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SVC Spring Viraemia Carp (Rhabdovirus carpio)

The fish disease is caused by Rhabdovirus (a group of viruses that also includes rabies) carpio. It occurs in Europe and Asia, each area having its own strain of virus. It has now been introduced into North America: the European strain was found in Wisconsin in 1989, and the Asian strain in North Carolina in 2002.

With classic SVC, infected fish may show a buildup of fluid in the abdominal cavity, which is suggestive of "dropsy". In this case, fluid accumulates in the body cavity, making the fish appear swollen, this pressure can build up so much that the anus may be partially forced out and prolapsed. The eyes may even bulge (exophthalmia) due to fluid in the eye sockets. Those tissues involved with the immune response, such as the spleen and the cranial kidney, often show marked reactive changes. Further changes that are often described are thought by many to be due to secondary bacterial infection and would include ulceration and accumulations of pus-like material inside the body cavity.

The virus will spread in the blood to the internal organs. Eggs can be infected at breeding. Fish lice (Argulus) and leeches (Pisciola) have been implicated with transmission of SVC from fish to fish, as have herons in transferring virus between ponds. Contaminated equipment may also help to spread infection.

A classic case of SVC occurs when the virus damages and so causes inflammation of the lining of blood vessels. This in turn makes them "leaky", allowing both fluid and red blood cells to leak out into the surrounding tissues. This means that SVC causes hemorrhages and edema (fluid buildup) in a number of tissues, including the heart, brain, and intestines. In the kidneys there is serious damage to the microscopic tubes, affecting their ability to function properly. Pancreatic tissue is often very inflamed.

Infected fish shed the virus into the surrounding water in their feces, often in mucous anal casts. Other fish then inhale viral particles suspended in the water, where they attach to the gills. Here they invade and replicate before spreading throughout the blood stream to infect the internal organs.

The disease is more severe in cool water temperatures; hence the prevalence of signs in spring, as the water temperature warms above 48°F (8°C).

Warming the pond water up to 68°F (20°C) will protect the fish from the virus. Ultraviolet irradiation of the pond water will inactivate the virus and help prevent its spread. Formalin added to the pond (25 mg/L) will also inactivate the virus in the water. An autogenous vaccine can be made from formalin inactivated virus and administered by intraperitoneal injection. This will not help already infected fish, but will prevent infection in exposed healthy fish. They should be vaccinated in the summer or fall to prevent disease outbreak in the spring.

Incubation time for SVC varies from 7 to 60 days, depending upon water temperature.

Affected fish will often show a darkened body color. Hemorrhages can be present both internally and externally. Infected fish are weak, breath shallowly, and may have a thick, mucous cast trailing from the anus. Gills are often pale.

There is as yet no effective treatment for SVC. In theory, supportive management would be appropriate. This would include antibiotics to prevent secondary infections and adding salt to the water to help with osmoregulation, but most fish exhibiting signs of the disease will die, especially at low temperatures.

There is no way of preventing the disease other than quarantining all new stock. Vaccination has been tried and appears to be reasonably effective at temperatures above 68°F, but these are unlikely to be made available in countries where SVC is notifiable, as vaccination may interfere by masking disease outbreaks.

This is a disease which you really hope you will never experience as it is a notifiable condition. This means that once it is identified, you must inform the relevant authorities who may well take the step of killing your stocks and disinfecting your whole system. SVC should not be a concern for Koi keepers as long as they do not introduce stocks from the wild, especially if they live in a country where SVC is present. In fact it is illegal in most instances to take and move wild stocks without first discussing your intentions with the appropriate authorities.

It is also wise to avoid importing Koi from a region or country which is known to have SVC. That is why strict licensing is in place in countries where SVC is not currently a problem.

SVC will cause symptoms of ulceration on the skin of your Koi, as well as areas of reddening as the virus progresses. Sometimes these are accompanied by swelling of the belly area, and a loss of color in the gills. Surprisingly SVC will not result in heavy loses of adult Koi, but if you have young or small Koi and they are infected, losses of 100% may be expected.

If you are unfortunate enough to encounter this disease in your fish, it is vital that the correct authorities are notified. In most cases the laboratory carrying out the testing will do this as a matter of course. Although the subsequent culling may be hard to handle, try to remember that it is not only in your best interests but also those of the whole Koi industry.

Fish will usually survive SVC; but their carrier state is unconfirmed.

References: Manual of Koi Health by: Keith Holmes and Tony Pitham

Mortality is usually high. Unlike KHV, which seems to affect only koi and carp, this disease affects goldfish and other Cyprinids, including wild populations and bait fish. The USDA is exploring methods of controlling the spread of SVC in this country including quarantine, health certificates, importation guidelines, vaccinations, testing programs etc. Official guidelines should be available soon.

Swim Bladder Inflammation (SBI)

In this form, only the swim bladder is targeted. Marked hemorrhages occur on the swim bladder surface, and there is serious damage to its lining. Such fish lose their balance and coordination before eventually dying. This disease is said to be due to a second rhabdovirus called SBI (Swim Bladder Inflammation) virus, although it is as yet impossible to distinguish it form the SVC virus.

In both cases there are often mass mortalities that can continue for weeks. Low or rapidly fluctuating temperatures can predispose fish to infection if the virus is present. Infected fish produce interferon as part of their antiviral response, but unfortunately, interferon production is temperature dependent. This means that above a temperature of 68°F, immunity is usually quite good. At 52° to 68°F, obvious clinical signs occur, but some fish are able to mount an immune response and survive. At temperatures below 50°F, the fish immune response is so sluggish that viral multiplication (which is itself reduced by the low temperatures) is able to continue unchecked, causing mortalities.

If SVC present itself as a swim bladder inflammation, infected fish will lose their balance and will be seen lying on their side at the bottom or floating beneath the surface. If the fish swim, their movements are wobbly and uncoordinated. Usually no hemorrhages can be seen. A definitive diagnosis can really be done only by a professional laboratory and involves isolating virus from infected tissue.

To find a Fish Vet in your Area

References: Advanced Koi Care by Nicholas Saint-Erne, DVM

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SVC kills 5-10% of the population of affected fish and seems endemic. KHV kills 90% of affected fish if untreated and is rare.

PCR Tests

PCR, or polymerase chain reaction, which is used to confirm the presence of both KHV and SVC 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.

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