| Summary: | Reiterative freshwater bathing is the main treatment to control amoebic gill disease (AGD) of farmed Atlantic salmon in Tasmania, Australia. Regular freshwater exposure appears to control ectoparasitic cymothoid isopods, which were only seen at high prevalence and intensity in summer when fish had not been treated for over 100 days. With the potential advent of non-freshwater AGD treatments or increased periods between freshwater bathing due to selective breeding for AGD resistance, it is possible that cymothoid parasitism will become an increasing threat on Tasmanian salmon farms. In order to establish whether isopods could be vectors of Neoparamoeba perurans Young et al. 2007 (the causative agent of AGD), gill isopods were collected from salmon that had not been bathed for seven months and carried a 95% prevalence of isopods, including Ceratothoa banksii (Leach, 1818) and Nerocila orbignyi (Guérin-Méneville, 1832). PCR analyses of gill swabs indicated that 82% of salmon were positive for N. perurans while 41% of the sampled isopods were positive for N. perurans on external surfaces. When internal material was analysed, only 9% of the isopods were positive for the amoeba, but in very low concentration. Quantitative analysis showed no correlation between the concentrations of N. perurans from gill swabs and the isopods from the same individual fish. Thus, it is unlikely that these isopods act as a significant reservoir or vector for N. perurans.
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