Parasite-Induced Changes in the Nervous System of the Shore Crab Hemigrapsus sanguineus

Trematodes are a class of parasitic flatworms with complex life cycles involving multiple hosts. They can influence the physiology and behavior of their intermediate hosts to improve the efficiency of transmission to definitive hosts, including by increasing host mortality. However, the mechanisms of their effect on the crustacean nervous system leading to neuronal dysregulation and its consequences have not been sufficiently elucidated. This study investigated the effects of infection by metacercariae of the trematode Cercaria fluviocinguli on the serotonin (5-HT) immunoreactivity, inflammation, and pathological processes in the nervous system of the shore crab Hemigrapsus sanguineus. The regions in the brain, the ventral nerve cords (VNC), and the nerves targeted by this parasite were examined by immunohistochemistry and confocal laser scanning microscopy methods. The cysts and the enclosed flukes caused compaction, compression, and distortion of many nerve fibers of the VNC and also impaired conduction in the mixed nerves of the thoracic ganglia controlling locomotion. The infection by C. fluviocinguli induced a decrease in 5-HT-like immunoreactivity in neurons and neuropils of the VNC, damage to nerve fibers and neurons of the VNC, inflammatory reactions, and apoptosis. In heavily infected crabs, the increased content of TUNEL-positive cells in the VNC was correlated with granuloma accumulation. Further investigations into host–parasite interaction mechanisms under controlled experimental conditions are needed to clarify the actual effects of encapsulated and non-encapsulated metacercariae on host behavior and their role in transmission to the next host.