Development and Application of a Novel q PCR Assay for Quantifying Flavophage F p V ‐4 Against Flavobacterium psychrophilum

Bacteriophage (phage) therapy is increasingly being used to control Flavobacterium psychrophilum impacts upon the health and production of salmonid aquaculture worldwide, and FpV-4 is a phage commonly utilised for the mitigation of F. psychrophilum outbreaks. While quantitative PCR (qPCR) methods are often used for quantifying F. psychrophilum, with its high-sensitivity and reproducibility offering a viable alternative to plate-based microbial quantification, there are few qPCR assays currently published for quantification of F. psychrophilum phages. We have developed and optimised a highly specific qPCR assay for FpV-4 quantification, capable of reliably detecting a minimum of seven copies. We applied the assay to intestinal content samples from FpV-4-fed rainbow trout (Oncorhynchus mykiss), showing consistent detection and quantification of the phage. This assay represents a new development for the evaluation of F. psychrophilum phage therapy, providing a fast quantification method that complements and reduces the potential uncertainty of plate-based enumeration.