Spring viremia of carp
Spring viremia of carp (SVC) is an acute, virulent, usually hemorrhagic viral disease of cultured carp caused by Rhabdovirus carpio.
The disease is listed as notifiable by the OIE. Historically, it was reported in Europe and the former USSR; however, an outbreak in the USA was confirmed in the spring of 2002.
The disease has been reported in common carp (or koi) (Cyprinus carpio), grass carp (Ctenopharyngodon idella), bighead carp (Aristichthys nobilis), silver carp (Hypophthalmichthys molitrix), and Crucian carp (Carassius carassius), a close relative of the goldfish. Recent evidence suggests that common goldfish (C. auratus) are also susceptible. .
Clinical signs of SVC are often non-specific and may include darkening of the skin, exophthalmia (pop-eye), ascites (dropsy), pale gills, hemorrhages in the gills, skin, and eye, and a protruding vent with a thick mucoid (white to yellowish) fecal cast. Internally, edema (fluid build up in organs and in the body cavity), inflammation, and pinpoint hemorrhages in many organs, including the swim bladder, may be present.
The presence of pinpoint hemorrhages in the swim bladder is considered an important indicator of this disease. The intestine is often severely inflamed and may contain significant amounts of mucus. The spleen is often enlarged.
Concurrent infection with bacteria, particularly Aeromonas (A. salmonicida or A. hydrophila), may confuse the diagnosis as fish will show signs of systemic infection such as ascites and hemorrhages.
Behaviorally, infected fish may appear lethargic, exhibit decreased respiration rate, and loss of equilibrium. Moribund fish have been reported to lie on their sides, often on the bottom of the tank, and when startled swim up but then return to the bottom. Fish are also reported to congregate where there is slow water flow and near pond banks.
Diagnosis of spring viraemia in fish is diagnosed by clinical signs and the virus is readily isolated in common fish cell lines and identified by serum neutralization and fluorescent antibody tests.
Antiviral drugs are not available to treat SVC or other viral diseases of cultured fish. Temperature manipulation is probably the most practical means of preventing or controlling mortality once an epizootic is in progress. Maintaining water temperature above 20°C (68°F) may prevent a potential outbreak.
In active outbreaks, efforts are directed at depopulating infected stock, and disinfecting all areas where infected fish were held. However, in some circumstances, this may be difficult. The virus can be infective in mud and water for up to 42 days.
The virus can be inactivated by formalin, ozone, sodium hypochlorite (chlorine at 500 ppm for ten minutes), organic iodophors, gamma and ultraviolet irradiation, pH extremes of < 4.0 or greater than 10.0, and heating at 60°C (140°F) for 15 minutes. All equipment and tanks, raceways, and ponds should be disinfected.
Fish that are exposed to physiological stressors such as crowding, handling, poor water quality, malnutrition, and sudden temperature changes are most susceptible, because of resulting immune system suppression.
Vaccine development has been attempted in the Czech Republic with promising results but further studies are necessary. The development of genetically resistant strains should also be pursued.
- ↑ Merck Veterinary Manual
- ↑ Fijan, N. (1972) Infectious dropsy in carp: a disease complex. Symposium of the Zoological Society of London 30:39-51
- ↑ Plumb, J.A. (1999) Health maintainence and principal microbial diseases of cultured fish. Iowa State University Press, Ames, IA, pp:77-90
- ↑ Smail, D.M. and Munro, L.S. (1989) The virology of teleosts. In: Roberts, R.J. (ed), Fish Pathology, Second edition. Balliere-Tindall, London, UK, pp:173-241
- ↑ Macura, B., Tesarcik, J., and Rehulka, J. 1983. Survey of methods of specific immunoprophylaxis of carp spring viremia in Czechoslovakia. Práce VÚRH (Vyzkumny ústav rybársky a hydrobiologicky) Vodnany (English = Papers of RIFH [Research Institute of Fishery and Hydrobiology] Vodnany) 12:50-56