Trypanosoma danilewskyi in Carassius auratus: the nature of protective immunity in recovered goldfish.

Trypanosoma danilewskyi is pathogenic to goldfish, Carassius auratus; the number that survived an infection was related to the size of inoculum, i.e., the mortality was low when the inoculum was small. Most fish that had prior exposure(s) to the parasite survived subsequent homologous challenge(s) and were protected for at least 89 days. The immunity was nonsterile as low numbers of parasites were detected in the blood of some recovered fish. Intraperitoneal injection of corticosteroid in recovered fish significantly increased the number of trypanosomes in peripheral blood of some fish. Although most recovered fish were protected from challenge, a few died from high parasitemia after challenge.