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

Abstract

There is a need for the development of alternative therapeutic treatments for amoebic gill disease (AGD) in Atlantic salmon Salmo salar L. to maintain the sustainability of the Tasmanian Atlantic salmon aquaculture industry. This study aimed to assess the effects of the mucolytic drug L-cysteine ethyl ester (LCEE) on marine Atlantic salmon mucus and whether or not it may have a therapeutic advantage for the alleviation of AGD when administered orally. We also aimed to document any physiological consequences of LCEE. Results showed that LCEE significantly decreased the viscosity of marine Atlantic salmon mucus both in vitro, where LCEE concentration showed a negative relationship to mucus viscosity (R 2 = 0.95 at 11.5 s -1 ), and in vivo. Oral administration of LCEE at 52.7 mg LCEE kg -1 fish d -1 over 2 wk significantly delayed the progression of AGDassociated pathology during an aggressive, cohabitation induced, laboratory infection. Medicated fish had approximately 50% less gill filaments affected by AGD than control fed fish at 3 d postinfection when assessed using histology. Palatability of medicated feed was shown to be approximately 65% of control feed. No osmoregulatory disturbance was seen in medicated fish, although blood and whole body flux data indicated a slight acidosis coinciding with an increased plasma total ammonia concentration. However, both variables were within a tolerable physiological range and returned to control levels 3 d post-cessation of medicated feed. LCEE holds potential as an in-feed additive when administered over 2 wk prior to infection to delay the progression of AGD associated pathology. From the parameters measured, LCEE seems to have minimal physiological consequences after 2 wk of administration. KEY WORDS: Fish gill disease · Salmonid · In-feed treatment · Oral therapeutant · Mucolytic drug · Mucus viscosity · Physiological effects Resale or republication not permitted without written consent of the publisherDis Aquat Org 66: [21][22][23][24][25][26][27][28] 2005 Roberts & Powell 2003a). It has been previously suggested that excessive branchial mucus impairs CO 2 excretion, leading to respiratory acidosis and subsequent death in AGD affected fish (Powell et al. 2000). The break-up and removal of mucus from the gills seems to be an important function of freshwater bathing as a treatment . 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 w...

Effect of Dietary Inclusion of N‐Acetyl Cysteine on Mucus Viscosity and Susceptibility of Rainbow Trout, Oncorhynchus mykiss, and Atlantic Salmon, Salmo salar, to Amoebic Gill Disease

“…These results clearly contrast those of Roberts and Powell (2005b) and Roberts (2004) who demonstrated that AGD was retarded with branchial lesions developing at approximately half as fast as in controls as a result of feeding LCEE at the same concentration and formulation as in our present study. One explanation may be that the period of feeding NAC prior to inoculation with Neoparamoeba spp.…”

“…One explanation may be that the period of feeding NAC prior to inoculation with Neoparamoeba spp. was inadequate (Roberts and Powell 2005b used a 14‐d prefeeding period). However, viscosity measurements of the cutaneous mucus from fish at the point of inoculation (Day 0) and end of the study (Days 14 and 19 for rainbow trout and Atlantic salmon, respectively) showed overall reduction in mucus viscosity.…”

“…These include the use of disinfectants such as chlorine dioxide, hydrogen peroxide, and chloramine‐T (Powell and Clark 2002, 2004; Harris et al 2004, 2005). The investigation of in‐feed treatments for the control of AGD has been limited, although recently Roberts and Powell (2005b) demonstrated that feeding fish the mucolytic agent L‐cysteine ethyl ester (LCEE) was able to retard AGD. Roberts (2004) also showed that prophylactic treatment with LCEE was able to reduce AGD severity (hyperplastic lesions on the gills) by approximately 50%.…”

Preliminary success using hydrogen peroxide to treat Atlantic salmon, Salmo salar L., affected with experimentally induced amoebic gill disease (AGD)

“…Previous in vitro screening of various antiparasitic drugs and oxidative disinfectants had identified several compounds of interest (Howard & Carson , ; Powell et al . , ; Roberts & Powell ). However, producing an unequivocal resolution of AGD (compared to FW treatment) that is transferable to the commercial farming environment has proven elusive.…”

Freshwater bathing alters the mucous layer of marine Atlantic salmon Salmo salar L.

“…The mechanism by which freshwater bathing produces a therapeutic effect, however, is poorly understood. Such knowledge would aid research and development into improved and alternative treatments for fish gill diseases, including mucolytic drugs (Yang & Albright, 1994; Roberts & Powell, 2005 a ).…”