Symptoms and Prevention and Control Methods of Hemorrhagic Disease in Grass Carp Aquaculture

Hemorrhagic disease is a fish disease caused by a reovirus. The virus particles are mostly spherical or hexagonal. The diameter of a complete virus particle is 68-72 nanometers, much smaller than the smallest bacterium.

(1) Symptoms The main symptom of diseased fish is congestion. External symptoms generally show a slight reddish tinge, and in small fry, subcutaneous congestion can be seen under sunlight or lamplight. Some diseased fish also show congestion in the mouth, lower jaw, operculum, and base of the fins. When the skin of a diseased fish is peeled back, the muscles may show punctate or patchy congestion; in some cases the entire body musculature is congested, the gills are anemic and show “white gills”, and patchy congestion of the gill lamellae may also occur, though some diseased fish show no such gill symptoms. The most common symptom in the internal organs is intestinal congestion; the whole intestine or part of it appears bright red due to congestion, and the mesentery and surrounding fat are also often accompanied by obvious punctate congestion, but the intestinal smooth muscle generally remains intact, with no rot or edema. A small number of diseased fish show grayish-white or locally congested liver, spleen, and kidney, and the swim bladder wall and gallbladder are often filled with blood streaks.

Based on the symptoms and pathological changes shown by diseased fish, they can be roughly divided into three types: “red muscle type”, “red fin and red operculum type”, and “enteritis type”.

Red muscle type. The diseased fish shows no obvious external bleeding, or only slight bleeding, but the muscles are obviously congested; some show congestion of the entire body musculature, while others show punctate congestion. At the same time, the gill lamellae are often severely anemic, showing “white gills” symptoms. This type is generally more common in smaller grass carp fry, that is, in grass carp fry of 7-10 cm in size.

Red fin and red operculum type. The operculum, fin rays, top of the head, mouth, and eye socket of the diseased fish show obvious congestion, and sometimes congestion also occurs under the scales, but muscle congestion is not obvious, or only local punctate congestion is shown. This type is generally more common in larger grass carp fry, that is, in grass carp fry of 13 cm or more in size.

Enteritis type. Its characteristic is that congestion of the body surface and muscles is not very obvious, but the intestine is severely congested; the entire intestine or part of it is bright red, and the mesentery, fat, and swim bladder wall sometimes show punctate congestion. This symptom can be encountered in both large and small grass carp fry.

The symptoms of the above three types cannot be completely separated, and sometimes two types, or even all three types, may be expressed together, appearing in mixed form.

(2) Epidemiology Hemorrhagic disease is the most harmful infectious disease during the rearing stage of grass carp fry, and it also harms second-year black carp. It is widespread, has a long disease season, and causes high mortality, posing a serious threat to the development of fishery production; in particular, the mortality of summer-cultured grass carp fry and second-year grass carp and black carp can reach about 70%, but sour, gill, carp, crucian carp, and bream are not infected. The disease season is mainly from April to October, being most severe in August to September, and it is most prevalent when the water temperature is 25-30 degrees. When the water temperature drops below 25 degrees, the disease disappears.

Artificial infection of healthy grass carp requires about 4-15 days from infection to onset of disease and death, generally 7-10 days, and the disease has three stages: incubation period, prodromal period, and development period.

Incubation period. 3-10 days. During this period, the fish shows no external symptoms, and activity and feeding are normal. The length of the incubation period is closely related to the water temperature. Below 24 degrees, the infection is latent; the optimal water temperature for infection is 25-30 degrees. If the pathogen concentration is high, the incubation period is short; conversely, if the concentration is low, the incubation period is long.

Prodromal period. The prodromal period is only about 1-2 days, and the fish's body color darkens and turns black. It swims alone away from the group and stops feeding.

Development period. The length of this period varies, generally 1-2 days, and the diseased fish shows congestion symptoms and dies.

From the situation of artificial infection of healthy grass carp fry, the prodromal period and development period of diseased fish are generally very short, and if treatment is given at this time, it is probably already too late.

(3) Prevention and control methods

Injecting inactivated vaccine into fry can produce relatively strong immunity, and immunity to hemorrhagic disease can last at least 14 months or more.

For every 10,000 fry, use 0.25-0.5 kg of rhubarb or maple leaves, grind into powder, decoct or soak overnight in hot boiled water, mix with bait and feed, for 5 consecutive days, then sprinkle diqilin over the whole pond, using 0.6 g of medicine per cubic meter of water; or use 0.7 g of copper sulfate per cubic meter, applying the medicine for two consecutive days, once a day, as one treatment course. Depending on the situation, two consecutive courses can be used, with a certain therapeutic effect.

Adopting ecological disease prevention aquaculture methods, focusing on the three aspects of seed, bait, and management, changing dense culture to sparse culture, and changing single-season to double-season breeding methods, can reduce the occurrence of hemorrhagic disease.