Most common fish viral diseases

- Prevention and control of viral diseases

➡️  Diseases prevention and control using biosecurity measures

• Application of biosecurity measures in the hatchery

              i.          Periodical screening to the broodstock and eggs

To prevent vertical transmission of infections, parent stocks must be regularly screened using laboratory assays to detect and eliminate infected fish, ensuring that only disease-free fish can be used for egg production in aquaculture. Individual fish from the parent stock should be tagged and examined for viral infections using sperm, ovarian fluid, and other materials for virus isolation and identification through cell culture and/or reverse transcription polymerase chain reaction (RT-PCR) (Watanabe et al., 2000).

            ii.           Application of standard operating procedures Standard Operating Procedures (SOPs) for Brood Stock Stations

According to Palić et al. (2015) SOPs should integrate biosecurity into daily operations, covering:

•     Facility design & flow: Clear separation of restricted and non-restricted areas, with defined movement paths for personnel and stock.

•     Disinfection protocols: Procedures for personnel, equipment, and vehicles to prevent pathogen transfer.

•     Water management: Treatment of inflow water and safe handling of wastewater.

•     Feeding & husbandry: Documented feeding regimes, verified feed sources, and use of immunostimulants or vaccines.

•     Routine screening: Protocols for sample collection, diagnostic testing, and scheduled health checks for viral diseases.

•     Outbreak response: Isolation of affected stock, disinfection, hygienic disposal of dead fishes, and mandatory reporting of notifiable diseases to national veterinary authorities.

•     Record keeping: Standardized forms capturing health status, disinfection logs, egg production, disease screening, diagnostic results, feed use, growth performance,

·     Implementation of Biosecurity on Aquaculture Farms

1. The biosecurity in the fish farms includes, newly acquired stock should be held in a separate facility for 4–6 weeks, with no sharing of equipment or personnel with resident fish. Vaccination before integration is recommended. Key transmission indices include regulating water temperature to reduce disease occurrence and enhance immune response, maintaining appropriate stocking densities to minimize stress and transmission, and treating inflow water with filtration and ultraviolet (UV) irradiation to prevent viral entry.
2. Chemical disinfectants (Timsen (n- Alkyl dimethyl benzyl ammonium chloride. It is broad-spectrum quaternary ammonium compound with powerful bactericide, fungicide, viricide and algicide activity), iodophores and H2O2) are essential for inactivating viral pathogens in aquaculture. Effective selection should consider efficacy, environmental impact, human safety, and effects on cultured species.

-Routine disinfection of equipment, facilities, and personnel is critical, and fish eggs should be treated with iodophor (25 ppm for 20 minutes or 50 ppm for 15 minutes) or sodium hypochlorite (4–6% for 5 minutes) (Russell Danner & Merrill, 2005).

-Eco-friendly antiviral compounds in aquaculture are big focus right now because the chemicals compounds hurt fish immunity, water and environment.

- Water quality must also be monitored, as high ammonia, nitrate, or low pH levels stress fish and increase susceptibility to infection. Adequate aeration is equally important to prevent hypoxia, which can be fatal to fish eggs.

Research by Amend & Pietsch (1972) further demonstrated that iodophors possess strong virucidal activity against major salmonid viruses, including IHNV, IPNV, and VHSV.

3. Biological Biosecurity Measures 

Biological processes are vital for protecting aquaculture species against viral infections. Key strategies include vaccination of fish, use of certified disease-free eggs or livestock, and adoption of disease-resistant strains. Policies such as “eggs only” combined with certified eggs have successfully prevented viral introductions in salmon farms, highlighting the higher risk of disease transmission through live fish transfers (Mweemba et al., 2024).

 

National biosecurity programs coordinate farm-level and brood-station disease control with international standards. Guidelines should cover outbreak management, eradication compliance, and restrictions on the movement of eggs, live fish, postlarvae shrimp, frozen shrimp, and other aquatic organisms (Scarfe, 2003). Authorities are responsible for routine surveillance, providing diagnostic services, supervising eradication programs, and regulating imports and exports of aquaculture products.

 

Although the profile of aquatic viral diseases varies among countries, many pathogens are transboundary, requiring common strategies for control. To coordinate these efforts, international organizations such as the Food and Agriculture Organization (FAO) and the World Organization for Animal Health (WOAH) provide global frameworks. The FAO issues guidelines for disease prevention in food animals, while the WOAH develops science-based standards for all animal health, including aquatic species. Through the Aquatic Animal Health Code and Manual of Diagnostic Tests for Aquatic Animals, the OIE ranks diseases by risk level and provides guidance on diagnosis, surveillance, disinfection, prevention, eradication, and restocking. These frameworks also support national biosecurity programs and ensure compliance with health certification requirements for international trade. In principle, both FAO and WOAH not only address transboundary diseases but also empower countries to establish their own effective biosecurity measures (Subasinghe et al., 2023).

 

➡️ Breeding resistant strains.

 

-   Selective breeding of diseases resistance

Efforts to breed fish resistant to viral diseases have gained momentum only in recent decades, as many viral pathogens were identified during this period. In Norway, selective breeding has been a key component of salmon programs since 1993, leading to the identification of strains highly susceptible to infectious pancreatic necrosis virus (IPNV). These strains have been valuable not only for developing resistant lines but also for optimizing challenge models in vaccine efficacy trials.

-   Genetic Selection of Disease-Resistant Strains

Advances in high-throughput genome mining have enabled the identification of genetic markers linked to disease resistance in aquaculture species. The most widely used approach is quantitative trait loci (QTL) analysis “ it is a statistical method used to identify specific region of the genome (DNA segments) associated with phenotypic variation in quantitative complex traits like height, yield or disease susceptibility” (Buchmann, 2022), which statistically links phenotypic traits such as post-challenge survival to specific genomic regions. By crossing fish with different susceptibility traits and exposing them to viral pathogens, researchers can identify QTLs associated with resistance or susceptibility. Common markers include single nucleotide polymorphisms (SNPs) and microsatellites, both of which have been successfully applied in salmonids to identify resistance to infectious pancreatic necrosis virus (IPNV).

➡️  Vaccination

Vaccination is one of the most effective ways to prevent viral diseases, as immunized fish are less likely to become infected. Successful vaccines depend on identifying pathogen surface antigens that trigger protective immune responses. Delivery systems may use inactivated or live attenuated viruses, or molecular methods that express immunogenic proteins. The choice of system depends on safety, cost, efficacy, and the type of immune response required.

➡️  Immunostimulants

Immunostimulants are widely used in aquaculture eco-friendly strengthen the immune system of fish and reduce the impact of viral infections. They do not directly destroy viruses, but they enhance both innate and adaptive immunity, making fish less susceptible to outbreaks.

Natural compounds such as β-glucans, chitosan, and plant extracts (garlic, turmeric, neem) stimulate macrophages and improve antiviral defense. Probiotics like Lactobacillus and Bacillus enhance gut immunity and help lower viral loads. Vitamins and minerals including vitamin C, vitamin E, selenium, and zinc act as antioxidants, supporting immune cell function. Synthetic agents such as levamisole and CpG oligonucleotides directly modulate immune responses and trigger antiviral signaling pathways.

The benefits of immunostimulants include reduced mortality during viral outbreaks such as Koi Herpesvirus (KHV), Infectious Pancreatic Necrosis (IPN), and Viral Hemorrhagic Septicemia (VHS). They also improve vaccine efficacy when used as adjuvants and provide a sustainable alternative to antibiotics, avoiding resistance and chemical residues.