What pathogenic effects does feed contamination have on fish?

Because some feed products currently contain bacteria at levels far exceeding the standard, mass fish deaths occur from time to time after fish eat contaminated feed. Based on the disease symptoms in fish, it can be roughly divided into two types: acute type (explosive) and chronic type.

1. Acute type (explosive type): After fish eat contaminated feed, disease onset occurs in as much as 3 to 5 days, or as little as 1 to 2 days, with symptoms appearing; severely affected fish have already died, and the number of dead fish shows a gradually increasing trend, but feeding is not affected, and most fish can eat normally. The symptoms of fish in this acute stage of disease resemble those of hemorrhagic septicemia. Diseased fish show bleeding all over the body, with large continuous patches of hemorrhagic spots on both sides of the body surface and the abdomen. The chest, pectoral fins, edges of the gill covers, and jaw bleed severely and are bright red, and the anus of diseased fish is red. Necropsy examination shows that diseased fish suffer from severe enteritis, the hepatopancreas is enlarged, and the gallbladder is very swollen, filled with dark green bile, with symptoms of cholestasis.

2. Chronic type: After fish eat a certain amount of contaminated feed or a certain amount of feed raw materials contaminated by something, in as much as one month or 40 days, or as little as 10 to 20 days, the fish slowly show symptoms; at first the symptoms are mild, and later they gradually worsen. At the beginning of onset, appetite is poor, they do not compete for food, feeding is weak, and the body color turns black. As time goes on, some fish do not come to the feeding platform to compete for food and only swim around nearby; individual fish eat as if startled, and just after taking food they suddenly swim away, causing patches of fish to also suddenly swim away and scatter, and the number of feeding fish gradually decreases. In severe cases, most fish may refuse to eat, and progressive deaths occur. Necropsy examination shows that most fish have typical symptoms of enteritis; the intestinal epithelial mucosa layer of diseased fish sloughs off and appears transparent, with colorless or pale yellow mucous or watery liquid inside, and containing some bubbly gas. In addition, in production, there are also subacute cases between the acute type and chronic type in terms of symptoms, course of disease, and amount of death loss. After fish eat contaminated feed, in as much as 10 to 20 days, or as little as about 7 days, disease onset and symptoms appear, and deaths occur. In fish with this type of disease, besides typical enteritis, other symptoms and the number of dead fish are less obvious than in the acute type. Contaminated feed can not only cause poisoning in vertebrate fish, but can also cause disease and death in the crustacean Chinese mitten crab. After fish eat contaminated feed, the symptoms shown in mild, severe, slow, and rapid cases and the number of dead fish have a certain correlation with the degree of feed contamination and the total bacterial content. The more severe the feed contamination and the higher the bacterial content, the more rapid the disease onset in fish, the more obvious the symptoms, the greater the number of dead fish, and the more serious the loss; otherwise, the opposite is true. Toxic and harmful components of feed contamination. The process in which feed is infected, colonized, and multiplied by microorganisms such as bacteria and molds during production, processing, storage, and transportation is called biological contamination of feed. Because the contamination damage results caused by microbial contamination of feed differ, the toxic and harmful components produced also differ. Feed contamination can generally be divided into three situations: 1. Feed is contaminated by a large number of saprophytic bacteria, the bacterial flora is complex, the total bacterial count is high, the nutritional quality of the feed declines, and various nutrients are degraded, decomposed, and hydrolyzed to produce many toxic and harmful substances, causing the feed to deteriorate and exert toxic effects on fish. High-protein animal feed: most of the protein in this type of feed can be decomposed by bacteria to produce harmful base nitrogen compounds, such as amines, hydroxylamine, hydrogen, etc., giving off a pungent ammonia-like odor. Feed with higher carbohydrate content such as flour, cornmeal, and grains: under the action of microorganisms, carbohydrate feed is hydrolyzed to produce hydroxyl OH, alcohols, aldehydes, CO_2, and other substances. In particular, grain feed is very prone to heating and spoilage when the weather is hot and the temperature is high, producing this hydrolysis reaction and giving off a sour odor. Oily feed: such as high-oil non-defatted fishmeal, cake-type feed, peanut cake, sesame cake, etc., can undergo rancidity reactions, producing a "rancid" odor, and producing ketones and aldehydes, which are strongly toxic to fish. 2. Feed contaminated by pathogenic bacteria becomes a carrier and transmitter of pathogens. For example, feed contaminated by terrestrial animal pathogens and Salmonella can also cause disease in fish. 3. Large amounts of toxin products produced by contamination with microorganisms, bacteria, and molds. For example, after mold - Aspergillus flavus contaminates feed, it can produce aflatoxins B1 and B2, which exert toxic effects on fish. Artificial compound feed composed of various raw materials eaten by fish. After feed contamination, the results of the above three types of changes are often biological, physical, chemical, and complex comprehensive processes, and the toxic and harmful substances produced are also diverse, composite, and act on each other, rather than being a single type. The symptoms shown after fish eat contaminated feed are also a very intricate biological process. They also have a certain correlation with the fish body's own nutrition, physical condition, and the quality of the water environment.