Comparison of assessment methods used to diagnose Hematodinium sp. infections in Cancer pagurus

Endoparasitic dinoflagellates of the genus Hematodinium have recently gained attention as significant pathogens of the brown crab Cancer pagurus in Ireland. Patent infections, which are characterized by a hyperpigmented carapace and moribund condition, are limited to the discrete periods when macroscopic identification is possible. Three methods are assessed for diagnosing Hematodinium sp. infections in brown crab at the times when macroscopic identification is not always possible. Haemolymph smears, histological sections of gill, heart, midgut, hepatopancreas, muscle, and gonad, and a polymerase chain reaction (PCR) assay provided virtually equivalent accuracy in gauging infection prevalence, regardless of season. Sequences of PCR amplicons from the 18S ribosomal RNA gene confirmed the identity of the parasite as belonging to the genus Hematodinium. Infection intensity values (<1-87%) obtained from haemolymph smears underscored infection levels within tissues, 90% of which contained advanced levels of infection. Alterations to tissues of infected crabs included haemocytopoenia, oedema, which caused dilation of the haemal sinuses resulting in pressure necrosis to the connective tissues around the oocytes, myocardial bundles, and hepatopancreatic tubules. The claw muscle of infected animals contained the fewest parasites.