What are the prevention and treatment methods for eel farming diseases?

Eel swim bladder nematode disease [Pathogen] is caused by eel swim bladder nematodes. There are three species of eel swim bladder nematodes parasitic in the swim bladder of eels: Anguillicola globiceps, Anguillicola crassa, and Anguillicola australiensis. Eel swim bladder nematodes are ovoviviparous; newly released larvae measure only 0.2 mm-0.35 mm, are covered with a transparent membrane, and have a slender tail.

[Symptoms and Diagnosis] Eel swim bladder nematodes mainly parasitize the swim bladder cavity, feeding on the blood of eels. Diseased eels infected with nematodes show poor feeding and stunted growth. They exhibit anemia, emaciation, and a darker body color. The swim bladder of diseased eels becomes swollen, internal organs are compressed, blood flow is obstructed, and the swim bladder tissue becomes inflamed. Diseased eels float on the water surface and have difficulty sinking, gradually become emaciated, and eventually die. Severe infection can cause the swim bladder wall to rupture, allowing nematodes to enter the abdominal cavity, causing peritonitis and leading to eel death. [Epidemiology and Harm] Eel swim bladder nematode disease is prevalent in eel farms in Taiwan, China, Guangdong, Fujian, Zhejiang, Jiangsu and other provinces. The infection rate of this disease is very high, but cases of mass mortality are generally rare. This disease occurs throughout the year, but is more common from June to September. [Prevention and Treatment] (1) Clearing ponds with lime or bleaching powder can kill larvae and intermediate hosts, providing a preventive effect. (2) Spraying trichlorfon (90% crystalline) over the entire pond at a final concentration of 1 mg/L in the water body has a good control effect. (3) Feeding antibacterial drugs to prevent bacterial infection. Edwardsiellosis [Pathogen] Edwardsiella tarda. Growth temperature 12°C-42°C, pH range 5.5-9. It can grow in a salinity range of 0-4%. [Symptoms and Diagnosis] Based on symptoms and pathological characteristics, this disease can be divided into two types: the liver type and the kidney type. Liver type symptoms: mainly manifested as swelling of the anterior abdomen of eels, congestion or hemorrhage, necrosis of abdominal wall muscles, and softening of the skin. In severe cases, the anterior abdomen perforates and the liver is exposed. Dissection reveals an enlarged liver with white purulent lesions. Kidney type symptoms mainly manifest as: red and swollen protruding anus, swelling of the kidney area before and after the anus, muscle necrosis, skin congestion, and purulent blood flowing out when the abdomen is pressed. Dissection reveals an enlarged spleen and kidney with small purulent lesions. In clinical diagnosis, the typical symptoms of this disease should be noted. Eels suffering from this disease show swelling and redness of the anterior abdomen or the area around the anus, softening of the skin, muscle necrosis, and even ulceration and perforation. [Epidemiology and Harm] This disease is prevalent in eel farms in Japan, Taiwan, China, and the southeastern coastal provinces of China; it causes significant harm. It is most likely to cause acute outbreaks especially about one week after feeding white elvers with “red worms” (bloodworms), resulting in mass mortality. [Prevention and Treatment] Prevention mainly involves strengthening feeding management, regularly removing residual bait and fecal waste, appropriately increasing water exchange, maintaining clean water quality, avoiding excessive stocking density, and washing red worms before feeding. (1) Use 50 g-100 g of Yufukang B-II per ton of eels, evenly mixed into the bait and fed once daily for 3-5 consecutive days. (2) Use 200 g of sulfamethoxazole per ton of eels mixed into the bait and fed, then switch to 100 g per day from the second day, feeding continuously for 5 days. Bacterial gill rot disease [Pathogen] Flexibacter columnaris. Growth temperature 5°C-35°C, optimal temperature 27°C-28°C; growth pH range 6.5-8.5, optimal pH around 7.5. The presence of 0.7% salt in the water body can inhibit its growth. [Symptoms and Diagnosis] This disease mainly causes necrosis of gill tissue. Diseased eels swim weakly at the water surface, sometimes stopping in corners with weak water flow or by the pond edge. Diseased eels have a slightly darker body color with no particularly obvious external symptoms. If the gills are gently pressed with a finger, reddened sticky pus will flow out from the gill openings. When the operculum is cut open, the gill filaments can be seen to be ulcerated starting from the edges, with gill tissue congestion, blood stasis, and abundant mucus. The ulcerated areas are yellowish and have many mud and debris particles adhering to them. In severe cases, most gill filaments can be seen to be decayed. The diagnostic method mainly involves observing whether there are necrotic lesions in the gill tissue. [Epidemiology and Harm] This disease has a long epidemic season and can be seen year-round, but is more severe during the high water temperature period in summer. It is a common disease, frequently occurring, and widely prevalent. It is prevalent in eel farms in Taiwan, China, Fujian, Jiangsu, Zhejiang, and Guangdong. [Prevention and Treatment] This disease is closely related to high stocking density, insufficient oxygen in the water, and excessive feeding. It has been found that Flexibacter columnaris grows well on the surface of pelleted compound feed for eels, so residual bait and feed dispersal provide favorable conditions for the massive proliferation of pathogenic bacteria. Therefore, to prevent this disease, special attention should be paid to feeding methods, avoiding excessive feeding, regularly removing residual bait, maintaining clean water quality, increasing the use of paddlewheel aerators, and appropriately increasing water exchange — all of which are effective prevention and control measures.

Tail rot disease [Pathogen] There are two types of pathogenic bacteria causing eel tail rot: one is Flexibacter columnaris, and the other is certain Aeromonas punctata strains. The causes of tail rot disease are relatively complex; besides pathogenic bacteria, it is also closely related to injury to eels, deterioration of water quality, and poor feeding management. [Symptoms and Diagnosis] The skin of the caudal peduncle, trunk, or head of diseased fish ulcerates, necroses and sloughs off; in severe cases, ulceration extends deep into the muscle layer, and some diseased eels have ulceration at the snout tip. This disease often occurs concurrently with saprolegniasis and hepatorenal disease. During low-temperature and rainy seasons, ulcerated areas are prone to developing saprolegnia. Continued progression of tail rot can lead to bacterial septicemia, with varying degrees of pathological changes in various organs. Notable characteristics: first, swelling of the neck region without redness; second, coagulated blood clots or complete absence of blood color in the gastrointestinal tract. [Epidemiology and Harm] This disease is a common disease in farmed eels in China and is prevalent to varying degrees in eel farms in the southeastern coastal provinces of China. During the high-temperature summer season, tail rot disease is most likely to occur when operations such as grading cause skin damage and loss of eel mucus. [Prevention and Treatment] (1) Handle eels carefully during pond transfer and grading to minimize injury. During pond transfer, pay attention to disinfecting fish bodies and ponds to prevent infection. (2) Bathing with 4%-5% salt for 1 minute-2 minutes has a good preventive and therapeutic effect on tail rot disease.

Red fin disease [Pathogen] Aeromonas hydrophila. Growth temperature range 5°C-40°C, optimal temperature 8°C; it can grow in a pH range of 6-11, with an optimal pH of 7.2-7.4. It can grow in salt solutions not exceeding 4% concentration, with an optimal salt concentration of 0.5%. [Symptoms and Diagnosis] Diseased eels initially show loss of appetite, then swim alone at the water surface away from the group or rest against the pond wall or stay motionless at the feeding platform. The anal fin, pectoral fin, and caudal fin of diseased eels become congested and red, the skin of the abdomen or body sides becomes congested, and the anus becomes red and swollen. In severe cases, hemorrhagic patches can be seen on the abdomen, body sides, and other areas. Sometimes local necrosis or saprolegnia infection occurs on the skin of the head, trunk, and tail. Dissection reveals obvious congestion and redness of the intestine. Pathogenic bacteria multiply in large numbers in the intestine, and the toxins they produce first cause intestinal inflammation, then invade the blood and cause systemic symptoms. As the disease progresses to a certain stage, bacteria invade the blood from the intestine, resulting in septicemia. [Epidemiology and Harm] Red fin disease is a common disease that can also occur as explosive outbreaks, causing serious harm. It is mainly prevalent in outdoor ponds, and eels of all sizes can contract this disease. It is prone to occur and spread during the spring water temperature rise period, the autumn water temperature decline period, and the plum rain season when weather is unstable; outbreaks are generally less common during high water temperature periods. [Prevention and Treatment] (1) Use 150 g-200 g of sulfamethoxazole or sulfonamide potentiator per ton of eels mixed into the bait and fed, then 100 g per day from the second day, feeding continuously for 5 days. (2) Use 100 g of oxytetracycline hydrochloride or tetracycline per ton of eels daily mixed into the bait and fed for 5 consecutive days

Vibriosis [Pathogen] Vibrio anguillarum disease. It is a Gram-negative short bacillus with a growth temperature of 10°C-35°C, optimal growth temperature around 25°C; optimal growth salt concentration around 1%. Growth pH range 6-10, optimal growth pH around 8. [Symptoms and Diagnosis] The main symptoms of eel vibriosis are hemorrhagic redness of all fin rays, red and swollen anus, and depigmentation, erosion, or swelling of the trunk skin. Some diseased fish develop hemorrhagic ulcers on the body surface. The intestine is usually congested. The liver is enlarged and yellowish-brown with hemorrhagic spots. Diagnosis of eel vibriosis based solely on symptom observation is difficult; serological methods or isolation and identification of the pathogen must be used. [Epidemiology and Harm] The occurrence of this disease, like red spot disease, is closely related to the salinity of the pond water, and it usually does not occur in eels farmed entirely in freshwater. Vibrio anguillarum is widely distributed in coastal seawater and on the seabed as a conditional pathogen. The occurrence and spread of eel vibriosis have certain predisposing factors, such as strong stimulation from adverse water quality factors and injury to fish bodies. Vibrio anguillarum is mainly introduced into freshwater eel farms by seed stock. [Prevention and Treatment] (1) Vaccination with Vibrio anguillarum vaccine. (2) Mix oxytetracycline hydrochloride into feed and feed. Use 100 g per ton of eels daily for 5 consecutive days.

Red spot disease [Pathogen] Pseudomonas anguilliseptica. It is a Gram-negative short bacillus with a growth temperature range of 5°C-30°C, optimal growth temperature 15°C-20°C; growth pH range 5.3-9.7, optimal growth pH 7-9. This bacterium dies within one day in freshwater, but can survive for over 200 days in seawater or diluted seawater. [Symptoms and Diagnosis] The main symptom of this disease is petechial hemorrhage on the body surface. The skin of diseased eels, especially the skin of the lower jaw, abdomen, or around the anus, shows the most obvious and numerous petechial hemorrhages. After invading the skin, pathogenic bacteria multiply in large numbers in the epidermal basement membrane and dermis, causing capillary congestion, which leads to exudative hemorrhage and ruptured patchy hemorrhage. In addition, fin ray hemorrhage, liver congestion, spleen depigmentation and atrophy, kidney atrophy, and petechial hemorrhage in the stomach, intestine, and mesentery can be seen. The most obvious characteristic of this disease is numerous petechial hemorrhages on the body surface, based on which a diagnosis can be made. [Epidemiology and Harm] This disease occurs in eel ponds containing salt. The epidemic periods are February-August and October-November, with April-May being the peak epidemic period. In early spring, the disease begins when water temperature rises above 10°C; the disease weakens when water temperature rises to 25°C, disappears in midsummer, and re-epidemics in autumn when water temperature drops below 25°C. [Prevention and Treatment] Red spot disease occurs at 10°C-25°C; therefore, in greenhouse eel farming, controlling water temperature at 26°C-27°C can prevent this disease. The occurrence of this disease is related to the salt content in the water, so special attention should be paid to eels raised in salt-containing ponds. Raising eels in salt-free freshwater is an important measure to prevent this disease; care should be taken to replace pond water used for saltwater baths and rinse once or twice. This disease has a long incubation period; diseased fish lose their appetite before symptoms appear, so oral medication treatment is difficult to effective. Furan bath at a final water concentration of 2 mg/L can be sprayed over the entire pond.

Open mouth disease [Pathogen] This disease is caused by a virus that parasitizes the cytoplasm of eel leukocytes. The virus can be cultured using eel gonadal cells and can cause cytopathic effects.

[Symptoms and Diagnosis] Diseased fish show cranial cavity hemorrhage, oral mucosa hemorrhage, head muscle hemorrhage, lymphocyte infiltration, oral muscle erosion, mouth open and unable to close, upper and lower jaw congestion and erosion, shortened upper jaw, and in severe cases, fracture of the lower jawbone and complete loss of ability to open and close the mouth. In severe cases, liver congestion, enlargement, and even ulceration can be seen. Diseased fish sometimes also show fin ray congestion. [Prevention and Treatment] There is currently no effective method

European eel summer frantic swimming disease [Pathogen] The cause is still unclear. It may be due to inability to adapt to changes in the living environment (such as water temperature, lighting, light quality, bait and drug stimulation, etc.), and prolonged environmental stress causes excessive stress response in the fish body, leading to imbalance of physiological functions, damage to the function of certain important organs, decreased disease resistance, and ultimately mass mortality of European eels. Or it may be mainly due to secondary pathological or bacterial infections caused by stress factors, causing mass mortality of European eels. To fully understand the cause, extensive and in-depth research is still needed. [Symptoms and Diagnosis] In the early stage of the disease, a small number of eels swim irregularly at the pond edge, swimming weakly, with intermittent head twitching, spinning and rolling, or backward movement. After a few days, a large number of eels show similar symptoms, with paroxysmal head trembling, frantic swimming at the water surface followed by rapid death. Newly dead eels are rigid with mouths open. The body surface of diseased fish is intact, but the lower jaw shows varying degrees of ulceration or congestion. Some diseased fish have congestion or ulcers in the oral cavity, anal fin, and tail. Most diseased fish have bright red gill filaments and pale red livers, and some have enlarged kidneys. Diseased eels show significantly elevated red blood cells, hemoglobin, and serum chloride ions, while blood urea nitrogen content significantly decreases, and serum cholinesterase decreases. Currently, due to the lack of in-depth understanding of this disease, diagnosis can only be based on the above main symptoms. [Epidemiology and Harm] This disease is a serious disease that occurred when European eels were introduced to Asia for farming, mainly prevalent during the high-temperature summer period, and can occur from black fry to adult eels. The disease strikes suddenly, with mortality rates reaching over 90% within one week, and in worse cases, almost all die. [Prevention and Treatment] There is currently no effective prevention or treatment method. The following preventive measures are provided: 1) Do not introduce European eel seed stock in summer, to avoid the stress on newly introduced seed stock during the high-temperature period. According to reports, the optimal water temperature for European eel growth is 20°C; continued rise in water temperature above 24°C is very unfavorable for European eels and should be noted. 2) Pay attention to maintaining relatively stable and clean water environment in eel ponds, preventing excessive changes in water quality and water temperature. 3) Set up shade structures at fixed points over eel ponds to avoid direct strong sunlight. 4) Control feeding amount during high-temperature periods; less is better than more. 5) Pay attention to timely prevention and treatment of parasitic diseases and other diseases; healthy fish bodies can enhance stress response capacity. 6) When necessary, use any one of the following therapeutic drugs as an external application and spray once over the entire pond.