Preventing Infectious Disease in Aquaculture: A Practical Fish-Health Approach

A disease outbreak can begin with a seemingly manageable problem: compromised water quality, stressed fish, an inadequately treated feed source or the introduction of new stock. Once pathogens gain access to a susceptible population, controlling the resulting disease can become considerably more difficult. 

For this reason, infectious disease management in aquaculture needs to focus on prevention and health protection, rather than relying primarily on treatment after fish become sick. 

Start with the water and diet 

Water can introduce and spread infectious diseases, making the use of pathogen-free water sources an important preventive measure. Treatment systems can include sand filtration, ultraviolet light, ozone, and chlorination-dechlorination. 

Domestic wastewater also requires careful consideration because it may contain human or animal pathogens. Appropriate treatment before use can reduce this potential risk^[1]. 

Feed is another possible route of pathogen introduction. Inadequately treated man-made diets containing animal protein may transmit viral, bacterial and protozoan pathogens. Pasteurisation of raw salmon viscera can eliminate Renibacterium salmoninarum, Ichthyophonus hoferi and Mycobacterium spp^[1].

Natural diets also require careful sourcing to minimise the risk of introducing infectious agents. 

Control the health status of incoming fish^[2] 

Fish can act as carriers or vectors for infectious agents. Broodstock should be maintained separately from production ponds in pathogen-free water to reduce microbial contamination. 

Wild fish can also serve as intermediate hosts for microorganisms, while parasites may act as vectors for viruses. This makes the health status of fish entering a production system an important part of disease prevention. 

Newly imported stocks should be quarantined until they are confirmed free of disease, particularly when they originate from countries where relevant diseases are endemic. 

Make routine health inspection part of management 

Disease prevention does not end once healthy stocks have entered the facility. 

Routine health inspections are essential for maintaining disease-free and pathogen-free fish stocks. Health inspections are also required before importation under relevant regulations. Fish should be examined twice a year as part of routine health monitoring, while ELISA testing can support easier, faster and more accurate diagnosis of important fish diseases^[1]. 

Regular surveillance therefore provides an opportunity to identify health problems before they become established across the production system. 

Protect the host by protecting the environment^[1] 

Poor environmental conditions can weaken natural resistance and immune responses, allowing pathogens and microorganisms to increase in susceptible hosts. 

Several factors can contribute to stress, including: 

• Poor pond design 
• Handling 
• Poor water quality 
• Low dissolved oxygen 
• Unfavourable temperature 

Maintaining appropriate environmental conditions is therefore not simply a production consideration. It is part of maintaining the fish's ability to resist infectious diseases. 

Where vaccination fits 

Vaccination can support the prevention of endemic diseases in aquaculture. Bacterial vaccines against Aeromonas salmonicida, Vibrio anguillarum and Yersinia ruckeri are in use, while vaccination against viral diseases is in preparation. Immunisation and long-term humoral immune responses have also been studied for viral diseases including IPNV, VHSV, IHNV, SVCV and CCVD^[3]. 

Vaccination therefore complements, rather than replaces, broader fish-health measures. 

Clinical bottom line 

Effective infectious disease prevention requires multiple points of control: safe water and feed, healthy and quarantined stocks, routine inspection, appropriate environmental conditions and immunisation where applicable. 

The overall objective is to reduce opportunities for host-pathogen contact and limit disease establishment. In aquaculture, prevention and control measures can be more practical than waiting until disease has already spread through a susceptible population. 

References:

1. Senthamarai MD, Rajan MR, Bharathi PV. Current risks of microbial infections in fish and their prevention methods: A review. Microbial pathogenesis. 2023 Dec 1;185:106400. https://www.researchgate.net/profile/Rajan-Muthuswami-Ruby/publication/375006029
2. Bartley DM, Bondad-Reantaso MG, Subasinghe RP. A risk analysis framework for aquatic animal health management in marine stock enhancement programmes. Fisheries Research. 2006 Aug 1;80(1):28-36. https://www.academia.edu/download/49869807/j.fishres.2006.03.01120161025-32542-14qalqz.pdf
3. Nasr-Eldahan S, Nabil-Adam A, Shreadah MA, Maher AM, El-Sayed Ali T. A review article on nanotechnology in aquaculture sustainability as a novel tool in fish disease control. Aquaculture International. 2021 Aug;29(4):1459-80. https://pmc.ncbi.nlm.nih.gov/articles/PMC7933385/pdf/10499_2021_Article_677.pdf