Early Detection and Scientific Control Are Key to Grass Carp Hemorrhagic Disease

Grass carp hemorrhagic disease is a viral infectious disease caused by the grass carp reovirus. It mainly affects grass carp fingerlings and juvenile fish, and is one of the common and highly damaging diseases in grass carp farming. The disease is most prevalent in hot seasons and spreads through water, diseased fish and contaminated tools. It comes on rapidly, and if not dealt with promptly it can easily cause significant losses.

The typical sign in affected grass carp is bleeding on the body surface and in internal organs. Common symptoms include congestion at the base of the fins, reddening of the gill cover and mouth, and pinpoint or patchy hemorrhages in the muscles. Depending on where the bleeding occurs, it is generally divided in production into the red muscle type, the red fin and red gill cover type, and the enteritis type. Diseased fish usually have a reduced appetite, swim slowly and often swim alone away from the school. After dissection, organs such as the intestine and hepatopancreas can be seen to be congested and red.

For control, prevention should come first. First, choose healthy fry and fingerlings, and thoroughly clear and disinfect the pond before stocking, commonly using quicklime and similar substances to treat the pond bottom and pond water. During farming, keeping the water clean, controlling stocking density at a reasonable level and avoiding injury to the fish during handling can all effectively reduce the probability of outbreaks. Where conditions allow, farms can also use vaccination to raise the resistance of the fish stock.

Once diseased fish are found, they should be removed promptly and isolated to reduce contact between healthy fish and the pathogen, and the whole pond should be disinfected to control the number of pathogens in the water. During an outbreak, feeding can be appropriately reduced to prevent leftover feed from spoiling the water quality, while measures to strengthen the fish's condition can help them recover. It should be noted that viral diseases are difficult to cure completely, and day-to-day management and water quality regulation are the key to reducing the occurrence of grass carp hemorrhagic disease.