Cellular responses of goldfish, Carassius auratus (L.), to metacercariae of Ribeiroia marini (Faust & Hoffman, 1934)

Goldfish, Carassius auratus (L.), mobilized an acute, non-specific cellular inflammatory response against experimental infection with metacercariae of the digenean Ribeiroia marini in lateral line scale canals. The host cellular response to primary infection consisted of a rapid infiltration of neutrophilic granulocytes and macrophages that adhered to the metacercial cyst wall. Granulocytes released cytoplasmic granules, forming a necrotic zone around the cyst. Outside the necrotic zone, a fibrocytic zone and epithelioid cells formed a matrix upon which reactive leucocytes were attached in a progressive granulomatous encapsulation. Interdigitating epitheliod cells formed a syncytial border separating the necrotic zone from the outer fibrocytic region of the granuloma. Acute inflammatory response preceded the expulsion of most metacercariae from the scale canals. Few cysts remained encapsulated in epidermal tissue. Cellular response to challenge infections was more intense, but no major differences in leucocyte composition between primary and challenge infections suggested a continuum of inflammation and cell-mediated responses. In late primary and challenge infections, the predominant eosinophilic granular cell released cytoplasmic granules. We propose that the eosinophilic granular cell of the goldfish has an antiparasitic function, releasing bioactive granules that alter the environment of the scale canal and cause the expulsion of parasites.