How can disease prevention and control be carried out properly in pollution-free ricefield eel farming?

    With the development of the aquaculture industry, people are paying increasing attention to the prevention and control of diseases in aquatic animals and to pollution-free aquaculture of aquatic animals. The trend in disease prevention and control is moving from a primary reliance on chemical drugs toward comprehensive prevention and control based mainly on biological agents and immunization methods, improving the immune function of farmed organisms, breeding disease-resistant varieties, and adopting ecological disease control, so that products become green products. 

    The ricefield eel (Monopterus albus) is a freshwater economic fish with tender flesh, a delicious taste and rich nutrition, and it also has medicinal and health-promoting functions. It is a delicacy and tonic health food much loved by consumers at home and abroad, and it sells very well on domestic and international markets. Artificial farming of ricefield eels has many advantages, such as requiring little land, being convenient to manage, having low costs and delivering significant economic returns, and it is increasingly favored by producers. However, with the development of intensification, eel diseases are also increasing. For this reason, this article introduces disease prevention and control of ricefield eels in the context of pollution-free farming, for the reference of ricefield eel farmers. 

    1 Disease prevention 

    In pollution-free ricefield eel farming, attention should first be paid to their disease-resistant immune capacity or to protecting their own inherent disease-resistant immune performance. The factors affecting the body defense functions of the ricefield eel's non-specific immunity mainly include surface barriers such as scales and skin, as well as the second line of defense formed by mucus and phagocytes. Scales, skin and mucus are important barriers for fish to resist infection and invasion by pathogens and parasitic organisms. Therefore, if the fish body is protected from injury and from being wounded by predators, pathogens cannot invade, and diseases such as red-skin disease, print disease and saprolegniasis will not occur. If the concentration of chemical substances in the farming water is too high, it will prompt the ricefield eel to secrete large amounts of mucus; excessive mucus secretion then fails to protect the fish body and cannot resist pathogen invasion. 

    In the early stages of disease, ricefield eels are difficult to detect as a group, so only by doing prevention work in advance can major economic losses be avoided. Disease prevention must run through the entire farming process. Here I mainly discuss the two aspects of seedling and fingerling selection and ecological disease prevention. 

    1.1 Seedling and fingerling selection 

    Seedling and fingerling selection is a key step in the entire farming process. In selecting ricefield eel seedlings and fingerlings, the following points should be noted: ① Injured or diseased ricefield eels with wounds or blood spots on the body surface, or red and swollen gill necks, are often caused by improper capture or improper temporary holding; generally, cage-caught ricefield eels should be retained for farming. ⑨ Eel fry in abnormal conditions are not suitable for farming. Purple tails and little or no mucus in ricefield eels are symptoms of saprolegniasis infection; obvious red sunken spots on the eel body, the size of soybeans, are symptoms of infection with putrid-skin disease; a large head, thin neck and emaciated body, and in severe cases a curled shape, are symptoms of capillary nematode disease, which is extremely contagious. ③ After long-term high-density holding, transport or temporary holding, ricefield eels often suffer from "fever" disease because of little water, much mucus and rising temperature; such ricefield eels often die one after another during artificial farming and are very difficult to cure. ④ Ricefield eels poisoned by drugs, for example harmed by pesticides, are difficult to identify by appearance, but after about 30 h (varying with temperature) their body color turns gray and the belly faces upward; they are not suitable for farming. ⑤ Ricefield eels that "drive piles" in relatively deep water often have poorer constitutions than those lying quietly on the bottom and are more likely to die, so they are not suitable for farming. 

    1.2 Ecological disease prevention 

    Eel disease prevention should focus mainly on ecological prevention. The main measures of ecological prevention include: ① Maintaining a good spatial environment, with reasonably built farms that meet the ricefield eel's ecological habits of preferring darkness, quiet and warmth. ② Strengthening water quality and water temperature management, keeping water quality and bottom conditions good, not making the temperature difference during water changes too large, and preventing water temperature from becoming too high. ③ Planting emergent plants or floating plants such as water hyacinth and alligator weed in the farm, and planting some climbing plants along the pond edges. ④ Stocking a small amount of loach in the ricefield eel pond to enliven the water body, and placing several toads in each pond to use their secretions to prevent eel disease. ⑤ Applying beneficial microbial preparations to improve water quality, maintain microbial balance and inhibit the reproduction of harmful microorganisms. ⑥ Diseased eels should be isolated and handled promptly. 

    In addition, in pollution-free ricefield eel farming, in order to maintain the ecological balance inside and outside the farming environment and the farmed organisms, and to inhibit or eliminate invasion and infection by harmful organisms, besides using beneficial microbial preparations as much as possible for biological control and creating a good ecological environment, it is also necessary to use disinfectants and antibacterial drugs correctly, reasonably and in a limited way. However, attention must be paid to the varieties, dosage and timing of these drugs. For example, prohibited drugs must never be used, excessive doses must not be used, and attention must be paid to pollution-free requirements such as prohibited periods and withdrawal periods. If used in excess, not only will the purpose of disease prevention and control not be achieved, but drug-induced deaths will also occur. Such deaths sometimes occur in large numbers within a short period, and sometimes occur continuously one after another during the farming process, causing quite serious losses. 

    2 Disease treatment 

    2.1 Red-skin disease 

    2.1.1 Pathogen: bacterial infection. 

    2.1.2 Symptoms: disease caused by bacteria invading the skin after the ricefield eel's skin is injured during capture or transport. The body surface of diseased eels shows local bleeding and inflammation, and the skin peels off, most obviously on the abdomen and both sides, appearing in patches. Sometimes the upper and lower jaws and opercula of ricefield eels also become congested and inflamed. Secondary saprolegnia infection often occurs at the diseased sites. Diseased eels become emaciated. It is more commonly prevalent in late spring and early summer. 

    2.1.3 Prevention and treatment methods: ① Operate carefully during capture and transport to avoid injuring the eel body; ② Before stocking, wash the eel body with 5-20 mg/L bleaching powder for about half an hour; ③ During the disease season, use bleaching powder hanging baskets for prevention. The usual dosage of bleaching powder is 0.4 g/L. ④ Splash 1.0-1.2 mg/L bleaching powder over the whole pond, and splash 2-4 mg/L gallnut over the whole pond, for 3 consecutive days; ⑤ Splash 0.05 g alum per sq m, and after 2 days splash quicklime dissolved in water at 25 g/cubic meter. 

    2.2 Enteritis 

    2.2.1 Pathogen: bacterial infection. 

    2.2.2 Symptoms: diseased eels swim alone away from the group, move slowly, and the eel body turns black, especially the head; red spots appear on the abdomen, and appetite decreases. When the intestinal tract is cut open, local congestion and inflammation can be seen in the intestines, with no food in the intestines and relatively large amounts of mucus. The disease is highly contagious, has a relatively short course and a high mortality rate. A water temperature of 25-30% is the suitable epidemic temperature for this disease. 

    2.2.3 Prevention and treatment methods: ① Strengthen feeding management, do not feed rotten or spoiled feed, remove residual bait promptly, and prevent water quality deterioration; ② During the disease season, mix 30 g garlic per 100 g of ricefield eels into the feed each d, for 3-5 consecutive days; ③ Treatment mainly combines oral and external drugs. External drugs commonly include 1×10×E-6 to 2×10×E-6 concentration bleaching powder, or 10×10×E-6 concentration quicklime, or 0.2×10×E-6 to 0.3×10×E-6 poultry erythromycin splashed over the whole pond. 

    2.3 Spotted disease 

    2.3.1 Pathogen: bacterial infection. 

    2.3.2 Symptoms: yellow round spots the size of broad beans appear on the back of diseased eels, and in severe cases they die. The disease is prevalent from June to August, peaking in mid-July.