Viral hemorrhagic disease in common carp

According to Fijan et al., 1971, it was termed Spring Viremia of Carp (SVC) because they successfully isolated a virus: Rhabdovirus from common carp affected by infectious dropsy of carp (IDC).

Pathogen

Viral hemorrhagic disease in common carp has long afflicted carp farming ponds across Europe. Hofer (1904) first described and named it as a bacterial hemorrhagic disease. Bauer and Fakto-novich in 1969 still supported the view that hemorrhage in common carp was caused by bacteria; red spot disease (hemorrhage) first attracted attention in the former Soviet Union in 1915. Otte, 1963 and numerous other scientists also agreed with this perspective. Schaperclaus first described the hemorrhagic disease in 1930. In Germany, Schaperclaus, 1979 still acknowledged the red spot disease described since 1927.

Hemorrhagic disease in common carp as a bacterial infection was investigated and treated in Western Europe. Bacteria identified from outbreaks were Gram-negative, but treatment with antibiotics did not achieve high efficacy. Therefore, the pathogen was either not bacteria or, at the very least, a specific bacterium consistently present during hemorrhagic (red spot) outbreaks. From this viewpoint, several scientists in Eastern Europe and Russia investigated and suggested that hemorrhagic disease (red spot) was caused by a virus, studying environmental fluctuations and the concurrent interaction between viral and bacterial pathogens.

^{Rhabdovirus carpio}

From two typical forms—acute hemorrhage and chronic ulceration—Fijan et al., 1971 isolated the causative agent as the virus Rhabdovirus carpio. Subsequently, a series of scientists in Europe, the United States, and Japan conducted extensive research on the hemorrhagic pathogen (spring virus disease) in common carp and various species within the Cyprinidae family, such as silver carp.

R. carpio has an RNA nucleic acid structure and a bullet-shaped, rod-like protein capsid with one rounded end, measuring 90-180 nm in length and 60-90 nm in width. It exhibits a 450-layer membrane, a porous state measuring 200 nm, but typically retains a 100 nm envelope. Antiserum diagnosis has confirmed Rhabdovirus in various fish species. Hill et al., 1975 demonstrated that the virus causing swim bladder inflammation in common carp is identical to the virus responsible for carp viral hemorrhage.

Clinical Symptoms

Behavior: Initial signs include fish suffocating, swimming at the water surface, dead fish sinking to the bottom, and loss of balance with disoriented swimming (swim bladder inflammation).

External signs: Hemorrhaging on gills and skin, sometimes in the eyes. Darkened skin color, inflamed areas covered in excess mucus, mild exophthalmia (pop-eye), pale gills with fused filaments. Thin, diluted blood exuding from the vent.

Internal organs: Abdominal distension (Figure 2A); hemorrhaging and fluid accumulation (dropsy/edema) in the abdominal cavity; hemorrhaging and atrophy of one chamber of the swim bladder (see Figures 2 B, C, D); enlarged spleen; hemorrhaging in heart, liver, kidneys, and intestines; abdominal cavity containing abundant viscous exudate; subcutaneous musculature displaying extensive hemorrhaging (Figure 2B).

Distribution and Transmission

The disease primarily occurs in common carp, affecting fish from fingerling to grow-out stages. In 1978-1979, swim bladder inflammation appeared in Hungarian common carp broodstock at the Research Institute for Aquaculture No. 1, causing high mortality. In addition, R. carpio has been isolated from silver carp, bighead carp, crucian carp, and European catfish (Silurus glanis), causing diseases with mortality rates exceeding 90%. The disease has been documented in Europe, the former Soviet Union, North America, China, and Vietnam.

^{Pathological conditions and signs in affected fish}

- A: Common carp infected with swim bladder inflammation, dark body coloration (specimen collected at RIA 1, April 2006 - according to Bui Quang Te)

- B: Hemorrhagic flesh of common carp (skinned carp A), posterior chamber of swim bladder showing atrophy

- C: Swim bladder showing atrophy in one chamber (specimen collected in Bac Ninh, 1996)

- CD: Common carp swim bladder with one atrophied chamber (top) versus normal swim bladder (bottom), (specimen collected in Hung Yen, May 2008).

Diagnosis

Diagnosis is based on clinical signs and seasonal occurrence, typically in spring and autumn. The disease progresses rapidly (acute form) with high mortality rates. When bacteria are absent upon isolation and external clinical signs are evident, viral etiology can be readily confirmed. Where laboratory conditions permit, diagnosis can be conducted using serum neutralization assays, virus isolation, and cell culture.

Disease Prevention

Several measures exist to prevent viral hemorrhage in common carp (SVC), each with limitations, predominantly due to incomplete understanding of viral reservoirs and aquaculture management specifics. Advanced virological methods have not yet been exhaustively applied to common carp viral hemorrhage. Nevertheless, neutralizing antibodies identified in broodstock serum are a notable marker, as these antibodies characterize fish that have not contracted the virus or exhibit delayed infection. Culturing carp in rivers and streams with abundant wild fish populations increases the likelihood of viral outbreaks. A primary biological preventive measure involves thermal manipulation by rearing fish at water temperatures above 20°C, because seasonal temperature drops are the primary trigger for disease proliferation (spring virus disease). Therefore, carp should ideally be raised in warmer water regimes where the pathogen's impact is minimized.

Selective breeding of carp strains resistant to viral hemorrhage is feasible but difficult to apply on an industrial scale. Since 1962, the former Soviet Union implemented selective breeding programs for disease-resistant common carp, confirming a survival variance of approximately 30% between resistant and susceptible carp stocks. In the United Kingdom, domestic cultured carp strains were found to be more resistant than wild carp. Wild carp exhibited mortality rates of 60-90%, whereas domestic carp suffered negligible losses.

In practical management, chemical prophylaxis and treatment for viral fish diseases provide little to no efficacy. Immunological prevention holds greater promise and has been tested against viral hemorrhage in carp. While immunological measures are developing rapidly in human and veterinary medicine, in fish they remain constrained by age-dependent immune competence and temperature thresholds (above 20°C) favorable for immune response. Furthermore, vaccines are currently not economically viable for large-scale commercial fish farming due to high manufacturing costs, making mass vaccination difficult to deploy in fingerling ponds.

common carp disease spring viremia of carp SVC virus Rhabdovirus carpio viral hemorrhage Carp