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Abstract

The protozoan parasite Neoparamoeba spp. infects the gills of marine cultured
Atlantic salmon in Tasmania, Australia causing amoebic gill disease (AGD) and
if untreated causes significant production losses. Knowledge of the interaction
between Neoparamoeba spp. and host immune factors or cells is limited; specific
anti-Neoparamoeba spp. antibodies are present in the serum of fish naturally
exposed and sheep anti-Neoparamoeba spp. antibodies do not affect the viability
of Neoparamoeba spp. This thesis investigates in vitro and in vivo interactions
between the host (Atlantic salmon) and Neoparamoeba spp. The effect of
Neoparamoeba spp. on in vitro innate immune cell function was investigated.
Sonicated Neoparamoeba spp. did not stimulate a respiratory burst response from
anterior kidney leukocytes (P> 0.05). However, there was evidence of priming
as cells incubated with the pathogen and then stimulated with phorbol myristate
acetate (PMA) produced more (P < 0.05) reactive oxygen species (ROS) than
cells stimulated with PMA alone. Neoparamoeba spp. culture supernatant did not
affect immune cell function (P> 0.05). Further in vitro investigations found that
there was no effect of host serum and mucus on parasite viability (P> 0.05). The
effect of Neoparamoeba spp. infection on in vitro gill cell function was assessed.
A technique was developed to isolate viable gill cells from perfused gills. Gill
cells were tested for their ability to produce ROS, engulf yeast cells and
chemotaxis. Cells did not produce ROS in the presence of PMA above basal
levels, did not chemotactically migrate and few cells were capable of
phagocytosis despite cell viability greater than 80 %. In light of these
observations further studies utilised anterior kidney cells. Anterior kidney cells
were used to investigate the effect of gill infection on innate immune cell function (ROS production, phagocytosis and chemotactic ability) ex vivo. There was a
significant (P <0.05) effect of infection on ROS production 8 and 11 days post
Neoparamoeba spp. exposure. A previous study reported increased resistance to
AGD in fish that had been infected with Neoparamoeba spp.. However, these
results were not consistent with the experiences of commercial aquaculture
ventures that have performed similar investigations; therefore the study was
repeated. It was found that recovered fish were equally susceptible (P> 0.05) to
disease upon re-infection as naïve fish. Immunohistochemistry was used to
identify Ig bearing cells in the gills of Atlantic salmon. The spatial and temporal
distribution of Ig bearing cells was not different between affected and un-affected
fish (P> 0.05). Together the data presented in this thesis identify that the
interactions between host and pathogen are complex. The parasite may elude
destructive immune cell defences by avoiding initiation of deleterious
mechanisms or by immunosuppression.

Item Type:	Thesis - PhD
Authors/Creators:	Gross, Kally Anne
Keywords:	Atlantic salmon, Atlantic salmon, Paramoebidae, Bacterial gill disease, Fishes
Copyright Holders:	The Author
Copyright Information:	Copyright 2007 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 (PhD)--University of Tasmania, 2007. Includes bibliographical references. Ch. 1. General introduction -- Ch. 2. Phagocytic behaviour of gill cells isolated from Neoparamoeba spp. infected Atlantic salmon (Salmo salar L.) -- Ch. 3. Identification and distribution of leucocytes in the gills of AGD-affected Atlantic salmon (Salmo salar L.) -- Ch. 4. In vitro interactions between Neoparamoeba spp. and salmonid leucocytes; the effect of parasite sonicate on anterior kidney leucocyte function -- Ch. 5. Changes in the innate immune response of Atlantic salmon (Salm o salar L.) exposed to experimental infection with Neoparamoeba sp. -- Ch. 6. Atlantic salmon (Salmo salar L.) previously infected with Neoparamoeba sp. are not resistant to re-infection and have suppressed phagocyte function -- Ch. 7. The effect of plasma and mucus from AGD-affected and unaffected Atlantic salmon (Salmo salar L.) on the in vitro viability of Neoparamoeba spp. -- Ch. 8. General discussion
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