to the OIE in July 2011.
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