The aquaculture industry faces a constant and evolving threat from emerging RNA viruses, pathogens that can cause significant ecological and economic consequences. Their high capacity for mutation and genetic adaptation drives their rapid evolution, cross-species transmission, and expansion of their host ranges, complicating disease management.
A recent scientific review article published by scientists from the University of Munich and Virginia Polytechnic Institute and State University delves into the emergence and significance of these viruses in aquatic animals over the last two decades, with a particular focus on biosecurity and vaccine development, crucial aspects for the sector’s sustainability.
The growing threat of RNA viruses in the aquatic world
RNA viruses are responsible for major outbreaks in farmed fish, while DNA viruses predominate in crustaceans. Even marine mammals are increasingly affected by morbilliviruses and highly pathogenic avian influenza (HPAI) H5N1, which has caused massive mortalities. The lack of effective antiviral treatments and the complexity of vaccine development underscore the urgent need to improve biosecurity measures.
The emergence of these viruses is driven by genetic mutations, recombination, and reassortment, resulting in new variants with altered pathogenicity and infectivity. In aquatic animals, these changes, also influenced by environmental factors and host immunity, are amplified in viral “hotspots” such as biodiverse regions or aquaculture facilities.
Biosecurity strategies: the first line of defense
Biosecurity is fundamental to preventing and controlling viral diseases in aquaculture. Although general principles apply to any infectious disease, each biosecurity plan must be unique to the specific situation and continuously updated.
The International Aquatic Veterinary Biosecurity Consortium (IAVBC) approach highlights several essential steps to develop a biosecurity plan adapted to a specific epidemiological unit (from an individual tank to an entire country):
- Hazard identification and prioritization: Recognizing which diseases pose a serious risk.
- Risk-impact assessment: Determining the risk to the farm and the operational impact of the disease.
- Critical Control Point (CCP) evaluation: Identifying where these hazardous diseases can enter.
- Mitigation, management & remediation of CCP risks: Establishing what can be done to prevent disease entry or escape.
- Contingency planning: Defining what to do if disease gets in.
- Clinical evaluation & diagnostic testing: Confirming if any of these diseases are on the farm.
- Ongoing disease surveillance & monitoring: Monitoring disease absence/presence.
- Veterinarian auditing & certification: Obtaining third-party recognition of disease freedom.
- Veterinary Authority (Gov’t) verification & endorsement: Formalizing the health status. Implementing these measures can be costly, so it’s important to consider more economical alternatives based on sound disease control principles when optimal methods are not practical.
Main emerging RNA viruses in aquatic animals
The article reviews a series of RNA viruses that have significantly impacted fish, crustaceans, and marine mammals.
- RNA viruses in farmed fish: Novirhabdoviruses (IHNV and VHSV), Spring Viremia of Carp Virus (SVCV), Infectious Salmon Anemia Virus (ISAV), Salmonid Alphavirus (SAV), and Tilapia Lake Virus (TiLV).
- RNA viruses in farmed crustaceans: Yellow Head Virus (YHV), Taura Syndrome Virus (TSV), Infectious Myonecrosis Virus (IMNV), Macrobrachium rosenbergii Nodavirus (MrNV).
- RNA viruses in marine mammals:
- Morbilliviruses: Include cetacean morbillivirus (CeMV), phocine distemper virus (PDV), and canine distemper virus (CDV). They cause severe outbreaks with high mortality. They spread primarily through horizontal transmission via aerosolized respiratory secretions.
- Influenza A Virus (IAV): Pinnipeds and cetaceans have increasingly been recognized as hosts for IAV. Highly pathogenic avian influenza A(H5N1) has caused significant mortality events in marine mammal populations worldwide since 2020.
- Caliciviruses: San Miguel Sea Lion Virus (SMSV) causes vesicular lesions in pinnipeds and has been associated with epizootic gastroenteritis. It has a broad host range and zoonotic potential.
Advances in vaccines against aquatic RNA viruses: challenges and promises
The high mutation rate and genetic variability of RNA viruses complicate vaccine development. In aquaculture, vaccine efficacy is limited by susceptibility in early life stages and difficulties in mass administration. Crustaceans lack adaptive immunity, making vaccine development largely ineffective.
Current approaches in aquatic vaccination:
- Whole pathogen vaccines (inactivated and attenuated): These are among the earliest approaches. Inactivated vaccines are safer but may require adjuvants and boosters. Attenuated vaccines generate strong, long-lasting immunity but carry risks of reversion to virulence.
- Subunit vaccines: Use purified viral antigens, being safer but with variable efficacy, needing adjuvants and repeated administration, often by intraperitoneal (IP) injection, which is costly and stressful.
- Nucleic acid vaccines (DNA and mRNA): Promising for inducing humoral and cellular immune responses. DNA vaccines have shown efficacy against IHNV and VHSV but raise biosafety concerns under genetically modified organism (GMO) regulations. mRNA vaccines face challenges such as high manufacturing costs and low thermostability.
- Live vector vaccines: Use non-pathogenic or attenuated viral or bacterial vectors to deliver antigens, inducing robust and long-lasting immunity. However, issues like pre-existing immunity and stability are obstacles.
Self-assembling protein nanoparticles (SAPN): an innovative platform
A promising strategy to improve antigen stability and immunogenicity, limitations of traditional subunit vaccines, is the presentation of antigenic epitopes on the surface of self-assembling nanoparticles, such as protein nanocages (SAPN).
Unlike virus-like particles (VLPs), protein nanocages function as scaffolds for the assembly of enveloped viruses, replacing the lipid membranes and matrix proteins. This makes them particularly suitable for emerging aquatic animal viruses, most of which are enveloped.
SAPN can:
- Improve antigen stability and immunogenicity.
- Allow for multivalent antigen presentation.
- Be produced cost-effectively and scalably.
- Induce durable and cross-protective immune responses.
- Facilitate oral administration, reducing stress in animals. Recently, an IHNV G-glycoprotein-ferritin fusion nanoparticle has been successfully developed using an E. coli system, demonstrating stability and innate antiviral activity, making it suitable for oral delivery in salmonids.
Implications for the aquaculture sector and future directions
RNA viruses remain a persistent threat. Although recent improvements in biosecurity have contributed to a decrease in some RNA virus outbreaks in crustaceans, the lack of commercial vaccines for most of these viruses is a problem.
Traditional vaccine platforms face regulatory, cost, and efficacy challenges. SAPNs represent a versatile and promising platform, combining safety, scalability, and adaptability. They allow for the rapid customization of autogenous vaccines to respond to local or emerging pathogens and support mass, stress-free (oral) administration, crucial capabilities for mitigating outbreaks.
Furthermore, the integration of immunostimulants, probiotics, and RNA interference (RNAi) therapies strengthens prevention strategies, especially for crustaceans. The emergence of HPAI H5 in marine mammals also requires international attention and collaboration.
Conclusion
RNA viruses continue to challenge aquatic animal health, necessitating a multifaceted approach that integrates robust biosecurity, vaccine innovation, and emerging biotechnological strategies. Advances in vaccine platforms like SAPNs, along with improved pathogen surveillance and sustainable aquaculture practices, will be essential in mitigating future outbreaks and protecting both aquaculture production and the health of aquatic ecosystems. International collaboration and continuous research are key to addressing these complex and constantly evolving viral threats.
Reference (open access)
Ahmadivand, S., Savage, A. C. N. P., & Palic, D. (2025). Biosecurity and Vaccines for Emerging Aquatic Animal RNA Viruses. Viruses, 17(6), 768. https://doi.org/10.3390/v17060768
Editor at the digital magazine AquaHoy. He holds a degree in Aquaculture Biology from the National University of Santa (UNS) and a Master’s degree in Science and Innovation Management from the Polytechnic University of Valencia, with postgraduate diplomas in Business Innovation and Innovation Management. He possesses extensive experience in the aquaculture and fisheries sector, having led the Fisheries Innovation Unit of the National Program for Innovation in Fisheries and Aquaculture (PNIPA). He has served as a senior consultant in technology watch, an innovation project formulator and advisor, and a lecturer at UNS. He is a member of the Peruvian College of Biologists and was recognized by the World Aquaculture Society (WAS) in 2016 for his contribution to aquaculture.
