Prevention and Treatment Methods for Liver Disease in Grass Carp Farming

1. Main Functions of the Liver

The liver of the grass carp is located in the abdominal cavity and is the largest digestive gland. It is roughly divided into left and right lobes; the right lobe is larger, extends to the first bend of the intestine, and occupies the entire space of the first bend. The liver is mainly composed of hepatic lobules, and pancreatic tissue is scattered within the liver tissue. The main functional unit of the hepatic lobules is the liver cell. The main functions of the liver are:

1. Bile secretion: it can emulsify fats and promote the digestion and absorption of fats. The gallbladder is where bile is stored.

2. Metabolic function: various nutrients absorbed from the digestive tract enter the liver through the portal vein, where they are processed and modified within the liver cells to synthesize important substances needed by the body, such as albumin, fibrin and some globulins. Liver cells can also synthesize and break down glycogen and participate in the metabolism of vitamins and hormones.

3. Detoxification: some toxic substances absorbed from the intestine and toxic substances from intermediate metabolism in the body are eliminated or their toxicity is reduced through transformation or conjugation by liver cells.

4. Defense function: liver macrophages have active phagocytic capacity and can eliminate bacteria, harmful substances and aged white blood cells.

2. How Liver Disease Is Caused

From the perspective of pathology, the causes of liver disease in grass carp can be mainly divided into two major categories: one is primary liver disease, mainly caused by bacteria and viruses; the other is secondary liver disease, mainly caused by other lesions, malnutrition and drugs. Based on what is currently known, primary liver disease occurs relatively rarely, and there have been no reports of successfully isolating the pathogenic bacteria or viruses of grass carp liver disease (for example, like the hepatitis A virus and hepatitis B virus in humans). In other words, the liver disease currently occurring is mainly secondary liver disease. From our investigations in areas with a high incidence of liver disease, the liver disease currently occurring has not been seen to be epidemic. We believe that the main factors causing the current high incidence of liver disease in grass carp are as follows:

1. Caused by drugs: one of the main functions of the liver is detoxification. Because grass carp farming at present is basically polyculture and mostly remains at a relatively low farming level, pond management techniques are relatively backward. In disinfection treatment and parasite prevention, drugs with relatively strong toxic side effects are basically used (for example, disinfectants are mostly chlorine-containing preparations, which after entering the water body produce carcinogenic amines; insecticides used are pyrethroids and organophosphates, which themselves have relatively high toxicity to fish). After these drugs are used, if the frequency of use is relatively high or the dosage used is relatively large, they will increase the burden on the liver, cause damage or even necrosis of liver cells, and thereby cause liver lesions.

2. Caused by the environment: in some farming areas, the water environment is extremely deteriorated, and coupled with unreasonable design of the pond water system, toxic and harmful substances in the water environment are often inadvertently added to the pond (for example, during water addition), causing deterioration of water quality in the pond and increasing the detoxification burden on the liver. At the same time, due to the backward level of pond management, especially the large-scale use of organic fertilizers and the unreasonable use of inorganic fertilizers, as well as failure to clean or timely clean residual matter in the pond, the pond water body can be deteriorated, causing toxic and harmful substances in the pond water body to rise, keeping fish in a relatively harsh environment for a long time, keeping the liver's detoxification function under a high load for a long time, causing damage or even necrosis of liver cells, and thereby causing liver lesions.

3. Caused by feed: in natural water bodies, grass carp basically feeds mainly on aquatic grasses and can also swallow some water fleas. Under artificial farming conditions, however, it is basically impossible to feed aquatic grasses, so terrestrial gramineous plants or artificial bait are used instead; in some areas, artificial bait is basically what is fed. Because the economic value of grass carp itself is not high, the artificial bait used is also some lower-priced varieties, and the feed varieties used are relatively single. In particular, in some areas, agricultural and sideline products are used, especially large amounts of rapeseed meal and cottonseed meal, and during use they have not undergone detoxification treatment, so large amounts of toxic and harmful substances are ingested by grass carp, thereby increasing the burden on the grass carp liver and causing liver lesions.

3. How to Prevent the Occurrence of Liver Disease in Grass Carp

To prevent the occurrence of liver disease in grass carp, efforts should mainly be made in the following aspects:

1. Improve the farming environment: pay attention to changes in the water source and changes in the pond water body, and add fresh water at any time and use microecological preparations to regulate water quality.

2. Use drugs reasonably: use drugs with relatively low toxicity to prevent and treat fish diseases, use drugs with definite efficacy, and reduce the frequency of use. Especially when carrying out prevention, the cost of medication should not be the only consideration.

3. Feed reasonably: combine grass feeding with artificial bait, appropriately change the varieties of feed, and especially when feeding agricultural and sideline products, be sure to pay attention to the properties of the product; those containing toxic and harmful substances must undergo detoxification treatment.