Trichodina truttae Mueller, 1937 (Ciliophora: Peritrichida) on Juvenile Chum Salmon (Oncorhynchus keta): Pathogenicity and Host-Parasite Interactions.

Abstract

A transmission experiment was carried out to evaluate the effects of Trichodina truttae infection on the growth and survival of juvenile chum salmon (Oncorhynchus keta) and the host's defense responses. The intensity of infection rose sharply to reach a peak of an average of 5, 730 (range 1, 770-12, 600) parasites per fish at week 3 post-infection, and then declined markedly to near zero by the end of the experiment (week 6). The parasite was site-specific to the skin of host fish and induced epidermal hyperplasia. Heavy infections caused intensive flashing among infected fish, which was accompanied by mass host mortalities with a cumulative loss of 56%. The growth and seawater adaptation of juveniles were, however, not significantly affected. In the course of infections, drastic changes occurred in the degree and histochemistry of the skin epidermal mucus secretion. The density of mucous cells containing acid mucopolysaccharides (alcian blue-positive, PAS-negative) decreased with increasing parasite intensity, while the parasite infections induced a marked increase of PAS-positive mucous cells, in turn, which may have caused a distinct reduction in parasite intensity. These results suggest that the increased production of PAS-positive mucous cells is an effective defense mechanism against trichodinid infections.