Repeated sublethal freshwater exposures reduce the amoebic gill disease parasite, Neoparamoeba perurans, on Atlantic salmon

Assigning cause for emerging diseases of aquatic organisms

Resolving the cause of disease (= aetiology) in aquatic organisms is a challenging but essential goal, heightened by increasing disease prevalence in a changing climate and an interconnected world of anthropogenic pathogen spread. Emerging diseases play important roles in evolutionary ecology, wildlife conservation, the seafood industry, recreation, cultural practices, and human health. As we emerge from a global pandemic of zoonotic origin, we must focus on timely diagnosis to confirm aetiology and enable response to diseases in aquatic ecosystems. Those systems' resilience, and our own sustainable use of seafood, depend on it. Synchronising traditional and recent advances in microbiology that span ecological, veterinary, and medical fields will enable definitive assignment of risk factors and causal agents for better biosecurity management and healthier aquatic ecosystems.

Evaluation of Low Temperature and Salinity as a Treatment of Atlantic Salmon against Amoebic Gill Disease

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.

Evaluation of the Infectious Potential of Neoparamoeba perurans Following Freshwater Bathing Treatments

Freshwater bathing for 2–3 h is the main treatment to control amoebic gill disease of marine-farmed Atlantic salmon. Recent in vitro studies have demonstrated that amoebae (Neoparamoeba perurans) detach when exposed to freshwater and that some eventually reattach to culture plates when returned to seawater. Here, we evaluated the potential for gill-detached N. perurans to survive a commercially relevant treatment and infect AGD-naïve fish and whether holding used bathwater for up to 6 h post treatment would lower infectivity. AGD-affected fish were bathed in freshwater for 2 h. Naïve salmon were exposed to aliquots of the used bathwater after 2, 4, 6 and 8 h. The inoculation was performed at 30 ppt for 2 h, followed by gradual dilution with seawater. Sampling at 20 days post inoculation (dpi) and 40 dpi confirmed rapid AGD development in fish inoculated in 2 h used bathwater, but a slower AGD development following exposure to 4 h bathwater. AGD signs were variable and reduced following longer bathwater holding times. These results suggest that viable amoebae are likely returned to seawater following commercial freshwater treatments, but that the risk of infection can be reduced by retention of bathwater before release.

Experimental Challenge Models and In Vitro Models to Investigate Efficacy of Treatments and Vaccines against Amoebic Gill Disease

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.

Searching for the sweet spot of amoebic gill disease of farmed Atlantic salmon: the potential role of glycan-lectin interactions in the adhesion of Neoparamoeba perurans

One of the first critical steps in the pathogenesis of amoebic gill disease (AGD) of farmed salmon is the adhesion of the causative amoeba to the host. The current study aimed to investigate the potential involvement of glycan-binding proteins expressed on the extracellular surface of Neoparamoeba perurans in gill tissue recognition and binding. The glycan-binding properties of the surface membrane of N. perurans and the carbohydrate binding profile of Atlantic salmon gill-derived epithelial cells were identified through the use of glycan and lectin microarrays, respectively. The occurrence of specific carbohydrate-mediated binding was then further assessed by in vitro attachment assays using microtitre plates pre-coated with the main glycan candidates. Adhesion assays were also performed in the presence of exogenous saccharides with the aim of blocking glycan-specific binding activity. Comparative analysis of the results from both lectin and glycan arrays showed significant overlap, as some glycans to which binding by the amoeba was seen were reflected as being present on the gill epithelial cells. The two main candidates proposed to be involved in amoeba attachment to the gills are mannobiose and N-acetylgalactosamine (GalNAc). Adhesion of amoebae significantly increased by 33.5 and 23% when cells were added to α1,3-Mannobiose-BSA and GalNAc-BSA coated plates. The observed increased in attachment was significantly reduced when the amoebae were incubated with exogenous glycans, further demonstrating the presence of mannobiose- and GalNAc-binding sites on the surfaces of the cells. We believe this study provides the first evidence for the presence of a highly specific carbohydrate recognition and binding system in N. perurans. These preliminary findings could be of extreme importance given that AGD is an external parasitic infestation and much of the current research on the development of alternative treatment strategies relies on either instant amoeba detachment or blocking parasite attachment.

Atlantic salmon Salmo salar and ballan wrasse Labrus bergylta display different susceptibility to clonal strains of Paramoeba perurans

Amoebic gill disease (AGD), caused by the marine amoeba Paramoeba perurans, is an important disease of farmed Atlantic salmon Salmo salar L. in Norway. The use of wrasse as cleaner fish in salmon net pens raises questions about interspecies transmission of pathogens such as P. perurans. In this study, cohabitant transmission of clonal isolates of P. perurans between Atlantic salmon and ballan wrasse Labrus bergylta Ascanius was examined, using isolates originating from both salmon and wrasse. The challenges resulted in AGD in both species, although less severely in wrasse. The amoeba isolate originating from ballan wrasse was more virulent than that originating from salmon, suggesting P. perurans strain-related virulence differences. The isolate originating from salmon showed limited proliferation in bath-challenged wrasse and salmon, and limited transfer to cohabitants. Our results support previous observations suggesting that salmon may be more susceptible to P. perurans and AGD than ballan wrasse. Treatment of P. perurans infection in wrasse is challenging, as it is a strictly marine fish species. In this study, brackish water (<15‰ seawater) treatment of AGD affected salmon and wrasse was examined. Both salmon and wrasse were treated for short periods (3 h and 24 h), and treatment of wrasse over longer periods (3-5 d) was also examined. Short exposure to brackish water was not enough to remove P. perurans, although the 24 h treatment reduced amoeba levels. It was not possible to culture or detect P. perurans from wrasse exposed to brackish water for 3 d, suggesting that this treatment would be effective in controlling the parasite.