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What aquaculture producers can use instead of antibiotics

A recent review funded by the FAO and the Norwegian Agency for Development Cooperation (Norad) has outlined potential alternatives to antibiotics in aquaculture.

Antibiotics are commonly used in aquaculture as therapeutics and prophylactics

Though deploying antibiotics can help with disease challenges, the industry faces a huge challenge from antimicrobial resistant bacteria and antibiotic residues in farmed stocks.

A team of researchers from FAO, the City University of Hong Kong, Nitte University, the National Aquaculture Group (NAQUA), the Norwegian Veterinary Institute and other scientific organisations have published a review outlining the scope of the problem with using antibiotics, the emergence of antimicrobial resistance in aquaculture and the viable alternatives at the industry’s disposal.

A closer look at the problem

The spread of diseases in aquaculture could be due to inadequate management and poor environmental conditions, including levels of feeding, harvesting stocking and inadequate nutrition.

Antibiotics are commonly used in aquaculture as therapeutic, prophylactic or meta phylactic agents; and the most used in the aquaculture industry worldwide are tetracycline, oxytetracycline (tetracyclines), oxolinic acid, flumequine, sarafloxacin, enrofloxacin (quinolones), amoxicillin (beta-lactams), erythromycin (macrolides), sulfadimethoxine (sulfonamides), ormethoprim (diaminopyrimidines) and florfenicol (amphenicols).

It is important to note that each country has its own legislation regarding approved antibiotics, use practices and residual limits in aquaculture products.

As a result of the increased use and abuse of antibiotics, mutations in bacterial DNA and horizontal gene acquisition have led to the survival and establishment of bacteria resistant to those specific antibiotics.

Approximately 80 percent of the antimicrobials used in aquaculture enter the environment with their activity intact.

Alternatives to antibiotics

The researchers outlined the alternatives to the use of antimicrobials in aquaculture, including vaccination strategies, phage therapy, Quorum quenching, probiotics, prebiotics, chicken egg yolk (IgY) antibodies, plant therapy and the use of “pathogen-free” seeds.

Alternative approaches to reduce the use of antimicrobials in aquaculture

© Source: Goodness et al, (2023)

Vaccines

Vaccines are preparations made of pathogenic microorganisms, eg, bacteria, viruses and their metabolites, which are artificially attenuated, inactivated or genetically modified to prevent infectious diseases.

These compounds are recognised as critical tools for the prevention and control of fish diseases and are considered an essential route to reducing the use of antibiotics in the aquaculture industry.

In recent years, several nucleic acid vaccines have been developed for use in aquaculture and appear to elicit strong humoral and cellular immunity. They consist of DNA or RNA-encoding antigens of interest and are relatively easy to manufacture and safe to administer.

Bacteriophages

Bacteriophages or phages are bacterial viruses that invade bacterial cells and, in the case of lytic phages, disrupt bacterial metabolism and cause lysis (or destruction) of the bacteria.

Phages are the most abundant microorganisms worldwide, particularly in marine and freshwater environments. Marine species are found near the surface and in deep benthic environments. Phage therapy has been used successfully to control bacterial infections in aquatic animals, but multiple phage therapy has proven more successful than single phage therapy.

There are many reports of phage therapy used against bacteria of the genus Vibrionaceae, which are abundant in the aquatic environment and are the genus of bacteria that cause disease in aquatic organisms.

Phages can be used to control bacteria such as Vibrio harveyi, V parahaemolyticus, V anguillarum, V alginolyticus, and V splendidus, which infect molluscs, crustaceans, echinoderms and fish.

Bacteriophages are bacterial viruses that invade and sometimes destroy bacterial cells

Phages are the most abundant microorganisms worldwide, particularly in marine and freshwater environments. © Dr Subhashini

Quorum quenching (QQ)

Quorum quenching refers to all processes that perturb quorum sensing (QS); the ability of bacteria to monitor their population density and regulate gene expression. Numerous species of bacteria can use quorum sensing signals to coordinate and synchronise behaviours under different environments, including microbe-microbe and host-microbe interactions.

Quorum quenching encompasses very diverse phenomena and mechanisms, and the molecular actors of QQ are also diverse in nature, ie enzymes, chemical compounds, mode of action, etc.

All major steps of the QS pathway, including the synthesis, diffusion, accumulation and perception of QA signals, can be affected. QS disruption is a field that is being developed and used for the biological control of bacterial diseases – it is also being trialled in aquaculture.

Bacteriocins

Bacteriocins, bioactive compounds produced by bacteria, have been proposed as a promising and sustainable alternative strategy to the use of antibiotics. According to the researchers, bacteriocins have antimicrobial properties due to their ability to inhibit or kill closely or distantly related microorganisms.

The benefits of bacteriocins include being environmentally friendly, biodegradable and non-lethal to the host or the environment while antagonising harmful intestinal pathogens and promoting beneficial bacteria.

Probiotic, prebiotics, symbiotics, parabiotic and postbiotics

In recent years, some publications have pointed out the importance of maintaining a healthy and stable gut microbiome in fish and shellfish to reduce the risk of disease occurrence. An affected microbiome is frequently related to a disease condition and is considered by some scientists as a biomarker to detect a pathological problem.

Probiotics are the most common and commercially available to positively influence microbiomes. They are live, non-pathogenic microorganisms administered to improve the microbial balance, particularly in the gastrointestinal tract.

Prebiotics are non-viable food ingredients, usually oligosaccharides, that are not digestible by the host, but are digestible to specific bacterial populations residing in the gut and therefore act as selective substrates for bacterial fermentation.

New scientific evidence reveals that parabiotics (for example, dead probiotic cells, also called ghost probiotics) and probiotics (probiotic culture supernatants, which contain soluble factors or metabolic by-products secreted by bacteria) also have a major impact on the microbiome and the appearance of diseases.

Saccharomyces cerevisiae, or baker’s yeast, is a common probiotic in aquaculture

Probiotics are live, non-pathogenic microorganisms administered to improve the microbial balance, particularly in the gastrointestinal tract.

Chicken egg yolk immunoglobulin

Chicken egg yolk immunoglobulin (IgY) is a useful antibody for passive immunisation due to the fact that high levels of pathogen-specific IgY are produced after immunisation of chickens and simple methods for IgY extraction have been developed for the egg yolk.

Chicken egg yolk immunoglobulin has been found to have effective therapeutic value in controlling various bacterial and viral pathogens in fish and other aquatic animals. IgY has been used to treat white spot disease (WSD) which affects shrimp and crayfish; Vibrio harveyi infection in Indian white shrimp (Fenneropenaeus indicus); V anguillarum and Yersinia ruckeri in rainbow trout (Oncorhynchus mykiss); V splendens in sea cucumber (Apostichopus japonicas); Aeromonas hydrophila in polyploid gibbelly carp (Carassius auratus gibelio) and Wuchang bream (Megalobrama amblycephala); A salmonicida in koi carp (Cyprinus carpio koi); and Edwardsiellosis in Japanese eel (Anguilla japonica) and small abalone (Haliotis diversicolor supertexta).

Medicinal plants

In recent years, medicinal plants and their derivatives have received considerable attention as alternatives to antibiotics. There is considerable interest in the use of medicinal plants due to their easy preparation, low cost, low risk of side effects and lower environmental impacts.

Medicinal plants include herbs, spices, seaweeds, herbal-extracted compounds, commercial plant-derived products and their active ingredients include secondary metabolites (phenols, essential oils, pigments, alkaloids, terpenoids, etc).

Alternatives to antibiotics have great potential; but whole some have shown benefits, others remain in the experimental stage. The researchers recommend increasing resources for research in the aquaculture sector that can focus on the health of aquatic organisms, with an emphasis on disease prevention, which could include increased knowledge of aquatic diseases, efficacy and safety of veterinary medicines in different environmental conditions.

By The Fish Site

Reference: https://thefishsite.com/articles/what-aquaculture-producers-can-use-instead-of-antibiotics

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