Amoebic gill disease (AGD) is a significant health issue for Atlantic salmon farmed in a marine environment. While the disease is currently managed using freshwater or hydrogen peroxide baths, there is a need to develop other treatments. The aims of this study were to examine the effect of salinity (0 ppt and 35 ppt) and temperature (3 °C and 15 °C) on attachment and survival of Neoparamoeba perurans in vitro over short exposure times (15 min and 2 h) and to assess the efficacy of reduced temperature (3 °C) as treatment for Atlantic salmon affected by AGD. In vitro freshwater 3 °C was at least as effective as freshwater 15 °C and the attachment was significantly lower after 2 h in freshwater 3 °C than freshwater 15 °C. In vivo there was no difference between the fish treated with freshwater 15 °C for 2 h or freshwater 3 °C. This study showed that despite exposure to low temperature reducing attachment of N. perurans to their substrate in vitro, 15 min cold-water bath treatment was not more effective at reducing AGD in Atlantic salmon than current commercial 2 h freshwater bath.

Amoebic Gill Disease (AGD) severely affects salmonid mariculture due to fish losses and costs associated with management of the disease. Continued research into management solutions, including new treatments and vaccine development, is highly important for the future of salmonid production worldwide. This requires both in vitro (both pathogen only and host-pathogen models) and in vivo (disease challenge) testing. Challenge models are still widely varied, in particular with regard to: infection methods (cohabitation or immersion), source of the pathogen (isolated from infected fish or cultured), infectious dose, environmental conditions (in particular temperature) and the endpoints across experimental treatment and vaccine studies which makes comparisons between studies difficult. This review summarises in vitro assays, the challenge methods and endpoints used in studies of experimental treatments and vaccines for AGD.

Reliance on lethal sampling to monitor Cardicola spp. infections in ranched Southern bluefin tuna (Thunnus maccoyii, SBT) constrains investigative time points and number of fish available for sampling, restricting health monitoring to commercial harvest periods. For these reasons six non-lethal sampling strategies from three biologically relevant sites in SBT were evaluated for their ability to detect Cardicola spp. infection severity relative to lethally collected gill necropsies, which are currently considered the diagnostic gold standard. From harvested SBT, gill biopsies demonstrated the highest sensitivity to reliably detect Cardicola spp., with higher infection prevalence than gill necropsies. Despite the appeal of blood and gill mucus as non-lethal samples, they were not effective for detecting Cardicola spp. in this study. This is likely related to the biology of the parasite, with DNA in these fluids representing migrating or residual stages of the parasite rather than established infections. The ability to assess fish health without sacrificing valuable stock represents an important methodological advance in aquaculture health monitoring and management. Gill biopsies are anticipated to be used in conjunction with other sampling techniques to enhance overall monitoring of SBT health, supporting improved blood fluke monitoring and assessments of treatment efficacy that inform management decisions. However, targeted welfare investigations are required prior to routine implementation.

Parasitic diseases can be a significant constraint on aquaculture industries, which continue to develop in response to the rise in global demand for sustainable protein sources. Blood flukes, Cardicola forsteri and Cardicola orientalis, are economically significant parasites of Southern bluefin tuna (Australia), Pacific bluefin tuna (Japan), and Atlantic bluefin tuna (Mediterranean) as they are responsible for blood vessel obstruction in the gills leading to branchitis and mortalities when untreated. Here, we have defined the mitochondrial genomes for these species—the first for any aporocotylids. Oxford nanopore long-read sequencing was used to sequence C. orientalis from a single individual. The mitochondrial genome of C. forsteri was assembled and curated from available sequence data. Both Cardicola spp. mitogenomes contained 12 protein coding, 2 ribosomal and 22 tRNA genes, with the gene order matching that of Asian schistosomes. A control region was identified for each species which contained long and short repeats; the region for C. forsteri was longest, and the overall pattern differed between the two species. A surprisingly high nucleotide diversity was observed between the two species, generating interest into the mitochondrial genes of related species. This paper provides a useful resource for future genetics-based research of aporocotylids and other flatworm parasites of socioeconomic significance.