Abstract
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.
Highlights
Parasites are ubiquitous in the aquatic environment, but in the wild their impact on fish health is generally kept in balance by host–parasite interactions, including parasite lifecycle, host age, immune function, and behavior
Advanced amoebic gill disease (AGD) was evident in the CIT fish that were used t perurans by freshwater bathing
AGD-affected salmon are held in freshwater for 2 h after the last fish is transferred to the treatment enclosure) is likely to release a concentration of infective amoebae back into the marine environment
Summary
Parasites are ubiquitous in the aquatic environment, but in the wild their impact on fish health is generally kept in balance by host–parasite interactions, including parasite lifecycle, host age, immune function, and behavior. Where fish are exposed to biotic or abiotic stressors, their susceptibility to parasitic disease can increase [1,2,3]. Fish reared in intensive sea pen monoculture are not co-evolved with local parasite populations and are susceptible to parasitic diseases due to high stocking density, in-pen environment and handling stressors [4]. Sea pen aquaculture generally favors ectoparasites and a reduction in parasite diversity because trophic transmission of parasites with complex lifecycles is generally reduced through limited access to wild prey and the use of manufactured feeds [5]. The economic impacts of parasitism to intensive aquaculture accrue directly through mortality, reduced growth, poor feed conversion and harvest quality, increased disease susceptibility of stressed fish and the costs and effects of husbandry and management practices, such as monitoring, treatment, fallowing, grading, and stock handling
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
