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

Barbara F Nowak,Andrew Bridle,Daryl Evans,Claire Webber,Roberto Bermudez,Mai Dang,Lukas Neumann

doi:10.3390/pathogens10010079

Barbara F Nowak, Andrew Bridle + Show 5 more

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https://doi.org/10.3390/pathogens10010079

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Abstract

Melanomacrophage centres (MMCs) are aggregates of macrophages accumulating various pigments. They have been proposed as an indicator of fish immune response. Blood flukes are common parasites in farmed fish. Two cohorts of wild Southern Bluefin Tuna (Thunnus maccoyi) were examined at transfer, before treatment against blood flukes (pre-treatment) and at harvest. MMCs were assessed in histological sections using image analysis, while Cardicola forsteri and Cardicola orientalis infection severity was determined using qPCR, count of adult flukes in heart flushes and count of eggs in gill filaments. Fish from both cohorts showed the same pattern in the changes in the surface area of MMCs. The surface area of splenic MMCs increased over the ranching duration and was positively correlated to the PCR determined copy numbers of Cardicola forsteri ITS2 rDNA in the gills of those fish. However, the infection with blood fluke was more variable, both between cohorts and individuals within the same cohort. Eggs of blood fluke were detected in renal MMCs using histology. Cardicola forsteri had a higher prevalence than Cardicola orientalis. This study contributes to our understanding of blood fluke infections in Southern Bluefin Tuna and their interactions with MMCs.

Highlights

Prevalence of DNA positive samples was similar at all sampling times; the copy number of C. forsteri ITS2 rDNA, measuring the parasite load in an individual fish, was greatest at harvest in both heart and gills, sometimes resulting in a statistically significant difference between pre-treatment and harvest and/or between the two cohorts (Table 1)
This is consistent with observations of positive correlation between splenic and renal Melanomacrophage centres (MMCs) surface area and blood fluke egg count in the gills observed in ranched Southern Bluefin Tuna (SBT) at harvest in another season [8]
Splenic MMC size was within the range previously reported for ranched SBT [7,8]

Summary

Introduction

Melanomacrophage centres (MMCs) are groups or clusters of pigmented macrophages present in many species of poikilotherms [1]. They can be found in a range of fish organs including kidney, spleen and liver (for review see Reference [2]). Their roles include destruction and recycling of exogenous and endogenous materials, storage of iron following erythrophagocytosis, retention of resistant pathogens and antigen processing during immune response [1,3]. MMCs have been proposed as histological indicators of immune function [2]

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