Prevention and Treatment Methods for Common Bacterial Diseases in Farmed Fish

1 Explosive Hemorrhagic Disease

Also known as bacterial septicemia, it is currently the infectious disease in China that harms the most fish species, covers the largest scope, has the widest epidemic areas, 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 develops, the fish body surface becomes congested, the anus becomes red and swollen, the abdomen becomes 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. The disease is sometimes accompanied by exophthalmos, erect scales, and rotting at the ends of the gill filaments.

This disease often occurs in ponds where the water has deteriorated, become aged, has low dissolved oxygen, and contains many harmful substances. If breeding management is poor, and moldy, low-quality feed is added, the occurrence and development of this disease are even more likely.

For the prevention of explosive hemorrhagic disease, first of all, feeding management 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 extracts (Chinese herbal medicine extracts) can be used to disinfect the water body, and then photosynthetic bacteria can be used to purify poor water quality. Zeolite, nitrite and other degrading agents are used to adsorb and degrade harmful physical and chemical factors such as ammonia nitrogen and nitrite in the water body; immunostimulants are added to the feed.

After the disease occurs, active diagnosis and treatment work must be taken. First, diagnose whether harmful parasites are present. If so, effective drugs should be used to kill the parasites, and then dibromohydantoin (0.2 mg/L) and bromochlorohydantoin (0.3 mg/L) should be used to disinfect the water body. Next, photosynthetic bacteria (5 mg/L) or Yubide (2 mg/L) 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 degrading agent (0.8 mg/L) should be splashed in time for treatment. While splashing the disinfectant, medicated feed should be fed, adding 1.5 g of bioactive additive per kilogram of feed, and feeding continuously for 5 days. If the condition is severe, feed medicated feed again for 3 days after a 2-day interval.

2 Enteritis Disease

The pathogen of this disease is Enteric Punctate Aeromonas, a conditional pathogen. Throughout the entire breeding process, this type of bacterium exists in the intestinal tract of fish. In healthy fish, this bacterial species is not dominant and will not cause disease. However, when affected by factors such as water quality deterioration, decreased dissolved oxygen, feeding deteriorated feed, and improper feeding, the resistance of the fish body decreases, so that the pathogenic bacteria are discharged into the water with the feces of diseased fish, contaminating the water quality and feed, and are orally transmitted to other fish. Therefore, this type of disease can occur and develop during the entire breeding cycle. When the water temperature in the pond 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 a rotting inner wall.

The prevention methods for enteritis disease are basically the same as those for explosive hemorrhagic disease. During treatment, a combination of external disinfection and oral administration should be adopted. Disinfectants such as bromochlorohydantoin (0.3 mg/L) or dibromohydantoin (0.2 mg/L) can be used to disinfect the breeding water area. At the same time, immune polysaccharide Haizhenbao (0.2%) or Yubide (0.2%-0.4%) can be added to the feed for feeding to enhance the body immunity of the fish. Continuous feeding can basically bring the disease under control.

3 Gill Rot Disease

The pathogen is Flexibacter columnaris. It is generally caused by direct contact between the fish body and the pathogen. In particular, when the gills of the fish are mechanically damaged, infection is more likely. The disease can occur within the water temperature range of 15-30 °C, and the higher the water temperature, the more likely it is to break out. The body color of diseased fish becomes dark, and appetite decreases. Opening the operculum of diseased fish shows congestion and inflammation of the skin on the inner surface of the operculum, and the middle part is often rotted into round transparent small windows, commonly known as "open skylight." The gill filaments are red and swollen, mucus increases, and the ends are defective. In most diseased fish, the color of the fin edges becomes 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 breeding process, disinfect the pond regularly. After the disease occurs, splashing Yubide (2 mg/L) or dibromohydantoin (0.3 mg/L) over the whole pond is sufficient.

4 Red Skin Disease

The pathogen is often Pseudomonas fluorescens, a conditional pathogen. When the fish body is injured due to manual operations such as fishing, transportation, and stocking, mechanical damage, or parasitic infestation, the pathogen takes the opportunity to invade and cause disease. The disease can occur in all four seasons, especially after overwintering, and is most likely to break out in spring (approximately from March to May). Diseased fish show surface bleeding, inflammation, scale loss, congestion at the base of the fins, and fin rays congested or eroded, appearing broom-like, often called "rotten fins," and are often accompanied by symptoms of gill rot.

The prevention and treatment methods for red skin disease are basically consistent with 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 mg/L) should be used to disinfect the pond water, and the water level should be deepened. At the same time, a one-time disinfectant (dibromohydantoin or bromochlorohydantoin, etc.) should be used to kill harmful pathogens in the water body to achieve a preventive effect. After the disease occurs, a combination of insecticides and disinfectants should be used in time to treat the water body, combined 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.

In summary, there are many other bacterial diseases of conventionally farmed fish, such as scale protrusion disease, stamp disease, white skin disease, and so on. All pathogenic bacteria share one common characteristic: they are conditional pathogens. Only when breeding density is too high, management is improper, causing water quality deterioration, fish body injury, decreased immunity, and other conditions, do pathogenic bacteria multiply in large numbers, invade the fish body, and cause disease. Therefore, when preventing bacterial diseases, disinfectants should be used to disinfect water quality, and then beneficial microbial preparations should be used to maintain a good water quality environment. At the same time, bioactive additives such as biological enzyme polysaccharides should be used in combination to improve the fish body's own immunity and other comprehensive methods, bringing effective economic benefits to aquaculture.