Amoebic gill disease: sequential pathology in cultured Atlantic salmon, Salmo salar L.

ABSTRACT:The relationship between salmonid gill bacteria and Neoparamoeba sp., the aetiological agent of amoebic gill disease (AGD) was determined in vivo. Fish were divided into 4 groups and were subjected to following experimental infections: Group 1, amoebae only; Group 2, Staphylococcus sp. and amoebae; Group 3, Winogradskyella sp. and amoebae; Group 4, no treatment (control). Fish (Groups 1, 2 and 3) were exposed to potassium permanganate to remove the natural gill microflora prior to either bacterial or amoebae exposure. AGD severity was quantified by histological analysis of gill sections to determine the percentage of lesioned filaments and the number of affected lamellae within each lesion. All amoebae infected groups developed AGD, with fish in Group 3 showing significantly more filaments with lesions than other groups. Typically lesion size averaged between 2 to 4 interlamellar units in all AGD infected groups. The results suggest that the ability of Neoparamoeba sp. to infect filaments and cause lesions might be enhanced in the presence of Winogradskyella sp. The possibility is proposed that the prevalence of more severe AGD is due to the occurrence of Winogradskyella sp. at high concentrations on the gills.KEY WORDS: Neoparamoeba · Winogradskyella · AGD · Potassium permanganate · Protozoan diseases · Salmon diseases · Amoeba · Bacteria Resale or republication not permitted without written consent of the publisherDis Aquat Org 67: [55][56][57][58][59][60] 2005 MATERIALS AND METHODS Fish. Atlantic salmon Salmo salar L. (n = 72; mean weight = 88 g) were acclimatised to sea water (35 ‰, 1 µm filtered) over a week in 6 identical recirculating systems each consisting of three 70 l tanks (n = 4 fish per tank) and a 70 l reservoir. A sentinel population (n = 12) of the same body weight was acclimatised in a static tank (210 l). Following acclimatisation, fish in the recirculating systems were divided into 3 treatment groups (n = 12 fish per treatment). Each treatment was duplicated. The 4th group was the sentinel population (n = 12). Fish in Group 1 were exposed to amoebae only (positive control); Group 2, Gram positive bacteria (Staphylococcus sp.) and amoebae; Group 3, Gram negative bacteria (Winogradskyella sp.) and amoebae; Group 4 did not receive any treatment. Sea water temperature was maintained at 16 ± 0.5°C, pH 8.2, dissolved oxygen 7.6 mg l -1 , salinity 35 ‰ and total ammonia-nitrogen below 0.2 mg l -1 . Sufficient air supply was maintained in the tanks throughout the experiment by using aerators.Neoparamoeba sp. isolation. Neoparamoeba sp. were harvested from the AGD affected Atlantic salmon held in the Aquaculture Centre, University of Tasmania, Launceston by a method described by Morrison et al. (2004). In brief, infected gills were removed from AGD affected Atlantic salmon after euthanasia (anaesthetic overdose at 20 ml l -1 Aqui-S ® ). Gills were transported to the laboratory in sterile sea water (SS) containing antibiotic and antimycotic solution (5% v/v 5000 IU ml -1 penicillin and 5 mg...

supporting

confidence: 92%

Influence of salmonid gill bacteria on development and severity of amoebic gill disease

Embar-Gopinath¹,

Bütler²,

Nowak³

2005

“…Fish returned to a similar level of infection 8 wk post-bath. Also, Adams & Nowak (2003) found AGD-affected fish from a commercial salmon farm to have 10 to 20% of gill filaments with an AGD lesion 18 wk after first being introduced to estuarine/ marine sites from freshwater. Differences in infection rates between laboratory and field situations occur due to optimal conditions for AGD in the laboratory, where no freshwater bathing treatment is administered, and there is the potential for continuous infection of gills under laboratory conditions because water is recirculated.…”

Section: Discussionmentioning

confidence: 99%

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

Roberts

Powell²

2005

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...

“…Increased mucus production, via significant elevations in mucous cell numbers, has also been noted in association with AGD (Munday et al 1990, Nowak & Munday 1994, Adams & Nowak 2003, Roberts & Powell 2003, and, although gill mucus production does not appear to greatly influence O 2 uptake in rainbow trout exposed to a chemical gill irritant, there is evidence to suggest that it does impair CO 2 excretion (Powell & Perry 1996, 1997, 1999, and this could lead to an acute acid -base disturbance. Although caudal blood pH was significantly elevated in Series II fish by Day 2, it appeared to fall from Day 7 onwards and consistently decreased over the remainder of the sampling period.…”

Section: Discussionmentioning

confidence: 95%

“…The actual effect of AGD lesions on gill blood flow is not currently understood; however, pathological studies of AGD have shown that amoeba presence and attachment to gill filaments initially results in hyperplasia of epithelial cells (Adams & Nowak 2003). Epithelial cells play a prominent role in respiratory gas transfer (Lin & Randall 1995); thus, any alterations in epithelial thickness can potentially decrease functional surface area for transfer of respiratory gases.…”

Section: Discussionmentioning

confidence: 99%

Respiratory pathogenesis of amoebic gill disease (AGD) in experimentally infected Atlantic salmon Salmo salar

Leef

Harris²,

Powell³

2005

The aim of this study was to investigate the respiratory responses of Atlantic salmon, Salmo salar, experimentally affected with amoebic gill disease (AGD). In Series I, arterial blood samples were taken over a 96 h period following amoebae addition to examine potential respiratory effects associated with initial exposure. No major significant treatment effects were found between fish exposed to amoebae and control (non-exposed) fish. Arterial pH (pH a ) was seen to be significantly elevated at 48 h in AGD fish relative to the 0 h time point. To investigate the long-term respiratory effects associated with infection, fish were similarly exposed to amoebae and sampled over a 16 d period. As for Series I, caudal blood pH was significantly elevated by Day 2 (48 h) compared to the pre (Day 0)-time point, suggesting that initial exposure to amoebae and/or amoebae attachment may have induced an initial respiratory alkalosis via increased ventilation frequency and/or amplitude. From Day 7 onwards, and coinciding with a significant increase in the percentage of affected gill filaments, blood pH decreased significantly, possibly indicating the onset of the characteristic respiratory acidosis that has previously been described for experimentally AGDaffected Atlantic salmon. Although fish in this study showed up to 90% AGD-affected filaments, the corresponding respiratory results do not reflect a major acid -base disturbance. Therefore, the findings from the present study support the contention that, although AGD only affects the gill, AGDassociated mortality in Atlantic salmon may not be primarily associated with respiratory failure.