Field investigations of amoebic gill disease in Atlantic salmon, Salmo salar L., in Tasmania

Abstract

Amoebic gill disease (AGD) is the most serious health problem in Atlantic salmon cultured in Tasmania. Our field investigation examined prevalence of AGD during 2 years, every year for up to 7 months after transfer to sea water. The relationship between environmental factors and AGD prevalence was determined. Additionally, effects of adding levamisole to freshwater baths were investigated in a field trial. AGD was recorded on all farms, except for farm A, which did not move salmon from a brackish site to a full‐salinity site during the study. The prevalence showed a bimodal distribution with the first larger peak in summer (usually in January) and the second smaller peak in autumn (between March and May). During both years the prevalence of AGD was significantly greater in January than any other month. Sampling month and the interaction between farm and month had a statistically significant effect on AGD prevalence. AGD was recorded at a minimum temperature of 10.6 °C and minimum salinity of 7.2 ppt. There was a positive relationship between the time since the freshwater bath and the prevalence of AGD for the first 30 days after the bath, with a dramatic increase in the AGD prevalence about 3 weeks after the bath. After 30 days, there was no statistically significant relationship between AGD prevalence and days since the last bath, except for the second bath. The addition of levamisole to the freshwater bath did not significantly increase the time between treatments. The relationship between diagnosis on the basis of gross signs and histological diagnosis was significant, however, the gross diagnosis was unreliable within the lower range, with 31.8% false negatives and 15.9% false positives and kappa value of 0.2742.

Validating the identity of Paramoeba invadens, the causative agent of recurrent mass mortality of sea urchins in Nova Scotia, Canada

“…Paramoebiasis in sea urchins off Nova Scotia occurs only above a threshold temperature of ~10°C, and rates of morbidity of sea urchins increase up to ~20°C, corresponding to the thermal optimum for growth of P. invadens in culture (Scheibling & Stephenson 1984, Jellett & Scheibling 1988a. Similarly, infection by Paramoeba/Neo paramoeba of farmed salmonids in Tasmania, Australia (Munday et al 1990, Clark & Nowak 1999, DouglasHelders et al 2001, and blue crab Callinectes sapidus in the northeastern US (Johnson 1977), occurs only above threshold temperatures of ~10 and 13°C, respectively. Furthermore, the only documented outbreak of AGD in Atlantic salmon in aquaculture in Norway occurred above a threshold of 11°C and was associated with sea temperatures 3.5°C above the seasonal average (Steinum et al 2008).…”

Evaluation of Non-destructive Molecular Diagnostics for the Detection of Neoparamoeba perurans

“…A score of 0 = normal gill; 1 = low pathology <10% of gill tissue affected; 2 = moderate pathology <50% of gill tissue affected and 3 = severe pathology > than 50% of gill tissue affected. Only histological sections where pathology was observed in the presence of amoeba were recorded as AGD infected (Clark and Nowak, 1999;Rodger, 2014).…”

“…The case definition for AGD is through histopathology, where amoebae are observed with associated pathology (Clark and Nowak, 1999;Rodger, 2014). By far the most widely used and practical method for ascertaining AGD severity and hence triggers for intervention (freshwater bathing, hydrogen peroxide treatments) is the gross gill score across all 16 hemibranchs, as described by Taylor et al (2009), which may be coupled with histopathology and fresh microscopy to confirm the presence of lesion-associated amoebae.…”

18S ribosomal DNA-based PCR identification of Neoparamoeba pemaquidensis, the agent of amoebic gill disease in sea-farmed salmonids

“…AGD remains the most important parasitic disease affecting sea-farmed Atlantic salmon Salmo salar L. and rainbow trout Oncorhynchus mykiss Walbaum in Tasmania, Australia (Munday et al 1990(Munday et al , 2001). Outbreaks of AGD have also been reported in Atlantic salmon farmed in Chile, France (Clark & Nowak 1999), Ireland (Rodger & McArdle 1996, Palmer et al 1997 and Spain (Munday et al 2001), and in coho salmon O. kisutch Walbaum in Washington State and California, USA (Kent et al 1988), chinook salmon O. tshawytscha Walbaum in New Zealand (C. Anderson pers. comm.…”