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Paramoebiasis of sea-farmed salmonids in Tasmania : a study of its aetiology, pathogenicity, and control


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Howard, Teresa Sylvia 2001 , 'Paramoebiasis of sea-farmed salmonids in Tasmania : a study of its aetiology, pathogenicity, and control', PhD thesis, University of Tasmania.

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Amoebic gill disease (AGD) is the most significant disease of farmed sea-caged
salmonids in Tasmania. The research reported here provides the first substantiated
evidence for a Paramoeba species as the cause of this economically important
disease. A total of 680 cultures of amoebae were prepared during an extensive
sampling programme of diseased Atlantic salmon, resulting in 61 successfully
purified and subcultured amoeba isolates. This library of amoebae comprised the
protozoan genera Platyamoeba, Vannella, Flabellula, Heteroamoeba, Vexillifera,
Acanthamoeba and Paramoeba. Fixed and frozen sections of gills from fish with
AGD were immunostained with polyclonal antisera against the predominant genera
associated with gills. Only Paramoeba was detected in large numbers on gill tissue
and always in close association with gill hyperplasia, a characteristic pathognomonic
of AGD. Antisera to Paramoeba were highly specific and did not cross react with
other genera of gill-associated amoebae. Specificity of the antisera has enabled the
development of a rapid and highly accurate immunofluorescent antibody test for the
identification of clinical cases of AGD in farmed fish and is now the major screening
tool of farmed Atlantic salmon in Tasmania.
Despite evidence that Paramoeba is the cause of AGD, fulfillment of Koch's
postulates could not be achieved when naive rainbow trout or Atlantic salmon were
exposed to freshly isolated cultures of Paramoeba with limited laboratory passage.
Infection could be established however in cohabitation trials when naive fish were
exposed to fish with AGD.
A sensitive enzyme linked immunosorbent assay was developed using Paramoeba
isolated from fish with AGD. In a preliminary survey of several Atlantic salmon
populations naturally exposed to Paramoeba, between 50-100% of fish had
circulating serum antibody to Paramoeba; no antibody could be detected in gill
mucus. The presence of anti-Paramoeba antibodies in the serum of fish exposed to
Paramoeba and/or infected with AGD provided additional evidence for the role of
Paramoeba in AGD.
AGD is controlled by bathing fish in freshwater. The standard treatment regime for
fish was validated by determining the rate of inactivation of Paramoeba in freshwater.
In addition, 37 potential anti-amoebic compounds were screened for their
amoebistatic and amoebicidal activity against Paramoeba using assays developed in
this study to determine contact and growth inhibition effects. From these in vitro trials
it was established that Paramoeba were totally inactivated by exposure to freshwater
within 120 minutes. Hydrogen peroxide at 100ppm inactivated Paramoeba within 30
minutes as did exposure to 0.lppm ozone for four hours. Of the remaining
compounds tested, 8-hydroxyquinoline, chloro-iodo-hydroxyquinoline and
pyrimethamine at 30µg/ml for four hours' exposure were able to inactivate
Paramoeba effectively and may have potential as medicated bath treatments for
AGD. These trials also identified several chemicals able to inhibit the growth of
Paramoeba at concentrations of <30µg/ml and may have potential as in-feed
treatments for AGD.

Item Type: Thesis - PhD
Authors/Creators:Howard, Teresa Sylvia
Keywords: Salmon industry, Tasmania, Salmonidae, Gills, diseases
Copyright Holders: The Author
Copyright Information:

Copyright 2001 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2001. Includes bibliographical references

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