Key Technical Points for the Prevention and Treatment of European Eel Depuration-Septicemia Syndrome

Changle City is a key area for European eel (Anguilla anguilla) farming and is the largest European eel farming region in China. Based on farming practices in recent years, European eel depuration-septicemia syndrome has occurred to varying degrees at every farm, with mortality rates of 5%-20%. European eel depuration-septicemia syndrome is a common and frequently occurring disease in European eel farming and is one of the most serious diseases affecting European eel farming in summer. If not handled properly and if sensitive antibacterial drugs cannot be promptly screened for effective treatment, it will bring serious economic losses to farmers. Therefore, how to correctly prevent and treat European eel depuration-septicemia syndrome is an important factor in improving the survival rate of European eel farming and increasing economic benefits.
I. Pathogen
It has been reported that the disease is caused by infection with non-O1 Vibrio cholerae and Aeromonas species, and is especially likely to be triggered when water quality is poor and stocking density is too high.
II. Epidemiological Characteristics
Since depuration disease and septicemia were first discovered in 1995, up until 1998 they occurred independently, and some basic treatment regimens were relatively easy to control. However, in recent years, septicemia often occurs as a complication after the onset of depuration disease, making it one of the most serious diseases in summer. This disease is most prevalent in late spring and during the high-temperature seasons of summer and autumn, with the peak epidemic period from June to August. The prevalent water temperature is generally 24-32°C, and the incidence rate is relatively low during low-temperature periods. During high-temperature periods, it is extremely prone to occur 3-5 days after grading and medication, especially after the use of drugs with strong irritant effects on European eels, such as formalin and high doses of chlorine-containing disinfectants. The disease is highly contagious and the bacterial strains are highly virulent. From the discovery of symptoms in individual eels to severe illness takes only 3-5 days, and it generally causes successive infection throughout the entire farm, with a disease course lasting more than one month and high mortality. If effective treatment is not administered promptly, the mortality rate can reach 5%-20%.
III. Symptoms and Lesions
1. Before the onset of the disease, European eels show no warning signs; it generally occurs suddenly. At the onset, depuration disease symptoms appear first: sick eels have weakened vitality and often swim slowly and weakly by the pond edge or against the water current, and some hang on the feeding tray. The water surface of affected ponds appears foamy, and as mucus in the water increases and water quality deteriorates, the foam continuously increases. At the onset, sick eels have mucus detachment patches on the body surface, which are more obvious on the trunk and flanks. In severe cases, ulcers form at the depuration lesion sites with red surroundings, usually accompanied by gill rot symptoms. About 3-5 days after depuration disease develops, septicemia begins to accompany it: opercular edema, congestion, gill filaments becoming anemic or brownish, and adhesion. Pectoral fins and anal fins are severely congested and red, the anus is red and swollen, and the abdominal skin has severe hemorrhage. Sometimes the lower jaw is congested, giving the head a "red head" appearance. On necropsy, most show abdominal cavity accumulation of bloody fluid, enlarged liver with pale or yellowish-brown color, distended gallbladder, congested and enlarged kidney, empty gastrointestinal tract, and inflamed, congested, or fluid-filled intestinal mucosa.
2. In the early stage of mortality, sick eels show obvious depuration or septicemia symptoms: eels have weakened vitality, large numbers hang on the feeding tray or gather around the drainage outlet, and they do not feed. Among dead eels, depuration symptoms are not numerous, while septicemia symptoms are more obvious. In the later stage, the number showing depuration or septicemia symptoms decreases, and red head or gill and lower jaw congestion and edema symptoms are more commonly seen, with other parts of the body surface basically intact; some have abdominal cavity filled with bloody fluid; there are blood clots, liver discoloration, and gill filaments turning white and swollen, with a phenomenon of discoloration and whitening similar to that after soaking in water. The number of deaths increases very rapidly, and the mortality rate is relatively high. The entire disease cycle in a single pond lasts 15-20 days.
It is worth noting that the ulcer symptoms appearing on the body surface of sick eels are often related to parasitism by myxosporidians (Myxidium). Moreover, in eels with severe septicemia, the kidney is usually parasitized by myxosporidians in large numbers, with several to dozens of white spots (cysts) visible to the naked eye. Therefore, whether myxosporidian parasitism of the kidney is a predisposing factor for the onset of disease in eels is worthy of study.
IV. Comprehensive Prevention
1. Disinfection of eel fry ponds. For newly built ponds, before use, soak with 1.0-1.5 mg/L oxalic acid to remove alkalinity, wash and change water multiple times, and soak in stored water for more than 20 days; or directly soak in clean water for more than 30-40 days. Before use, measure the pH value, and put into use after testing is qualified. Seven days before stocking fry, soak and disinfect with 20 mg/L potassium permanganate for 3 days, drain and wash clean, then add fresh water and heat, and make the pH value of the pond water less than 8, then set aside for use. Old ponds should be fully sun-dried, and 200 g/cubic m quicklime or 20 g/cubic m bleaching powder should be splashed over the entire pond for disinfection for 10-15 days, then washed clean with fresh water; seven days before stocking fry, soak with 20 mg/L potassium permanganate for 3 days, drain and wash clean, then set aside for use. One day before releasing fry, fill the pond with water to a depth of 30-35 cm.
2. Stocking density should be reasonably set according to the different stages of European eels. General stocking densities should be controlled as follows: at the glass eel (white fry) stage, 500 individuals/sq m; at the black fry stage of 150-20 individuals/kg, stocking density controlled at 300-30 individuals/sq m; at the young eel and adult eel stage of 10-20 individuals/kg, stocking density controlled below 30 individuals/sq m.
V. Drug Treatment
Diagnosis should be made as early as possible based on the symptoms of the eels, and corresponding measures should be taken. In the early stage of the disease, using antibiotics cannot obviously inhibit the progression of mortality, and sometimes may instead accelerate the death of sick eels. Therefore, in the early stage of the disease, conventional disinfectants such as bromochlorohydantoin, chlorine dioxide, and Blue Angel + Virus Net can be chosen to first control disease progression. At this time, disinfection of the water body must also use drugs with low irritation. After a relatively large number of deaths occur, sensitive drugs should then be selected for treatment. During high-temperature periods, attention should be paid to changes in water temperature and water quality. Generally, ventilation should be strengthened to increase dissolved oxygen in the water, and when necessary, the water level for medication should be raised to prevent a sharp change in water temperature during medication from causing increased eel mortality. In order to allow the eels remaining in the pond to feed as early as possible to restore their strength, medicated feed can be administered, and at the same time Haizhongbao, multivitamins, glucose, etc. can be used for water bath treatment of the eels to eliminate pathogenic bacteria in the eel body and promote feeding.
After the onset of this disease, when necessary, samples should be sent for bacterial drug sensitivity testing so that the right drug can be used promptly. In recent years, the following drug treatment regimens have all achieved good results.
① Splash eel-use electrolytic vitamin at 4-8 g/cubic m for 3 hours, then bathe with Manleling V at 8-10 g/cubic m for 24 hours, then change water and replenish the drug, repeating 2-3 times. ② Bathe with Manleling V at 5 g/cubic m plus Nongda High-Stability C at 3 g/cubic m plus Jundusan at 20 g/cubic m for 48 hours. ③ Bathe with 3‰-5‰ table salt plus Jundusan at 10 mg/L plus Manleling V at 5 mg/L plus Nongda High-Stability C at 3 g/cubic m, with daily drainage, water change, and drug replenishment, for 2-3 consecutive days. ④ Sodium chloride (table salt) 4‰ + sodium bicarbonate 4‰ + chlorine-containing disinfectant.
After choosing any one of the above regimens for treatment and the disease condition is brought under control, it is still necessary to bathe with Youdiansu at 2-3 mg/L + Shadu Xianfeng at 0.3-0.5 mg/L for 2-3 consecutive treatments.
The recovery period is relatively long, so do not be impatient. Use Youdiansu at 1.5-3 mg/L for water body disinfection for 2 consecutive treatments; alternatively, chlorine-containing disinfectants can be used alternately; for oral administration, Manleling, vitamin C, eel-use electrolytic vitamin, etc. can be used continuously for about 10 days.
VI. Analysis and Discussion
When treating European eel depuration-septicemia syndrome with drugs, do not use drugs indiscriminately in a panic, and never use prohibited drugs, which could cause drug residues and affect the quality of adult eels. Because medication is needed for a long time, it easily causes water quality deterioration. Therefore, after each medication cycle ends, it is best to carry out a large water change to keep the water environment clean.
During high-temperature periods, attention should be paid to changes in water temperature and water quality. Generally, ventilation should be strengthened to increase dissolved oxygen in the water, and when necessary, the water level for medication should be raised to prevent a sharp change in water temperature during medication from causing increased eel mortality. In the early stage of the disease, try not to use antibiotics; conventional disinfectants such as bromochlorohydantoin, chlorine dioxide, and Blue Angel + Virus Net can be chosen to first control disease progression, and after a relatively large number of deaths occur, then use drugs for treatment.
After the peak mortality period has passed, the eels remaining in the pond must be allowed to feed as early as possible to restore their strength. Medicated feed can be administered to eliminate pathogenic bacteria in the eel body. At this time, disinfection of the water body should use drugs with low irritation, and at the same time Haizhongbao, multivitamins, glucose, etc. can be used for immersion bathing of the eels to promote feeding. While dealing with bacteria, attention should also be paid to checking for gill mold and parasitic diseases. If present, they should be treated first, especially gill mold, which must be killed first. Otherwise, using antibiotics will trigger an outbreak of gill mold disease and cause massive deaths. If the number of parasites is not large, treatment can be temporarily deferred. European eel summer syndrome is a hurdle; once past it, everything will go smoothly.