Reduced total hardness of fresh water enhances the efficacy of bathing as a treatment for amoebic gill disease in Atlantic salmon, Salmo salar L.

Abstract

The current treatment for amoebic gill disease (AGD)-affected Atlantic salmon involves bathing sea-caged fish in fresh water, often sourced from local dams, for 3-4 h. In both a small-scale laboratory and an on-farm field experiment, the effects of water hardness on the efficacy of freshwater bathing were assessed. Results showed that soft fresh water (19.3-37.4 mg L(-1) CaCO3), whether it be naturally soft city mains water or artificially softened dam water, was more efficacious at alleviating AGD in affected fish than hard fresh water (173-236.3 mg L(-1) CaCO3). Soft freshwater bathing significantly reduced viable gill amoebae numbers (from 73.9 to 40.9% of total count) and significantly alleviated gill pathology, both gross and histological. Following bathing, gross gill pathological scores of soft freshwater bathed fish lagged 2 weeks behind hard freshwater bathed fish. Significant gill lesion fragmentation, and shedding of lesion-associated hyperplastic tissue, was accompanied by a significant reduction in AGD-affected gill filaments in soft freshwater bathed fish. Furthermore, soft freshwater bathing alleviated the blood plasma electrolyte imbalance seen in control (sea water) and hard freshwater bathed fish. This study showed that the use of soft fresh water for bathing AGD-affected Atlantic salmon could be an improvement to the current method of treatment. Not only does it reduce gill amoeba numbers, but also, it is of a therapeutic advantage with the potential to reduce bathing frequency.

Oral l-cysteine ethyl ester (LCEE) reduces amoebic gill disease (AGD) in Atlantic salmon Salmo salar

“…Comparatively, field outbreaks of clinical AGD on commercial salmon farms exhibit much less severe infection rates than laboratory-induced infections. Roberts & Powell (2003b) showed that AGD-affected fish in a field experiment on a commercial salmon farm had 5% of gill filaments with an AGD lesion, which coincided with a gross gill score that initiated freshwater bathing. Fish returned to a similar level of infection 8 wk post-bath.…”

“…Briefly, AGD had been maintained in this tank for several months, with introduced fish exhibiting gross signs of AGD within 3 d of cohabitation. The presence of Neoparamoeba pemaquidensis, the putative agent of AGD, was routinely confirmed using a modified indirect fluorescent antibody test (Roberts & Powell 2003b), PCR (Wong et al 2004), immunohistochemistry (Bridle et al 2003 and by visually identifying parasomes within isolated gill amoebae (Morrison et al 2004). All fish were fed to satiation daily with non-medicated pellets, and further lethally sampled 3 and 5 d into their cohabitation with AGD-affected fish.…”

“…Indeed, freshwater bathing has been shown to significantly decrease mucus viscosity of marine Atlantic salmon (Roberts 2004). Thinner and more removable mucus is thought to aid the significant lesion fragmentation and shedding of AGD lesion-associated hyperplastic gill tissue, as well as sloughing of gill-associated amoebae in soft, freshwater-bathed fish (Roberts & Powell 2003b).…”

“…The current commercial treatment for AGD is freshwater bathing whole pens of fish (Parsons et al 2001), preferably in water from a soft freshwater source (Roberts & Powell 2003b). Recently, we found that water hardness (mg l -1 CaCO 3 ) is a limiting factor for the efficacy of freshwater bathing (Roberts & Powell 2003b).…”

“…Indeed, freshwater bathing has been shown to significantly decrease mucus viscosity of marine Atlantic salmon (Roberts 2004). Thinner and more removable mucus is thought to aid the significant lesion fragmentation and shedding of AGD lesion-associated hyperplastic gill tissue, as well as sloughing of gill-associated amoebae in soft, freshwater-bathed fish (Roberts & Powell 2003b).Therapeutic mucolytic drugs have been extensively used for the alleviation of respiratory diseases associated with excessive production and accumulation of mucus in humans and domestic animals. N-acetylcysteine (NAC) is a widely used mucolytic agent that by virtue of its free sulfhydryl group reacts with and ruptures the disulfide bridges of mucoproteins leading to a decreased mucus viscosity (Walters et al 1985, Holdiness 1991, Houtmeyers et al 1999.…”