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Epidemiology of blood fluke, Cardicola forsteri, infection in southern bluefin tuna, Thunnus maccoyii

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Aiken, H (2009) Epidemiology of blood fluke, Cardicola forsteri, infection in southern bluefin tuna, Thunnus maccoyii. PhD thesis, University of Tasmania.

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Abstract

Cardicola forsteri Cribb, Daintith and Munday, 2000 (Digenea: Sanguinicolidae), is
a digenean parasite of southern bluefin tuna, Thunnus maccoyii (Castelnau) that can
cause disease in an aquaculture environment. The aim of this research was to gain
information about factors affecting the epidemiology of blood fluke, C. forsteri,
infection in farmed southern bluefin tuna in South Australia. Comparative analysis of
blood fluke of other bluefin tunas was undertaken to determine the host range of C.
forsteri. We found, through comparisons of ITS2 and partial 28S rDNA, C. forsteri
from multiple hosts and localities: southern bluefin tuna (T maccoyii) from South
Australia and northern bluefin tuna (T. thynnus) from two localities in the
Mediterranean (Spain and Croatia). Host migration seems likely to be responsible for
the widespread occurrence of C. forsteri, although it is also possible that C. forsteri
was translocated recently by the spread of infected intermediate hosts in international
shipping, either as biofouling attached to hulls, or in ballast waters. C. forsteri, was examined in cultured southern bluefin tuna, T. maccoyii, over a six
month growout season in Port Lincoln, South Australia. C. forsteri infections
declined after an initial peak two months after transfer from the wild and no effect
was observed on tuna condition index despite high intensities being recorded. It is
concluded that T maccoyii may be able to control blood fluke infection.
Stochastic models were developed to describe the infection pattern of Cardicola
forsteri in farmed southern bluefin tuna, T maccoyii. Observed field data on the
lengths of flukes over a growout season were used as the basis for the models. An
estimated time of infection was produced from the models and it was shown that
infections mostly occurred after introduction to sea-cages from the wild indicating
the presence of the intermediate host in the farming environment. Factors influencing blood fluke intensity, abundance and prevalence were
investigated by examining southern bluefin tuna collected from commercial harvests
over a three-year period. Blood fluke prevalence was observed to be approximately
60% in tuna over a growout season. Annual means of intensity were fixed around six
fluke per infected host and annual means of abundance between three and five fluke
per host. A universal factor in explaining variation in C. forsteri intensity, abundance
and prevalence was company. Year did not influence intensity or abundance
although a decrease in prevalence in 2005 was evident. Tuna harvested in winter had
a significantly greater abundance and prevalence of blood fluke than harvest in
autumn. Interestingly, the period of time that tuna are in captivity does not
significantly influence intensity of infection even though it does affect blood fluke
abundance and prevalence. Intensity or abundance did not affect the condition of
tuna. An adaptive immune response was investigated by developing an ELISA to detect
and quantify an antibody response against the blood fluke in southern bluefin tuna
serum. Antibody titres and seroprevalence increased during the growout period.
Parasitological and serological values from were compared from a cohort introduced
to the tuna farming zone in 2005 to a cohort introduced in 2006 to determine if prior
infection in the 2005 cohort elicited any protection against infection in 2006.
Although significant differences were not observed in intensities between cohorts it
was shown that the cohort that had a history of infection had significantly lower
abundances and prevalences of blood fluke infection than the naïve cohort
demonstrating the development of acquired resistance against C. forsteri.
The accuracies of a gross gill pathology test and a histopathological test for detecting
C. forsteri were evaluated. The sensitivity of gross gill pathology was the only high
estimate of diagnostic accuracy. Although the other estimates of diagnostic accuracy
were low, the high sensitivity of gross gill pathology suggests that this may be a
useful tool for future epidemiology studies.
A Bayesian approach to the estimation of prevalence was carried out using two
populations of tuna and two different diagnostic tests, an ELISA and parasitological
examination. The prevalence of infection was shown to be higher than previously
thought. ELISA was shown to have poor estimates of accuracy whereas a high
sensitivity for parasitological examination was demonstrated. Parasitological
examination is probably the best current method for detecting blood fluke infections.

Item Type: Thesis (PhD)
Copyright Holders: The Author
Copyright Information:

Copyright 2009 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:

Chapter 2 appears to be the equivalent of the peer reviewed version of the following article: Aiken, H. M., Bott, N. J., Mladineo, I., Montero, F. E., Nowak, B. F., Hayward, C. J., 2007, Molecular evidence for cosmopolitan distribution of platyhelminth parasites of tunas (Thunnus spp.), Fish and fisheries 8(3), 167-180, which has been published in final form at 10.1111/j.1467-2679.2007.00248.x. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.

Chapter 3 appears to be the equivalent of a post-print version of an article published as: Aiken. H. M., Hayward, C. J., Nowak, B. F., 2006, An epizootic and its decline of a blood fluke, Cardicola forsteri, in farmed southern bluefin tuna, Thunnus maccoyii, Aquaculture 254(1-4), 40-45.

Chapter 4 appears to be the equivalent of a post-print version of an article published as: Aiken. H. M., Hayward, C. J., Cameron, A., Nowak, B. F. 2009, Simulating blood fluke, Cardicola forsteri, infection in farmed southern bluefin tuna, Thunnus maccoyii using stochastic models, Aquaculture, 293(3-4), 204-210.

Chapter 6 appears to be the equivalent of a post-print version of an article published as: Aiken. H. M., Hayward, C. J., Crosbie, P., Watts, M., Nowak, B. F., 2008, Serological evidence of an antibody response in farmed southern bluefin tuna naturally infected with the blood fluke Cardicola forsteri. Fish and shellfish immunology 25(1-2), 66-75.

Date Deposited: 25 Nov 2014 00:57
Last Modified: 27 Sep 2016 23:30
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