Detailed Explanation of Key Technical Points for Eel Disease Prevention and Control
Eel is the collective term for species belonging to the order Anguilliformes. It is a type of fish with an elongated, snake-like appearance that has the basic characteristics of fish. Similar to salmon, it has migratory characteristics and no scales, and is generally found in waters where saltwater and freshwater meet. It is highly nutritious and has excellent aquaculture prospects. Let us take a look at eel disease prevention and control techniques below!
Branchionephritis
[Symptoms] Branchionephritis shows no obvious external symptoms. The gills are swollen and dark red, the gill lamellae are thickened and adherent, and the gill filaments hemorrhage. Laboratory blood plasma testing shows that chloride ions are significantly lower than in healthy eels. When this disease is prevalent, it can cause devastating mortality.
[Prevention and Control Methods] Antibiotic drugs are basically ineffective against branchionephritis. Adding salt at the early stage of the disease to bring the pond water to a concentration of 0.4%-0.7% is effective, and introducing seawater from nearby coastal areas is also effective. When complicated by other diseases, other drugs should be applied for concurrent treatment.
Liver and Kidney Disease
[Symptoms] Liver and kidney disease, also known as Edwardsiellosis, is caused by bacteria of the genus Edwardsiella. The onset temperature is 15°C, and the peak onset temperature is 25-30°C. It is a common disease and relatively serious. The bacteria invade the liver and kidneys, and it is divided into two types: the liver type and the kidney type. The kidney type is more common in the white fry and black fry stages, while the liver type is more common in the adult eel stage. Dissection reveals an enlarged or ulcerated liver, ulceration and perforation of the skin and muscle, and obviously enlarged and firm kidneys.
[Prevention and Control Methods] (1) Thoroughly disinfect during the white fry feeding-training stage and wash the tubifex worms. (2) Periodically bathe with 30 mg/L formalin and 3-5 mg/L furazolidone. (3) During the white fry stage, the temperature can be lowered to below 25°C, and compound feed should be introduced as much as possible. (4) Mix antibiotics into bait and feed, 30-50 g per ton of eel, for 5-7 days. (5) Feed allicin and norfloxacin as medicated bait at 3-5 g per kg of bait, for 5-7 consecutive days.
Red Fin Disease
[Symptoms] Red fin disease is caused by Aeromonas hydrophila and Aeromonas punctata. The skin and fins of diseased eels become congested and hemorrhagic, appearing distinctly red, and in severe cases ulceration occurs. The liver and spleen become enlarged, and the mortality rate is high. It occurs in all four seasons, with more severe occurrences in spring and summer, and mostly affects eel fingerlings and adult eels.
[Prevention and Control Methods] (1) When clearing the pond, thoroughly remove the sludge at the bottom, and sprinkle 30%-40% limewater over the entire pond bottom and walls, or soak and wash the pond with 2 mg/L bleaching powder for 5-7 hours. (2) Before transferring eel fingerlings to a pond, soak them in 3%-5% salt water for 10-15 minutes, or soak them in 5-6 mg/L nitrofurazone for more than 30 minutes. (3) For affected ponds, splash 25-30 mg/L formalin to disinfect the pond, or splash 1-2 mg/L nitrofuran preparations for disinfection. (4) Administer oral antibiotics to the eels. Use 100 mg of sulfonamide drugs per kg of fish body weight (double the dose for the first administration), 20 mg each of tetracycline, oxytetracycline, and chloramphenicol, and 4 mg of nitrofuran preparations, mixed into the feed and fed once daily for 5-7 consecutive days.
Gill Rot Disease
[Symptoms] Gill rot disease is caused by bacteria or certain parasites invading the gills. Diseased eels show almost no external abnormalities. Affected fish swim alone away from the group and gasp at the surface. When the gill plates are opened, the gill filament tissue is damaged with large defects. As the disease progresses, pressing the gills of diseased eels lightly with a finger causes bloody mucus to flow out. The eels are anemic and appear white, and the liver turns yellowish-white. It occurs mostly during high-temperature periods and mainly harms adult eels, rarely affecting fingerlings, causing scattered deaths. Excessive feeding and overeating are predisposing factors for the occurrence of this disease.
[Prevention and Control Methods] (1) Prevent overfeeding. During outbreaks, stop feeding or reduce the amount of compound feed; where live bait is available, switch to feeding live bait. (2) Use antibiotics, sulfonamide drugs, nitrofuran preparations, and other drugs mixed into feed for oral administration, with the same dosage as for red fin disease. (3) Splash nitrofuran preparations and formalin, splash the pond with 0.8-1 mg/L potassium permanganate, or bathe with 0.5% salt water, which also has certain therapeutic effects. When examination of the gills reveals damage from nematodes or other parasites, splash 0.2-0.4 mg/L trichlorfon, and repeat once after 2-3 days, for 2-3 consecutive treatments.