Epidemiology of blood fluke, Cardicola forsteri, infection in southern bluefin tuna, Thunnus maccoyii
Creators: name: Aiken, H
Publisher: name: University Of Tasmania
Year: 2023

description: 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 na‚àövòve 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.