Common Fish Diseases in Ponds and Their Prevention: Viral Diseases and Their Prevention

Common fish diseases can generally be categorized into viral diseases, bacterial diseases, fungal diseases, and parasitic diseases. Diagnosis and treatment of fish diseases should strictly follow medical advice, adhering closely to prescribed medication dosages without arbitrary increases or decreases.

Common viral diseases include hemorrhagic disease of grass carp, hematopoietic organ necrosis disease of crucian carp, herpesviral dermatitis of carp, spring viremia of carp, infectious pancreatic necrosis, infectious hematopoietic tissue necrosis, and viral hemorrhagic septicemia.

Hemorrhagic Disease of Grass Carp is primarily caused by grass carp reovirus. The disease worsens with poor water quality and is most severe under prolonged low oxygen conditions. Methods for prevention and treatment include pond disinfection, pre-stocking medication baths, artificial immunization, medication therapy, water disinfection, and eradication of viral pathogens in the water.

Aquatic pond bottom improvement and disinfection mainly involve removing excessive sediment, improving pond aquaculture environment, and using quicklime and bleach for disinfection.

Pre-stocking medication baths can use 2%~3% salt for 5~10 minutes or a 10 ppm polyvinylpyrrolidone-iodine solution for 6~8 minutes, or a 60 mg/L polyvinylpyrrolidone-iodine (PVP-I) bath for about 25 minutes.

Artificial immunization focuses on stringent quarantine of seedlings to prevent viral transmission.

Medication therapy can involve copper sulfate. Copper sulfate can be applied at a concentration of 0.7 mg/L over the entire pond, repeated every other day for two applications.

Water disinfection methods include full pond application of quicklime for disinfection and water quality improvement, or potassium hydrogen sulfate complex dissolved and applied for water disinfection.

To eradicate viral pathogens in the water, iodine preparations can be sprayed. For ponds with hemorrhagic disease in grass carp, polyvinylpyrrolidone-iodine or quaternary ammonium iodine complexes (0.3-0.5 ml per cubic water) can be sprayed 2-3 times every other day.

Hematopoietic Organ Necrosis Disease of Crucian Carp is caused by koi herpesvirus II. Prevention and treatment include:

(1). Regular quarantine of parent fish at fish farms to prevent breeding of infected parent fish. When purchasing crucian carp seedlings, ensure they are inspected or inquire about the disease history of the seedling source to avoid buying virus-infected seedlings.

(2). Use of photosynthetic bacteria, Bacillus spp., and denitrifying bacteria as microbial agents, along with substrate amendments, to effectively maintain stable aquaculture water environment. Additionally, maintaining adequate water depth, ensuring high water transparency, and increasing water self-circulation and external circulation are beneficial for maintaining water environment stability.

Herpesviral Dermatitis of Carp is another disease caused by herpesvirus. Prevention and control measures include:

(1) Enhanced comprehensive prevention measures and strict quarantine systems. Isolate diseased fish and avoid using them as parent fish.

(2) Thorough pond disinfection using quicklime in fish ponds, and disinfecting water areas with diseased fish or pathogens should also be thoroughly treated, preferably avoiding use as a water source.

(3) Water quality improvement can involve adjusting pond water pH with quicklime to maintain it above 8. Full pond application of dibromide or bromide can be used for water disinfection. Alternatively, full pond application of povidone-iodine, compound iodine solution, 10% povidone-iodine solution, or 10% povidone-iodine powder can all achieve water disinfection effects.

Spring Viremia of Carp is caused by spring viremia virus (SVCV), for which there is currently no effective treatment. Prevention methods include alternating use of quicklime or bleach for full pond application, chlorinated disinfectants, or effective disinfectants such as povidone-iodine and quaternary ammonium salts for water disinfection to prevent outbreaks.

Infectious Pancreatic Necrosis is caused by infectious pancreatic necrosis virus, primarily affecting cold-water fish. Early-stage treatment involves feeding with povidone-iodine solution (calculated as 10% effective iodine) at 1.64-1.91 g per kg fish body weight daily for 10-15 days.

Infectious Hematopoietic Tissue Necrosis is caused by infectious hematopoietic tissue necrosis virus, also primarily affecting cold-water fish. Prevention involves strict disinfection of aquaculture facilities and tools. Fish eggs should be hatched at 17-20°C and washed with 50 mg/L polyvinylpyrrolidone-iodine (PVP-I, containing 1% effective iodine) for 15 minutes. Concentration can be increased to 60 mg/L when pH is alkaline, as the efficacy of PVP-I decreases under alkaline conditions.

Viral Hemorrhagic Septicemia is caused by Novirhabdovirus in the Rhabdoviridae family, a single-stranded RNA virus. Currently, there is no effective treatment, so prevention is crucial. During the eyed egg period, soak eggs in iodine for 15 minutes. In the early stages of disease, feeding with iodine can reduce mortality.