Pronephric phagocytes from common carp Cyprinus carpjo L. infected with the gut-dwelling coccidian Goussia carpelli showed enhanced phagocytic activity during the period of merogonic and gamogonic development of the parasite from Days 10 to 15 post exposure (PE) to the parasite. The respiratory burst activity was enhanced during excretion of oocysts at Days 15 to 25 PE. The addltion of parasite lysates to assays resulted in an increase of phagocyte activity in cells from carp infected with gamogonic and sporogonic stages of the parasite and from carp which, at Day 35 PE, had recovered from a G, carpel11 infection. The addition of immune serum sampled from fish at Days 5. 10, 15. 20, and 25 PE had no clear stimulatory effect on phagocyte activity This suggests an activation of pronephric phagocytes on infection with the parasite. In carp with gamogonic and sporogonic parasite stages, activated macrophages show an increased cytotoxic reactivity, and activated macrophages rather than non-activated resident macrophages can be enhanced by opsonization with parasite molecules.

Goussia carpelli causes enteritic coccidiosis in juvenile carp. In nature it affects carp fry (1 to 2 mo old) and fish (3 to 4 mo old) which are subjected to environmental stress. Carp treated with corticosteroids or subjected to temperature stress in the laboratory produced higher numbers of oocysts during the primary infection. Resistance to reinfection via the fecal contamination route, however, was not reduced by the application of hydrocortisone, dexamethasone, or X-ray irradiation given both prior to and concurrently with reinfection. The administration of hydrocortisole or irradiation did not induce a relapse of a previous infection. Carp which had been imlnunosuppressed by hydrocortisole injection during a primary infection were also refractory to a secondary infection via fecal contamination. The results of these experiments suggest that the mechanisms which are responsible for the resistance of carp to secondary infections with Goussia carpelli were not affected by hydrocortisole, dexamethasone or X-ray treatment.