Prevention and Treatment Methods for Bacterial Diseases in Conventional Farmed Fish

In recent years, because the problem of drug residues has dealt a blow to China's aquatic product exports, the government has carried out a large-scale rectification of the entire fishery drug market, successively announcing a large number of banned drugs for aquaculture, such as chloramphenicol, furazolidone, malachite green, sodium pentachlorophenate, etc. Because special aquatic products have high economic value and a relatively large export market, through the joint efforts of all parties, the use of banned drugs has been well controlled. In the process of conventional fish farming, bacterial diseases occur relatively frequently and over a relatively large area. However, limited by their commercial value and export market, farmers still often choose some old methods for disease prevention and treatment, and even use some banned drugs, such as malachite green, furazolidone, chloramphenicol, etc., to control diseases. Such prevention and treatment methods not only seriously affect the quality of aquatic products; the long-term use of drugs will also produce relatively strong drug resistance, leading to increased drug dosage; moreover, the discharged aquaculture wastewater directly or indirectly pollutes the natural environment, bringing serious negative effects to the sustainable development of society. Therefore, in the disease prevention and treatment of conventional fish, it is extremely necessary to explore and promote some new methods and new drugs.

I. Fulminant Hemorrhagic Disease

Also known as bacterial septicemia, it is currently the infectious disease in China that harms the largest number of fish species, covers the largest range, has the widest epidemic areas, has the longest epidemic season, and causes the greatest losses. The disease is mainly caused by two pathogenic bacteria, Aeromonas hydrophila and Aeromonas sobria. After the disease occurs, the body surface of the fish is congested, the anus is red and swollen, the abdomen is distended, and a large amount of ascites accumulates in the abdominal cavity with hemolysis; there is no food in the intestinal tract, but there is a lot of mucus. Diseased fish sometimes also show exophthalmos, erect scales, and rotting ends of gill filaments.

This disease often occurs in ponds where the pond water and bottom sediment are deteriorated and aged, dissolved oxygen is low, and harmful substances are abundant. If aquaculture management is poor and moldy, inferior feed is also fed, it is even more likely to cause the occurrence and development of this disease.

For the prevention of fulminant hemorrhagic disease, first of all, the feeding management of aquaculture should be strengthened, and feeding should be carried out according to the "four fixed" feeding method; drugs should be used regularly for comprehensive prevention. In production, disinfectants such as dibromohydantoin, bromochlorohydantoin, and natural plant extract (Chinese herbal medicine extract) preparations can be used to disinfect the water body; then beneficial bacteria such as photosynthetic bacteria can be used to purify poor water quality. Immune enhancers, such as natural plant extract feed additives with no drug residues, should be added to the feed. Zeolite powder and nitrite degraders should be used to adsorb and degrade harmful physical and chemical factors such as ammonia nitrogen and nitrite in the water.

After the disease occurs, active diagnosis and treatment work should be taken. First, diagnose whether there is parasitic infection. If so, effective drugs should be used to kill the parasites, and then dibromohydantoin (dosage 0.2 ppm) and bromochlorohydantoin (0.3 ppm) should be used to disinfect the water body; second, photosynthetic bacteria (dosage 5 ppm) or Yubide (dosage 2 ppm) should be used to improve the water quality environment; if testing shows that the nitrite content in the water body is too high, a nitrite degrader (0.8 ppm) should also be splashed for treatment; third, while splashing the disinfectant, medicated feed should be fed, adding 15 grams of bioactive additive per kilogram of feed, and feeding continuously for 5 days. If the condition is relatively severe, feed medicated feed again for 3 days after an interval of 2 days.

II. Enteritis Disease

The pathogen of this disease is intestinal punctate Aeromonas, a conditional pathogenic bacterium. This type of bacterium exists in the intestinal tract of fish throughout the entire farming process. In healthy fish, this bacterial species is not dominant and will not cause disease, but when affected by reasons such as deterioration of the water quality environment, reduced dissolved oxygen, feeding deteriorated feed, and irregular and unquantified feeding, the resistance of the fish body decreases, so the pathogenic bacteria are discharged into the water with the feces of diseased fish, contaminating the water quality and feed, and are transmitted orally to other fish and fingerlings. Therefore, this type of disease may occur and develop at any time throughout the entire farming cycle. When the pond water temperature is above 18°C, the disease can become epidemic, generally occurring from April to September. Diseased fish generally have a distended abdomen with red spots, a red and swollen anus, and when the abdomen is gently squeezed, yellow mucus flows out from the anus. Dissection shows congestion and inflammation of the intestinal wall, no food in the intestinal tract, only pale yellow mucus, and erosion of the inner wall.

The prevention methods for enteritis disease are basically the same as those for fulminant hemorrhagic disease. During treatment, a method combining external disinfection and oral administration should be adopted. Disinfectants such as bromochlorohydantoin (0.3 ppm) or dibromohydantoin (0.2 ppm) can be used to disinfect the aquaculture water area, and at the same time, immune polysaccharide Haizhenbao (0.2%) or Yubide (0.2-0.4%) should be added to the feed for feeding to enhance the immunity of the fish body. With continuous feeding, the condition can basically be controlled.

III. Gill Rot Disease

The pathogen is Flexibacter columnaris. It is generally caused by direct contact between the fish body and the pathogen, and infection is especially likely after mechanical damage to the gills of the fish. The disease can occur within a water temperature range of 15-30°C, and the higher the water temperature, the more likely it is to break out. Diseased fish have a darkened body color and reduced appetite. Opening the operculum of a diseased fish shows congestion and inflammation of the skin on the inner surface of the operculum, and the middle part is often eroded into a round transparent small window, commonly known as "open skylight." The gill filaments are swollen, mucus increases, and the ends are damaged. The fin edges of most diseased fish become white and faded.

Prevention methods for gill rot disease: during operations, be gentle and quick to avoid injuring the fish body; when releasing fry, soak and disinfect them with medicated water. During the farming process, disinfect the pond regularly. After the disease occurs, splash Yubide at 2 ppm or dibromohydantoin at 0.3 ppm over the whole pond.

IV. Red Skin Disease

The pathogen is Pseudomonas fluorescens, a conditional pathogenic bacterium. When the fish body is injured due to manual operations such as capture, transportation, and stocking, or mechanical damage, frostbite, or parasitism by parasites, the pathogen takes the opportunity to invade and cause disease. The disease can become epidemic and occur in all four seasons. It is especially most likely to break out in spring after overwintering (around March to May). Diseased fish show bleeding and inflammation on the body surface, shedding of scales, congestion at the base of the fins, and fin rays congested or eroded in a broom-like shape, often called "fin rot," and are also often accompanied by symptoms of gill rot.

The prevention and treatment methods for red skin disease are basically the same as those for gill rot disease. When catching and stocking, movements should be gentle and quick to avoid injuring the fish body. Before overwintering, disinfectants such as bromochlorohydantoin (0.3 ppm) should be used to disinfect the pond water, and the pond water should be deepened to ensure that the fish body is not frostbitten. After early spring of the following year, start feeding as early as possible to improve the immunity of the fish body. At the same time, use a disinfectant once (dibromohydantoin or bromochlorohydantoin, etc.) to kill harmful pathogens in the water body, which can achieve a preventive effect. After the disease occurs, promptly use a method combining insecticides and disinfectants to treat the water body, and combine it with oral feed drugs such as natural plant extracts for treatment. For specific operations, refer to the prevention and treatment methods for gill rot disease.

V. Tilapia Ulcer Disease

The pathogen is Aeromonas hydrophila, a conditional pathogenic bacterium that is commonly present throughout the farming period and in water areas, but it can cause disease in tilapia only under high-density farming conditions and improper feeding management. Diseased fish have congestion on the body surface, and surrounding scales become loose and fall off, gradually ulcerating into blood-red spotted depressions. The lesions have no specific location and may occur on all parts of the body, including the head and trunk. Dissection of diseased fish shows a brown liver and an enlarged dark green gallbladder.

Prevention methods for tilapia ulcer disease: use appropriate farming density, move into warm sheds in time to prevent frostbite, regularly use disinfectants such as dibromohydantoin and bromochlorohydantoin for disinfection, and combine with feeding medicated feed such as Yubide and Haizhenbao. Some green feed should also be appropriately fed to improve the immunity of the fish body. After the disease occurs, promptly remove diseased fish and move them into excellent water quality for temporary holding. At the same time, splash bromochlorohydantoin at 0.3 ppm or Yubide at 2 ppm in the pond to disinfect the water body, and try not to use highly irritating drugs to avoid further stimulating the lesions. While using external drugs, combine with oral Florfenicol (0.15%) medicated feed for 5-7 days.

In summary, there are still many bacterial diseases of conventional farmed fish, such as erect scale disease, stamp disease, white skin disease, and so on. However, all pathogens share one common characteristic: they are all conditional pathogenic bacteria. Only when farming density is too high, improper management causes water deterioration, fish bodies are injured, immunity decreases, and under such conditions do pathogenic bacteria multiply in large numbers and invade the fish body to cause disease. Therefore, when preventing bacterial diseases, disinfectants should be used to disinfect the water body, and then beneficial microbial preparations should be used to maintain a good water quality environment. At the same time, a series of comprehensive methods such as using biological enzymes and polysaccharide active additives to improve the fish body's own immunity should be combined to bring relatively high economic benefits to aquaculture.