Grass Carp Reovirus (GCRV) Haemorrhagic Disease: A Disease Guide

Grass carp reovirus causes haemorrhagic disease with high mortality in young grass carp, especially in warm water. There is no antiviral cure, but vaccines are used in China.

Overview

Grass carp haemorrhagic disease is one of the most damaging viral diseases of Chinese freshwater aquaculture. It is caused by grass carp reovirus (GCRV), a non-enveloped, double-stranded RNA virus of the genus Aquareovirus, family Reoviridae, whose genome is divided into 11 segments enclosed in multiple concentric protein capsids. GCRV is regarded as the most pathogenic aquareovirus and causes severe haemorrhage and high mortality, particularly in young fish.

Affected species

The main host is the grass carp (Ctenopharyngodon idella), one of the most important farmed freshwater fish in China. Fingerlings and yearlings are the most severely affected, and outbreaks cause major losses in grass carp culture. Different GCRV genotypes vary in virulence, with some highly virulent strains causing mortality above 80% and others producing milder disease.

Clinical signs

• Widespread haemorrhage, including reddening of the muscle ('red muscle')
• Haemorrhage and congestion of the gills, intestine, and mesentery
• Haemorrhage at the fin bases, mouth, and body surface
• Lethargy and reduced feeding
• High mortality in fingerlings, in severe cases above 80%

Transmission and temperature

GCRV spreads horizontally through water and contact, entering fish through routes including the gills, skin, and olfactory system. Temperature is a key factor: natural outbreaks are most prevalent in warm water, around 25-30 degrees C, which favors viral replication. This makes the disease especially dangerous during the warm growing season for young fish.

Diagnosis

Diagnosis is based on the characteristic haemorrhagic signs together with laboratory confirmation by RT-PCR and virus isolation in cell culture, supported by histopathology. Because haemorrhagic signs can resemble bacterial septicaemia, molecular confirmation of the virus is important.

Treatment, control and prevention

There is no specific antiviral treatment for GCRV, so vaccination is the primary control method, alongside good husbandry. Both attenuated (live) and inactivated vaccines have been developed and used in China, where a live vaccine received manufacturing approval.

• Vaccinate fingerlings with attenuated or inactivated GCRV vaccines where available
• Source healthy, screened fingerlings and avoid introducing infected stock
• Maintain good water quality and reduce crowding and handling stress
• Manage warm-season stocking and density to limit outbreak risk
• Disinfect equipment and remove and dispose of dead fish