In fish seed production, one of the most concerning threats today is viral nervous necrosis in fish (VNN), caused by a virus. The disease severely damages the nervous system and vision, causing fish to lose direction, swim in circles, and die in large numbers. To help farmers recognize it early, prevent it, and control it effectively, Vemedim would like to share scientific information and practical measures for handling diseased fish in the article below.
The causative agent of nervous necrosis in fish
In today’s intensive aquaculture environment, the emergence of viral diseases is always a major threat to farmers. Among them, Viral Nervous Necrosis in fish (VNN) is considered one of the most dangerous diseases, capable of causing mass mortality in fry and juvenile fish stages.
The causative agent is Nervous Necrosis Virus (NNV), a member of the Nodaviridae family, which can directly attack the central nervous system and retina, causing movement disorders, loss of orientation, and gradual exhaustion. The disease spreads rapidly under high-density farming conditions, large temperature fluctuations, or when fish are stressed.
Many economically valuable species such as grouper, red snapper, tilapia, tra and basa catfish, and cage-cultured marine fish can be severely affected if not detected and treated early. The pathogen can persist for a long time in pond environments, adhering to nets, hatchery tanks, or farming equipment. Common transmission routes include:
- Vertical transmission: infected broodstock can pass the virus to eggs or fry.
- Horizontal transmission: healthy fish come into contact with infected fish, water, or virus-contaminated waste.
- Equipment, water supply, and intermediate organisms (such as small crustaceans) are also concerning sources of spread.
>> See also:
- Gill rot disease in fish – Causes and effective treatment
- Tuberculosis in fish – Signs, treatment, and prevention
How to recognize nervous necrosis in fish
Larval fish and fry under 2 months of age are the most susceptible group because their immune systems are not yet fully developed. Early detection is decisive in controlling VNN, because the virus spreads very quickly in water environments. Fish infected with nervous necrosis typically show the following characteristic signs:
- Swimming in circles, losing balance, head pointing downward, or spinning continuously.
- Bulging eyes, cloudy pupils, poor response to light.
- Weak fish, loss of appetite, fading skin color, and emaciated body.
- Mortality rate rises rapidly, especially within the first 3–5 days after symptoms appear.
- When examining brain or retinal tissue under a microscope, necrotic and vacuolated tissue areas may be seen—typical signs of the disease.
The harmful effects and severity of nervous necrosis in fish for the stock:
- Mortality can reach 80–100% in fry stocks if not treated.
- Slows growth and causes serious production losses.
- Easily mistaken for parasitic diseases or nutritional deficiencies, leading to incorrect treatment.
Preventive measures and effective control of nervous necrosis in fish
Prevention of Nervous Necrosis must be carried out comprehensively, from seed selection and environmental management to strengthening fish resistance. Specifically:
Seed management
- Only select broodstock and fry that test negative for NNV (PCR).
- Prioritize hatcheries that are quarantined and have clear traceability.
- Isolate newly introduced fish for at least 14 days before stocking.
Farming environment management
- Maintain stable water parameters: pH 7.5–8.2, temperature 26–30°C, dissolved oxygen > 4 mg/L.
- Reduce stocking density, change water regularly, and siphon the bottom to limit waste accumulation.
- Periodically disinfect ponds, cages, and equipment with products such as Vemedim Bitech Yucca or Vemedim Zeolite - Yucca according to the recommended dosage.
Strengthening fish resistance
- Supplement vitamin C, E, and beta-glucan through biological products such as Vemedim ADE B.complex, Vemedim ANTI – SHOCK, Vemedim GLUSOME 115 into daily feed.
- Use Anti Edward to enhance immunity and stabilize the gut, helping fish stay healthy, reduce stress, and improve resistance to pathogens.
Applying biosecurity
- Do not move fish, water, or equipment between different ponds/cages.
- Dispose of diseased fish properly, avoiding dumping carcasses into natural water sources.
- Restrict visitors from entering the farming area during disease outbreaks.
Handling when disease is detected
When signs of suspected nervous necrosis in fish appear, you should:
- Isolate the diseased stock, stop feeding, and remove dead fish daily.
- Disinfect water and equipment with appropriate disinfectant solutions.
- Do not stock new fish in the infected area until it has been thoroughly treated.
- Report to the veterinary authority or local aquaculture extension center for technical support.
Thus, it can be seen that nervous necrosis in fish is a potential threat that can cause serious losses if not detected and handled promptly. Maintaining clean water, healthy seed, and proactive disease prevention procedures is the key to protecting the stock. With scientific solutions and specialized support products from Vemedim, farmers can feel more secure in controlling disease, improving productivity, and moving toward sustainable farming. Contact Vemedim now for detailed advice!
