Development of a non‐lethal hydrogen peroxide treatment for surveillance of Gyrodactylus salaris on trout farms and its application to testing wild salmon populations

This study documents the development of a non‐lethal sampling method to recover gyrodactylid parasites from large numbers of fish that will underpin an improved surveillance strategy for Gyrodactylus salaris. A review of published literature identified over 80 compounds that have previously been tested against gyrodactylids or closely related parasite species. Five safe and relatively fast‐acting compounds were selected for testing to determine their efficiency in removing gyrodactylids from host fish in small‐scale aquaria trials using three‐spined stickleback infected with Gyrodactylus gasterostei as a model host–parasite system. The most effective compound was hydrogen peroxide; short‐duration exposure (3 min) achieved a parasite detection sensitivity of 80%–89%. The practicality of exposing farmed salmonids to hydrogen peroxide for G. salaris surveillance was tested in the field by conducting a parasite recovery trial using a brown trout stock endemically infected with G. truttae and G. derjavinoides and comparing this to the whole‐body examination procedure currently conducted by UK authorities. Significantly more parasites were recovered after exposing fish to hydrogen peroxide and filtering the treatment solution than by direct whole‐body examination of killed fish (mean: 225 vs. 138 parasites per fish). The gyrodactylid recovery rate of the two methods was 84.6% and 51.9%, respectively. A comparison of timings for the two methods indicated scope for significant time savings in adopting the chemical screening method. The study demonstrated that hydrogen peroxide bath treatment may be successfully applied to the surveillance of gyrodactylid parasites and established as a non‐lethal method for sampling farmed and wild fish. This approach has the potential to reduce resources required to collect and isolate parasites for diagnostic testing and improve the sensitivity and confidence of surveillance programmes designed to demonstrate freedom from disease, thus underpinning a robust and defensible surveillance strategy for G. salaris for the UK aquatic animal disease contingency plan.