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Abstract

Amoebic gill disease (AGD) is an ectoparasitic condition of some marine fish.
Atlantic salmon are highly susceptible to AGD and the cost of mitigation is a
significant financial burden for the aquaculture industry, particularly in Tasmania,
Australia. Despite a considerable research effort over the past 20 years, two
fundamental questions remain unanswered:
1. Which species of amoeba causes AGD? Two species of amoebae,
Neoparamoeba pemaquidensis and Neoparamoeba branchiphila have been
isolated from the gills of AGD-affected fish. Based on morphology alone,
either species may be associated with AGD yet all attempts to experimentally
induce AGD with cultured strains of either species have been unsuccessful.
2. Why are Atlantic salmon highly susceptible to AGD and is this susceptibility
linked to aberrant innate and adaptive immune responses?
In this thesis, the aetiology of AGD was resolved by identifying and characterising a
new species of amoeba, Neoparamoeba perurans and demonstrating that this species
is the only known aetiological agent of AGD globally. In so doing, the phylogeny of
Neoparamoeba was resolved and the strict co-evolution of Neoparamoeba species
with their endosymbiont, Perkinsela amoebae-like organism was confirmed as a
defining characteristic amongst all members of the Neoparamoeba.
Following this, the mechanisms that underpin the susceptibility of Atlantic salmon to
AGD were assessed. Global gene expression profiling the gills of AGD-affected fish
revealed that transcripts associated with the immune response were almost
universally down-regulated in AGD-lesions specifically. In AGD-affected tissue,

significant, coordinated down-regulation of the major histocompatibility complex
class I (MHC I), and possibly the MHC II pathway-related genes occurred during the
later stages of infection and appeared to be mediated by down-regulation of
interferon-regulatory factor (IRF)-1, independent of interferon-á, interferon-y (IFN-
ã) and IRF-2 expression. Stimulating AGD-lesions ex vivo with recombinant IFN-y
failed to restore the expression of IRF-1 and the MHC I receptor molecule, thus
confirming earlier observations that the MHC I antigen presentation pathway appears
to be modulated independently of IFN-y in AGD lesions. Within the AGD lesion
microenvironment, suppression of the MHC I and possibly the MHC II pathways
may inhibit the development of acquired immunity and could explain the unusually
high susceptibility of Atlantic salmon to AGD. Whilst the data are preliminary, the
immunologically unresponsiveness of the AGD lesion microenvironment is possibly
linked with a disruption in the NF-kB signalling pathway which may permit N.
perurans to evade the host immune response. Finally it is proposed that an
understanding of the mechanisms of localised immunosuppression will be
particularly important for the development of new treatments for AGD since
systemic immunostimulation may be ineffective without simultaneous disruption of
the immune privilege-like microenvironment within AGD lesions.

Item Type:	Thesis - PhD
Authors/Creators:	Young, ND
Copyright Information:	Copyright © 2009 the Author
Additional Information:	Chapter 2 appears to be the equivalent of a post-print article published as: Young N. D., Crosbie P. B. B., Adams M. B., Nowak B. F. and Morrison R. N. (2007) Neoparamoeba perurans n. sp., an agent of amoebic gill disease of Atlantic salmon (Salmo salar L.). International Journal for Parasitology 37, 1469-1481, doi:10.1016/j.ijpara.2007.04.018 Chapter 3 appears to be the equivalent of a post-print article published as: Young N. D., Dyková I., Snekvik K., Nowak B. F. and Morrison R. N. (2008) Neoparamoeba perurans is a cosmopolitan aetiological agent of amoebic gill disease. Diseases of Aquatic Organisms 78, 217-223. doi:10.3354/dao01869 Chapter 5 appears to be the equivalent of the peer reviewed version of the following article: Young N. D., Dyková I., Nowak B. F. and Morrison R. N. (2008) Development of a diagnostic PCR to detect Neoparamoeba perurans, agent of amoebic gill disease (AGD). Journal of Fish Diseases 31, 285-295 which has been published in final form at DOI:10.1111/j.1365-2761.2008.00903.x This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving. Chapter 6 appears to be the equivalent of a post-print article published as: Young N. D., Cooper G. A., Nowak B. F., Koop B. F. and Morrison R. N. (2008) Coordinated down-regulation of the antigen processing machinery in the gills of amoebic gill disease-affected Atlantic salmon (Salmo salar L.). Molecular Immunology 45, 2581-2597. doi:10.1016/j.molimm.2007.12.023
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