Perspectives on phage therapy for health management in aquaculture

Though biosecurity and best management practices (BMPs), vaccines, immunostimulants, probiotics and prebiotics are often used for health management in aquaculture, the use of chemotherapeutics/antibiotics often becomes the preferred method of choice once the infectious disease outbreak occurs. Phage-based control of bacterial pathogens (phage therapy) has recently re-emerged as an attractive therapeutic alternative due to the global emergence of antimicrobial-resistant bacterial pathogens. Target specificity with minimal disruption of natural microbiota, auto-dosing, safety, no production of toxic metabolites/residues and relatively inexpensive production are some of the distinct advantages of phages over antibiotics. In vivo, experimental studies have demonstrated the efficacy of phage therapy through immersion, oral, injection, topical application, and anal intubation routes against Aeromonas hydrophila, A. salmonicida, Edwardsiella tarda, Flavobacterium columnare, Pseudomonas aeruginosa, Streptococcus iniae, Vibrio anguillarum, V. harveyi, V. parahaemolyticus, etc. in aquaculture. Several factors such as phage selection, therapeutic dose, age of fish, specific targeting of the pathogen, disease condition of fish, environmental conditions, and administration route influence the efficacy of phage therapy in aquaculture. The application of a mixture of phages (phage cocktail) has also been suggested to overcome the narrow host range of phages and the development of phage resistance in bacteria. At present, a few commercial phage cocktail-based products are also available for control of Yersinia ruckeri, Aeromonas spp., Pseudomonas spp., and shrimp pathogenic vibrios in aquaculture. To successfully implement phage therapy, there is a need to develop region-specific/localized phage repositories with associated safety and efficacy data which could be used to quickly formulate effective phage cocktails after the identification of pathogenic bacterial strains in specific aquaculture areas.