Analysis of the Causes of Eel Diseases and Prevention and Treatment Methods
Under certain temperature and humidity conditions, many molds can multiply in large numbers in feed ingredients or finished feed, producing mycotoxins that have carcinogenic, cytotoxic or neurotoxic effects, among which aflatoxin is relatively widespread and highly toxic. Aflatoxin can cause damage to the liver and kidneys of eels and has carcinogenic and lethal effects, but the median lethal dose (LD50) for eels has not yet been reported. Because eels are relatively expensive, eel farmers generally do not feed moldy feed. However, when mixing ingredients, feed mills must take care that the raw materials used are not contaminated by Aspergillus flavus. Pesticides, polychlorinated biphenyls, crude oil and hydrocarbons cause water pollution through residual pesticides applied in farmland, discharges from chemical plants, and leaks from oil refineries. These substances are highly toxic to eels and can cause liver enlargement and abnormal development, and have carcinogenic and lethal effects. Among them, organophosphorus and organochlorine pesticides can both disrupt the normal physiological function of the eel nervous system, inhibit the transmission of nerve energy and block ion transport. Although the direct toxic effect of organochlorine pesticides on eels is not as obvious as that of organophosphorus pesticides, they are more stable than organophosphorus pesticides and can accumulate in the eel body, creating serious hidden dangers.
Heavy metals mainly refer to mercury, lead, zinc, cadmium, nickel, etc. The harm of heavy metals to eels includes external toxicity and internal toxicity. External toxicity is when they combine with the mucus secreted by the gills and body surface to form protein complexes, covering the entire gills and body surface and filling the gaps between gill filaments, making normal movement of the gill filaments difficult and causing the eel to suffocate to death; internal toxicity is when heavy metal ions enter the eel body through the gills and body surface and combine with sulfhydryl groups in the catalytic active sites of major enzymes in the body to form insoluble sulfates, thereby inhibiting enzyme activity, hindering the body's metabolism and causing eel death.
Therefore, when choosing water for eel farming, attention should be paid to water quality analysis, and water sources free from pollution should be selected as far as possible. At the same time, the water environment should be protected and the environmental awareness of the whole population should be strengthened.
The main raw material of eel feed——fish meal undergoes protein decomposition during transportation and storage, producing a series of toxins such as histamine, putrescine, cadaverine, tyramine, and agmatine, which are very harmful to fish. Existing research has focused more on histamine. Histamine is a decomposition product of histidine; it is an amine substance formed after histidine loses its hydroxyl group under the action of histamine dehydrogenase in the presence of bacteria such as Morganella morganii and Achromobacter histaminicum. Eels will suffer poisoning after ingesting a certain amount of histamine. Therefore, the freshness of fish meal is crucial to the quality of eel feed and the growth of eels. Histamine content and volatile basic nitrogen (VBN) can be used as indicators for measuring the freshness of fish meal. Generally, fish meal of good freshness has a histamine mass concentration of 100mg/cubic decimeter and VBN<60×0.00001m/m.