Hemorrhagic symptoms and prevention of farmed grass carp

Hemorrhagic disease is a fish disease caused by reovirus. Most of the virus particles are spherical or hexagonal. The intact virus particles are 68-72 nm in diameter, much smaller than the smallest bacteria.

The main results are as follows: (1) the diseased fish are mainly hyperemia. External symptoms are generally slightly red, small fish in the sun or light perspective, you can see subcutaneous hyperemia. Some of the diseased fish also showed hyperemia in the mouth, jaw, operculum and the base of the fin. Peel off the skin of diseased fish, some muscles show punctate or massive hyperemia, some systemic muscles show hyperemia, Gill anemia, the emergence of "white Gill", Gill may be speckled hyperemia, but some diseased fish Gill does not have this symptom. The common symptoms of internal organs are intestinal congestion, the whole intestine or part is bright red because of congestion, mesentery and surrounding fat, often accompanied by obvious punctate hyperemia, but intestinal smooth muscle is generally intact without decay or edema. A few diseased fish liver, spleen, kidney and so on appear gray or local congestion, ethereal walls and gallbladder are often full of blood.

According to the symptoms and pathological changes of diseased fish, they can be divided into three types: "red muscle type", "red fin red Gill cover type" and "enteritis type".

Red muscle type. The diseased fish showed no obvious bleeding or only slight bleeding, but the muscles were obviously congested, some showed systemic muscle hyperemia, and some showed speckled hyperemia. At the same time, the Gill flap is often severely anemic, showing "white Gill" symptoms. This type is generally common in smaller grass carp species, that is, grass carp species with sizes of 7 to 10 centimeters.

Red fin red operculum type. The Gill cover, fin, head, mouth and eye cavity of the disease show obvious hyperemia, sometimes there is hyperemia under the scale, but muscle congestion is not obvious, or only local punctate hyperemia. This type is generally found in the larger grass carp species. That is, it is more common in grass carp species with specifications of more than 13 centimeters.

Enteritis type. Its characteristic is that the body surface and muscle congestion phenomenon is not obvious, but the intestinal tract is seriously congested, all or part of the intestinal tract is bright red, mesentery, fat, sometimes a little hyperemia. This symptom can be found in both large and small grass carp.

The above three types of symptoms, can not be completely separated, sometimes two types, or even three types are shown, showing mixed appearance.

(2) epidemic hemorrhagic disease is not only the most harmful infectious disease in the breeding stage of grass carp, but also harmful to two-year-old herring. It has a wide epidemic, a long morbidity season and a high mortality rate, which poses a serious threat to the development of fishery production. In particular, the morbidity and mortality rate of summer grass carp, two-year-old grass carp and herring can be as high as 70%, but acid, Gill, carp, crucian carp, braid and so on are not infected. The disease season is mainly from April to October, especially from August to September, and it is the most popular 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 takes about 15 days from infection to death, usually 7 to 10 days. The disease can be divided into three stages: incubation period, prechemotaxis stage and development stage.

Incubation period. During this period, the appearance of the fish did not show any symptoms, and the activity and feeding were normal. The incubation period is closely related to the water temperature. There is recessive infection below 24 degrees; the optimum water temperature of infection is 25Mel and 30 degrees. If the concentration of pathogen is high, the incubation period is short; on the contrary, if the concentration is low, the incubation period is long.

Antecedent period. The pre-trend period is only about 2 days, and the body color of the fish darkens and darkens. Travel alone from the group and stop feeding.

Period of development. The duration varies, usually for 2 days, and the diseased fish show symptoms of congestion and die.

From the point of view of artificial infection of healthy grass carp species, the pretending period and development period of diseased fish are generally very short. If we treat them again at this time, I am afraid it will be too late.

(3) Prevention and control methods.

Injecting inactivated vaccine into fish species can produce strong immunity, and the immunity to hemorrhagic disease can be maintained for at least 14 months.

Every ten thousand fish species were powdered with 0.25 kilograms of rhubarb or maple leaves, boiled or soaked in hot water overnight, mixed with bait for 5 days, and then sprinkled with carbendazim, 0.6 grams per cubic meter of water, or 0.7 grams of copper sulfate per cubic meter, once a day for two consecutive days, as a course of treatment. Depending on the situation, you can use two courses of treatment, which has a certain curative effect.

The occurrence of hemorrhagic disease can be reduced by using ecological disease prevention and breeding method, focusing on species, bait and management, changing dense culture to sparse culture and changing single season to double season breeding method.