Prevention and Treatment of Complications Such as Enteritis and Gill Rot in Grass Carp Fingerling Culture

Symptoms The diseased fish swim slowly and have reduced appetite; the scales on the fish body are basically intact; there are one or several varying furuncles on both sides of the dorsal fin, which feel relatively soft to the touch and have a swollen sensation, and when the furuncles are cut open, purulent and bloody fluid flows out; the anus is slightly red and swollen; the gill filaments are red and swollen, with dirt attached, the ends are decayed, exposing cartilage, and the inner epidermis of the operculum is congested and inflamed, and in severe cases is corroded, appearing like an “open skylight”. When the fish body is dissected, mesenteric bleeding and intestinal wall inflammation can be seen, and in severe cases it is purplish red, with basically no food in the intestine, and there is yellow viscous matter with blood streaks; the liver and spleen are slightly swollen, the liver is earth-yellow, with bleeding points, and the texture is brittle and easily broken. Microscopic examination of the gill filaments shows a large number of Trichodina parasites, occasionally Dactylogyrus parasites, and the hindgut has Balantidium parasites. The mortality rate of mildly affected fish is 7%, and the mortality rate of severe cases is about 30%.

According to the author's diagnosis, the grass carp fingerlings were suffering from complications such as trichodiniasis, gill rot disease, furunculosis, and enteritis.

Cause analysis (1) The breeding density is relatively high, the water color is relatively dense, and the water quality is poor. Although some water quality is good, it is relatively lean. Some ponds have no independent inlet and drainage system. (2) The feed quality is poor, and the nutrition is unbalanced, resulting in a decline in the fish body's resistance and stress resistance. (3) The main green feed——fine Azolla is mainly collected from fields, ditches, ponds, and bloodworm breeding fields, and is directly fed to the fish without cleaning and disinfection. (4) The fertilizer applied by some farmers has not been fermented or is not fully fermented, causing the water quality to deteriorate and producing toxic gases, which worsens the aquaculture water environment and causes the fingerlings to become ill.

Prevention and treatment (1) Improve water quality and increase the amount of water exchange. (2) Feed compound feed and green feed together (each accounting for 50%), or feed compound feed and green feed alternately, or after continuously feeding compound feed for 4 days, stop feeding for 1 day, and then continuously feed green feed for 2 days. The fine Azolla to be fed should be cleaned thoroughly, disinfected with 3% bleaching powder solution or 10 mg/L potassium permanganate solution, and then fed promptly. Apply an appropriate amount of fermented and decomposed fertilizer or a small amount of chemical fertilizer. (3) Stop feeding when the disease occurs. On the first day, use 0.7 g of a mixture of copper sulfate and ferrous sulfate (5:2) per cubic meter of water, dissolve it in water, and splash it over the whole pond. On the second day, use 0.5 g of chlorine dioxide per cubic meter of water, dissolve it in water, and splash it over the whole pond. After stopping the drug for 2 days, use it once more according to the above method. (4) While using external drugs, mix oxytetracycline or sulfadiazine into the feed and feed it. For every 100 kg of fish body weight, use 20 g of the drug on the first day, and halve the dosage from the second day to the sixth day. 6 days is 1 treatment course. Mild cases are basically cured after 1 treatment course, and severe cases are basically cured after 2 treatment courses.