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Detailed Explanation of the Main Symptoms and Onset Patterns of Eel Hyperactivity Disease

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Caused by Eel Coronavirus-like Virus. Main symptoms: 1) Diseased eels dart erratically up and down in the water at the pond edge, or roll and spin, or swim backwards, with occasional paroxysmal spasmodic trembling or twisting of the head, mouth open, followed by death shortly after. There are no obvious external lesions, though slight congestion or ulceration may sometimes be seen in the oral cavity, anal fin, and pectoral fin areas; necropsy may reveal swelling of the liver or kidney. 2) In the early stage of onset, 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 trembling of the head, and after frantic swimming at the water surface they die quickly; newly dead eels are rigid with the mouth 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 ulceration in the oral cavity, anal fin, and tail. Most diseased fish have bright red gill filaments and pale red liver, and some have swollen kidneys. In diseased eels, red blood cells, hemoglobin, and serum chloride ions are significantly elevated, while blood urea nitrogen content is significantly decreased, and serum cholinesterase is reduced. At present, because understanding of this disease is still lacking, diagnosis can only be based on the above main symptoms. It mainly occurs in European eels and African eels, with the high-temperature summer period as the peak epidemic season, hence it is also called European eel summer hyperactivity disease. From the start of onset to the peak period is about 7 days, and mass mortality occurs about 15 days after onset; the speed of spread and transmission is very fast, and it readily forms outbreaks and epidemics.

It is caused by the Eel Coronavirus-like Virus. Main symptoms: (1) Diseased eels dart back and forth erratically in the water at the pond edge, or roll and spin, or swim backwards, sometimes with paroxysmal spasmodic trembling or twisting of the head, with the mouth open, and die shortly afterwards. There are no obvious external symptoms; sometimes slight congestion or ulceration can be seen on the mouth, anal fin or pectoral fin, and necropsy may reveal swelling of the liver or kidney. (2) 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, rolling or backward movement. After a few days, large numbers of eels show similar symptoms, with paroxysmal trembling of the head, and die quickly after frantic swimming at the water surface; newly dead eels are rigid with the mouth 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 on the mouth, anal fin or tail. In most diseased fish the gill filaments are bright red and the liver is pale red, and some diseased fish have an enlarged kidney. In diseased eels, red blood cells, hemoglobin and serum chloride ions are significantly elevated, while blood urea nitrogen content is significantly decreased and serum cholinesterase is reduced. At present, because understanding of this disease is still lacking, diagnosis can only be based on the above main symptoms. It occurs mainly in European eels and African eels, with the peak epidemic season in the high-temperature period of summer, so it is also called European eel summer frantic swimming disease. From the onset of disease to the peak takes about 7 days, and mass mortality occurs about 15 days after onset; it spreads and extends very quickly and easily forms an explosive epidemic.

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