Experimental evidence on prevention of infection by the ectoparasitic protozoans Ichthyobodo salmonis and Trichodina truttae in juvenile chum salmon using ultraviolet disinfection of rearing water

In northern Japan, juvenile chum salmon Oncorhynchus keta (Walbaum) are released from hatcheries to enhance the fishery resource. Infections with ectoparasitic protozoans, particularly the flagellate Ichthyobodo salmonis and the ciliate Trichodina truttae, occasionally cause severe mortality among hatchery-reared juveniles. This study examined the susceptibility of the two parasites to wide-ranging UV irradiation (experiment 1) and then investigated whether UV disinfection of the rearing water using a commercial device was useful for preventing infections among juveniles in a small-scale rearing system over a 28-day period (experiment 2). In experiment 1, parasite mortality reached 100% with UV irradiation doses of ≥9.60 × 10⁵  μW s/cm² for I. salmonis and ≥8.40 × 10⁵  μW s/cm² for T. truttae. In experiment 2, disinfection of the rearing water at a UV irradiation dose of 2.2 × 10⁶  μW s/cm² succeeded in complete prevention of both parasites in the juvenile salmon. These results elucidate the minimum dose of UV irradiation for inactivation of I. salmonis and T. truttae, and demonstrate the usefulness of water disinfection using a commercial UV irradiation device to prevent infections by these parasites in hatchery-reared juvenile chum salmon.

Effects of dietary supplementation with oregano essential oil on prevention of the ectoparasitic protozoans Ichthyobodo salmonis and Trichodina truttae in juvenile chum salmon Oncorhynchus keta

The present study performed three experiments to establish a practical prevention strategy for the ectoparasitic flagellate Ichthyobodo salmonis and ciliate Trichodina truttae in hatchery-reared juvenile chum salmon Oncorhynchus keta using dietary supplementation with oregano essential oil. Experiment 1 showed that a diet supplemented for 3 weeks with 0.02% oregano essential oil significantly prevented infection with I. salmonis and T. truttae in juveniles reared in small tanks. Experiment 2, in outdoor hatchery ponds, demonstrated that the oregano treatment completely prevented I. salmonis infection for 52 days and T. truttae infection for 38 days. Oregano-treated juvenile mortality attributable to infection with these protozoans also decreased to 7.6% of control juvenile mortality, confirming the utility of this treatment in cultured O. keta. Physiological analyses of the oregano-treated juveniles elucidated the treatment's safety in relation to their metabolism, osmoregulation, natural immunity and olfactory responses and also detected carvacrol (a major component of oregano essential oil which shows antimicrobial activity) on the skin. In experiment 3, exposure of the two protozoans to oregano essential oil revealed a weak antiparasitic action on the body surface of the juvenile O. keta. The overall results demonstrate that dietary oregano supplementation is a practical prevention strategy for I. salmonis and T. truttae in hatchery-reared juvenile O. keta and suggest the possibility that its anti-parasitic action is attributable to a component of the oil that emerges onto the skin of the body of the fish.

Epizootiology of the ectoparasitic protozoans Ichthyobodo salmonis and Trichodina truttae on wild chum salmon Oncorhynchus keta

Infestations of the ectoparasitic flagellate Ichthyobodo salmonis and the ciliate Trichodina truttae have caused acute mortalities of hatchery-reared juvenile chum salmon Oncorhynchus keta in Hokkaido, northern Japan. This study examined the epizootiology of I. salmonis and T. truttae on wild chum salmon as a possible infection source of the 2 parasitic protozoans in hatcheries. Infestations by both ectoparasites were detected on freshwater-adapted adult and juvenile chum salmon in all 4 rivers examined. This is the first study of an anadromous Pacific salmonid to report infestation of I. salmonis and T. truttae in adults returning for spawning. Among the marine-inhabiting phase of chum salmon, infestation with I. salmonis, but not T. truttae, was observed on adults and juveniles. The 2 protozoans were experimentally transmitted at the same time from wild to hatchery-reared chum salmon juveniles, and caused a high rate of mortality in the hatchery fish. In freshwater, the proliferation rate of T. truttae was greater than that of I. salmonis. These observations show that the euryhaline ectoparasite I. salmonis can infest chum salmon throughout their life cycle, in both river and ocean habitats, whereas T. truttae is able to infest these salmonids only in freshwater. Furthermore, wild chum salmon were shown to be a potential infestation source for both T. truttae and I. salmonis in hatchery fish.

Differential rejection of salmon lice by pink and chum salmon: disease consequences and expression of proinflammatory genes

The consequences of high (735 copepodids fish-1) and low (243 copepodids fish-1) level exposures of size-matched juvenile pink and chum salmon to Lepeophtheirus salmonis copepodids were examined. At both levels of exposure the prevalence and abundance of L. salmonis was significantly higher on chum salmon. In addition, the weight of exposed chum salmon following the high exposure was significantly less than that of unexposed chum salmon. At both exposures, the haematocrit of exposed chum salmon was significantly less than that of unexposed chum. Neither weight nor haematocrit of pink salmon was affected by exposures at these levels. Despite the presence of microscopic inflammatory lesions associated with attachment of L. salmonis on the epithelium of gill and fin of both salmon species, there were no mortalities following either exposure. A transient cortisol response was observed in chum salmon 21 d after low exposure. An earlier and quantitatively higher expression of the proinflammatory genes interleukin-8 (IL-8), tumour necrosis factor alpha-1 (TNFalpha-1) and interleukin-1beta (IL-1beta) in fin and head kidney of pink salmon suggested a mechanism of more rapid louse rejection in this species. Together, these observations indicate a relatively enhanced innate resistance to L. salmonis in the juvenile pink salmon compared with the juvenile chum salmon.

Differences in host selection of actinospores of two myxosporeans, Myxobolus arcticus and Thelohanellus hovorkai

We investigated the host selection mechanism of actinospore stages of 2 myxosporeans, Myxobolus arcticus and Thelohanellus hovorkai, infecting masu salmon (Oncorhynchus masou) and common carp (Cyprinus carpio), respectively. Discharge of the polar filaments and sporoplasm release by M. arcticus actinospores occurred within the first 5 min of exposure to skin mucus of masu salmon. The actinospores also reacted to the mucus of nonsusceptible fish, i.e., sockeye salmon (Oncorhynchus nerka) and goldfish (Carassius auratus), although the reactivity was comparatively lower. After exposure of masu, and sockeye and chum salmon (Oncorhynchus keta) to M. arcticus actinospores, the penetration of sporoplasms was observed in the fins and gills of masu and sockeye salmon to a similar extent and to a lesser extent in chum salmon. Thelohanellus hovorkai actinospores exhibited a slow response of sporoplasm release to common carp mucus as well as penetration into the gills of common carp. Neither chemoresponse to mucus of nonsusceptible fish (goldfish and sockeye salmon) nor sporoplasm invasion in goldfish was observed for T. hovorkai actinospores. These results indicate notable differences in the host selection at the time of entry between M. arcticus and T. hovorkai; the former responds quickly to fish mucus with low host specificity, whereas the latter was highly host specific in a dilatory reaction.

Putative Diphyllobothrium nihonkaiense acquired from a Pacific salmon (Oncorhynchus keta) eaten in France; genomic identification and case report

We report here a likely case of Diphyllobothrium nihonkaiense contracted in France through the consumption of a Pacific salmon imported from Canada. The species diagnosis was made by molecular analysis of two mitochondrial genes (COI & ND3). This case is rather unusual in that D. nihonkaiense has never been reported along the Pacific coast of North America.

[Peter-the-Great Gulf fishes are a source of human invasion of the nematode Anisakis simplex (the family Anisakidae)]

The paper presents the results of the rate of infection with the nematode Anisakis simplex in Peter-the-Great gulf fishes that are a source of human invasion. Five of 19 fish types were found to be nematode-free, and the extensiveness and intensity of invasion were observed in Siberian salmons (Oncorhynchus keta) (61% and 2-700 worms, respectively), Alaska pollacks (Theragra chalcogramma) (75.7% and 1-31 worms), and brown rock trouts (Hexagrammos octogrammus) (100% and 1-60 worms). Autopsy showed that the distribution of larvae was ambiguous in different fish species. Data on the survival of Anisakis were also obtained under experimental conditions. The nematodes were ascertained to die at -8 degrees C at the moment of complete fish freezing. Anisakis may be alive in the unfrozen fish for more than 40 days. The death of worms in the salted fish occurs within 3-8 days only when 6-10/1000 concentrations of NaCl salts are achieved at the sites of worms.

[Infection of coastal catching fishes with a nematode of Anisakis simplex (Rud., 1809), L. in the Primor'e region]

The paper presents data on the infection of fishes from the Gulf of Peter the Great with an A. simplex nematode. A total of 308 fishes of 19 species were examined for parasites. Of them 5 species were found to be free from nematodes. The highest infection rates were noted in fishes of the species Oncorhynchus keta, Hexagrammos octogrammus, and Theragra chalcogramma. Differences in the infection rates of fishes of the gulf were explained by the specific features of their vital activity and by the saturation of the environment with an invasive agent. Based on the infection of secondary intermediate hosts with the nematoda, it may be suggested that there are the populations of fishes whose representatives accomplish their life cycle under the conditions of the internal waters of the gulf. The infection rate of these fishes with the parasite is low. The altered situation--an increase in the invasion rate--was associated with the seasonal migration of fishes of these species from the open marine spaces to the cost of the Gulf of Peter the Great. The opposite values of the infection rates of deep-sea (herrings) and bottom-dwelling (bases, greenlings) fishes make it possible to assume that bottom-dwelling invertebrates function as the first intermediate hosts in the life cycle of the nematode.