The biological and immunological aspect of the host-parasite relationship of goldfish, Carassius auratus, infected with Trypanosoma danilewskyi

Trypanosoma danilewskyi is a parasitic protozoan that lives in tissues of fish from Europe and Asia. In aquaculture industry, the prevalence of T. danilewskyi infection has approached 100% in some populations. Because little is known about the biological association between the host and the parasite, it is difficult to establish appropriate control measures.; The objective of this research was to: (1) develop reliable in vitro procedures for cultivation of trypanosomes; and (2) examine components of the host-parasite relationship that influence the susceptibility of goldfish, Carassius auratus.; Serum-related factors in fish blood were required for long-term cultivation of T. danilewskyi, because medium containing mammalian serum (10% bovine or horse) or no serum failed to support growth. Bloodstream-forms were the predominant stage in cultures containing goldfish serum. Medium supplemented with soluble cell products, collected from macrophages isolated from infected or uninfected fish, supported parasite growth better (P < 0.05) than control medium. Similar experiments with macrophage (mammalian) or fibroblast cells (fish or mammalian) revealed that growth-enhancing activity was not specific to species or cell type.; As previously reported, fish acquired resistance after alleviation of the acute primary infection. Further, passive transfer of immune serum to naive fish conferred protection against trypanosomes. These corroborative results led to an in vitro experiment that determined that blood leukocytes, derived from convalescent fish, were responsive to stimulation with trypanosorne extract. Subsequently, I studied active immunization of goldfish with trypanosome extracts (whole dead, lysate, water-solubilized molecules, detergent-solubilized molecules, and excretory-secretory products) via different vehicles and routes. The main conclusions drawn from these experiments were: (1) intraperitoneal administration of excretory-secretory products in Freund's complete adjuvant conferred protection (P < 0.05); (2) administration of other extracts incurred insignificant levels of protection or enhanced the susceptibility to the parasite; (3) the role of parasite-specific antibodies in eliminating trypanosomes seems unlikely, as there was no relationship between protection and anti-parasite antibody; (4) administration route (intravenous, intramuscular, or intraperitoneal) of immunization did not influence the outcome of the infection; and (5) of the five parameters examined, the prevalence of infection and parasite abundance were the best indicators of protection.