The effect of different treatment strategies on Cardicola spp. (Trematoda: Aporocotylidae) infection in ranched Southern Bluefin Tuna (Thunnus maccoyii) from Port Lincoln, South Australia

Changes in the Splenic Melanomacrophage Centre Surface Area in Southern Bluefin Tuna (Thunnus maccoyii) Are Associated with Blood Fluke Infections

“…DNA presence corresponded with the presence of eggs in histological sections of heart and gills. C. forsteri was the most common species which is consistent with results since 2013 [ 9 , 13 ] but in contrast with reports from 2008 to 2012 [ 14 ] suggesting that there has been a change in the blood fluke dynamics infection, possibly caused by praziquantel treatment used by the industry since 2013 [ 9 ]. The increase in C. forsteri in gills at harvest as measured by qPCR indicates an increase in the number of eggs in the gills.…”

“…This is due to reinfection after treatment and the eggs being resistant to praziquantel treatment [ 15 ] and in contrast to qPCR results for heart where there was no significant difference between transfer and harvest for each cohort. Overall, the severity of infection was within the range reported previously from ranched SBT [ 9 , 13 ].…”

“…A recent study showed a significant positive correlation between MMC size (determined as percentage surface area) and Cardicola spp. in the spleen for gill egg counts, and in the kidney for C. forsteri DNA from SBT heart [ 9 ]. However, that study focused on harvest fish only (sampling one timepoint during production cycle) and changes in MMCs during ranching season or variability between different SBT cohorts have not been determined in that study [ 9 ].…”

The Southern Bluefin Tuna Mucosal Microbiome Is Influenced by Husbandry Method, Net Pen Location, and Anti-parasite Treatment

“…Fish were measured for total length (mm) and mass (grams) and Fulton’s condition factor “K-factor” was calculated ( Fulton, 1904 ; Froese, 2006 ). Fish ( n = 45) from four of the six pens were treated with the antiparasitic drug, anthelmintic drug praziquantel (PZQ) orally (injected into baitfish) at a dose of 30 mg/kg bodyweight 4–5 weeks after transfer to pontoons in a single treatment over 2 consecutive days by the prescribing veterinarian (APVMA Permit Number-Per 85738) ( Power et al, 2019 ) while fish in the other two pens did not get treated with PZQ ( n = 20). Presence of adult flukes in heart and egg densities within the gill were measured per fish as previously described ( Aiken et al, 2006 ; Power et al, 2019 ).…”

“…PZQ treatment has previously been shown to reduce Cardicola spp. parasite egg abundances in the gills ( Power et al, 2019 ). To better establish the impact of antiparasitic treatment on microbiome composition, we analyzed mucosal microbiome composition as a function of parasite infection and PZQ treatment.…”