An acute to chronic vesceroptropic disease of young (fingerling to yearling) rainbow trout at 100C or lower. Also in adult trouts and salmonids, but not usual.
Etiology: VHSV (Viral Haemorrhagic Septicemia Virus)
Viral characteristics:
- Bullet-shaped rhabdovirus, enveloped.
- Length- 180 nm, width- 70-85 nm, apical end rounded, other end flat.
- ss RNA, replication in cytoplasm.
- Sensitive to lipid solvents, detergents and proteolytic enzymes.
Distribution:
Trouts farms in European countries: Denmark, Italy, Germany, Switzerland and Spain (not in UK and USA).
Signs and pathology:
Acute:
- Poor feeding.
- High mortality.
- Dark skin, exophthalmia.
- Pale gills, anaemia.
- Widespread internal haemorrhages in skeletal muscles and viscera.
- Hyperemic and swollen kidney and liver.
- Empty intestine.
- Hemorrhages in muscle and kidney.
- Focal necrosis in liver.
Chronic:
- Low mortality.
- Dark skin.
- Exophthalmia.
- Swollen abdomen and kidney.
- Pale liver.
- Increased lymphoid cells.
- Decreased renal pigmentation.
Nervous:
- Circular swimming.
- Poor balance.
- Retracted abdomen.
- Anemia.
- No internal signs.
Epizootiology:
- Spread via virus-laden water-lateral transmission.
- Possible sources: escape fish, virus shedding into river water, diseased fry, diseased eggs, infected water, washings from brood fish eggs.
- Re-isolation of virus after one year in rainbow trout carriers at 40C is possible.
Diagnosis:
- VHSV is grown on RTG-2, or CHSE-214 cell lines.
- 140C is the optimum temperature.
- VHSV can be confirmed in infected culture by FAT or IFAT.
- Clinical diagnosis is easy when mortality breaks out among fingerling or yearling rainbow trouts during low temperature showing major signs and behaviors.
- Epizootiological features are helpful for diagnosis, e.g., VHS is now in Europe, or place having VHS history or are exposed to potential known sources of VHSV.
- Diagnosis is reinforced by histopathologic changes.
Significance:
Historically, VHSV has been the most serious viral disease affecting European trout culture. With its detection along the northwest coast of North America in 1988, new emphasis has been placed on its potential impact on salmon culture throughout the world. It is now recognized that where VHSV occurs, there is the potential for a highly communicable disease outbreak.
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