Comparative study of infection with Tetrahymena of different ornamental fish species.

Tetrahymena is a ciliated protozoan that can infect a wide range of fish species, although it is most commonly reported in guppies (Poecilia reticulata). The aim of this study was to compare the susceptibility to infection with Tetrahymena of five different ornamental fish species from two different super orders. The species examined were platy (Xiphophorus), molly (Poecilia sphenops) and angelfish (Pterophyllum scalare) of the Acanthopterygii super order (which also includes guppies) and goldfish (Carassius auratus auratus) and koi carp (Cyprinus carpio) of the Ostariophysi super order. These two super orders are phylogenetically distant from each other. Infection with Tetrahymena resulted in parasite invasion of internal organs, skin and muscle in all fish species. A relatively strong inflammatory response was observed in infected goldfish and koi, with negligible response in fish species of the Acanthopterygii super order. Guppies were the most susceptible to Tetrahymena infection, exhibiting a mortality rate of 87% and 100% in two separate experiments. A high mortality rate was also observed in platy (77%), while that of molly and angelfish was significantly lower (23% and 33%, respectively). Goldfish and koi carp were less susceptible to infection compared with guppies (24% and 59% mortality, respectively). Immunization studies revealed that the Tetrahymena are immunogenic, since infection of koi carp increased their Tetrahymena immobilization response by approximately three-fold at 3 weeks post infection, while immunization with Tetrahymena plus adjuvant increased their immobilization response by approximately 30-fold.

Treatment development for systemic Tetrahymena sp. infection in guppies, Poecilia reticulata Peters.

Antibacterial and antiparasitic agents and a cysteine protease inhibitor (E-64) were tested against Tetrahymena infection, a serious problem in guppy production worldwide. Chemicals were tested in vitro by a colorimetric assay for Tetrahymena survival. The most effective were niclosamide, albendazole and chloroquine, with 23%, 35% and 60% survival, respectively, following 2-h exposure to 100 ppm. Longer incubation periods resulted in greater reductions in survival. Niclosamide was further studied in vivo at different dosages, administered orally to Tetrahymena-infected guppies. Mortality rates were significantly lower in all treatment groups; in trial I, 30% and 33% mortality in 5 and 40 mg kg(-1) niclosamide-fed fish vs. 59% mortality in controls; in trial II, 35%, 13% and 10% in 50, 100 and 200 mg kg(-1) niclosamide-fed fish vs. 64% in controls. The effect of the cysteine protease inhibitor E64 was tested in tissue culture, by measuring histolytic activity of the parasite (Tet-NI) on a guppy-fin cell line, based on cell depletion. Tet-NI feeding activity was significantly reduced following pretreatment with E-64 relative to non-treated Tet-NI. E-64-pretreated Tet-NI was injected i.p. into guppies: recorded mortality rates were significantly lower (35%) than that in non-treated Tet-NI (60%), suggesting inhibition of the parasite's cysteine protease as a possible therapeutic approach.

Protective immunization against Tetrahymena sp. infection in guppies (Poecilia reticulata).

Systemic tetrahymenosis constitutes a serious problem in guppy (Poecilia reticulata) production worldwide and no therapeutic solution is available for this disease. Three immunization trials were conducted, testing the effectiveness of different Tetrahymena preparations applied by intraperitoneal injection (IP) with or without Freund's complete adjuvant (FCA) and with or without booster dose. In trial 1, immunization with the pathogenic Tet-NI 6 lysate and live attenuated Tet-NI 1 did not provide significant protection from infection, although infection rates were significantly lower in the Tet-NI 6-immunized group than in controls. In trial 2, mortality in Tet-NI 6 + FCA-immunized fish was 10%, significantly lower than in all other treatment groups, including Tet-NI 6 lysate, live attenuated Tet-NI 1 and controls (77, 67 and 73%, respectively). In trial 3, the lowest mortality rates were obtained in the Tet-NI 6 + FCA + booster-immunized group (15%). These levels were lower but not significantly different from the non-boostered Tet-NI 6-immunized group (28%) and the groups immunized with Tet-NI 1, with and without booster (32 and 34%, respectively). Mortality in these four groups was significantly lower than in controls, including adjuvant- and PBS-injected groups (72 and 81%, respectively). Body homogenates of immunized fish immobilized Tetrahymena in-vitro, as compared to no or very little immobilization in controls. Lysozyme levels in the Tet-NI 6 + FCA + booster group were significantly higher than in all other treatments in trial 2 and controls in trial 3. There was no significant difference in anti-protease activity among the differently immunized fish. We conclude that immunization with Tetrahymena lysates in FCA confers a high degree of protection from infection, suggesting this preparation as a basis for vaccine development.