The epizootiology and pathogenesis of thyroid hyperplasia in coho salmon (Oncorhynchus kisutch) in Lake Ontario.
The thyroid glands of coho salmon collected at different stages of their anadromous migration exhibited progressive and extensive hyperplasia and hypertrophy. The incidence of overt nodule formation rose from 5% in fish collected in August to 24% in fish collected in October. The histological picture of the goiters was similar to that found in thiourea-treated teleosts and thiouracil-treated mammals. There was a concomitant, significant decrease in serum thyroxine and triiodothyronine values between September and October (thyroxine, 1.0+/-0.3 mug/100 ml and 0.4 mug/100 ml in September and October, respectively; triiodothyronine, 400.3+/-51.6 ng/100 ml and 80.2 ng/100 ml in September and October, respectively) and marked hypertrophy and hyperplasia of thyrotrophs. These data indicate a progressive hypothyroid condition which, although it may be linked to iodide deficiency, may well be enhanced by other environmental factors. The evidence for involvement of other factors is discussed. (+info
Molecular differentiation of Renibacterium salmoninarum isolates from worldwide locations.
Renibacterium salmoninarum is a genospecies that is an obligate pathogen of salmonid fish and is capable of intracellular survival. Conventional typing systems have failed to differentiate isolates of R. salmoninarum. We used two methods to assess the extent of molecular variation which was present in isolates from different geographic locations. In one analysis we investigated possible polymorphisms in a specific region of the genome, the intergenic spacer (ITS) region between the 16S and 23S rRNA genes. In the other analysis we analyzed differences throughout the genome by using randomly amplified polymorphic DNA (RAPD). We amplified the spacer region of 74 isolates by using PCR and performed a DNA sequence analysis with 14 geographically distinct samples. The results showed that the 16S-23S ribosomal DNA spacer region of R. salmoninarum is highly conserved and suggested that only a single copy of the rRNA operon is present in this slowly growing pathogen. DNA sequencing of the spacer region showed that it was the same length in all 14 isolates examined, and the same nucleotide sequence, sequevar 1, was obtained for 11 of these isolates. Two other sequevars were found. No tRNA genes were found. We found that RAPD analysis allows reproducible differentiation between isolates of R. salmoninarum obtained from different hosts and different geographic regions. By using RAPD analysis it was possible to differentiate between isolates with identical ITS sequences. (+info
Inhibition of vibrio anguillarum by Pseudomonas fluorescens AH2, a possible probiotic treatment of fish.
To study the possible use of probiotics in fish farming, we evaluated the in vitro and in vivo antagonism of antibacterial strain Pseudomonas fluorescens strain AH2 against the fish-pathogenic bacterium Vibrio anguillarum. As iron is important in virulence and bacterial interactions, the effect of P. fluorescens AH2 was studied under iron-rich and iron-limited conditions. Sterile-filtered culture supernatants from iron-limited P. fluorescens AH2 inhibited the growth of V. anguillarum, whereas sterile-filtered supernatants from iron-replete cultures of P. fluorescens AH2 did not. P. fluorescens AH2 inhibited the growth of V. anguillarum during coculture, independently of the iron concentration, when the initial count of the antagonist was 100 to 1, 000 times greater that of the fish pathogen. These in vitro results were successfully repeated in vivo. A probiotic effect in vivo was tested by exposing rainbow trout (Oncorynchus mykiss Walbaum) to P. fluorescens AH2 at a density of 10(5) CFU/ml for 5 days before a challenge with V. anguillarum at 10(4) to 10(5) CFU/ml for 1 h. Some fish were also exposed to P. fluorescens AH2 at 10(7) CFU/ml during the 1-h infection. The combined probiotic treatment resulted in a 46% reduction of calculated accumulated mortality; accumulated mortality was 25% after 7 days at 12 degrees C in the probiotic-treated fish, whereas mortality was 47% in fish not treated with the probiont. (+info
Biodiversity of Lactococcus garvieae strains isolated from fish in Europe, Asia, and Australia.
Lactococcus garvieae (junior synonym, Enterococcus seriolicida) is a major pathogen of fish, producing fatal septicemia among fish species living in very diverse environments. The phenotypic traits of L. garvieae strains collected from three different continents (Asia, Europe, and Australia) indicated phenotypic heterogeneity. On the basis of the acidification of D-tagatose and sucrose, three biotypes were defined. DNA relatedness values and a specific PCR assay showed that all the biotypes belonged to the same genospecies, L. garvieae. All of the L. garvieae strains were serotyped as Lancefield group N. Ribotyping proved that one clone was found both in Japan, where it probably originated, and in Italy, where it was probably imported. PCR of environmental samples did not reveal the source of the contamination of the fish in Italy. Specific clones (ribotypes) were found in outbreaks in Spain and in Italy. The L. garvieae reference strain, isolated in the United Kingdom from a cow, belonged to a unique ribotype. L. garvieae is a rising zoonotic agent. The biotyping scheme, the ribotyping analysis, and the PCR assay described in this work allowed the proper identification of L. garvieae and the description of the origin and of the source of contamination of strains involved in outbreaks or in sporadic cases. (+info
Whirling disease: host specificity and interaction between the actinosporean stage of Myxobolus cerebralis and rainbow trout Oncorhynchus mykiss.
Scanning electron microscopic studies were conducted on rainbow trout Oncorhynchus mykiss in the first 60 min after their exposure to the triactinomyxon spores of Myxobolus cerebralis. The results demonstrated that as early as 1 min post exposure the whole process, from the attachment of the triactinomyxon spores to the complete penetration of their sporoplasm germs, had occurred. The triactinomyxon spores sought out the secretory openings of mucous cells of the epidermis, the respiratory epithelium and the buccal cavity of trout and used them as portals of entry. Exposure experiments of the triactinomyxon spores of M. cerebralis to non-salmonid fish, such as goldfish Carassius auratus, carp Cyprinus carpio, nose Chondrostoma nasus, medaka Oryzias latipes, guppy Poecilia reticulata and also the amphibian tadpole Rana pipiens as well as to rainbow trout fry indicated a specificity for salmonids. Attempts to activate the triactinomyxon spores by exposure to mucus prepared from cyprinid and salmonid fish showed no significant differences from those conducted in tap water. The results suggest that the simultaneous presence of both mechano- and chemotactic stimuli was required for finding the salmonid fish host. (+info
Rainbow trout leucocyte activity: influence on the ectoparasitic monogenean Gyrodactylus derjavini.
The ectoparasitic monogenean Gyrodactylus derjavini from rainbow trout Oncorhynchus mykiss was exposed in vitro to macrophages isolated as peritoneal exudate cells or as pronephros cells from the host. Cells colonized the parasite especially in the mannose-rich regions in the cephalic ducts where ciliated structures were abundant. Opsonization with fresh serum, in contrast to heat-inactivated serum, enhanced colonization also on other body parts. The adverse effect of the activated macrophages towards G. derjavini was associated with a heat-labile component released from these cells to the culture medium. Analysis of substances released from the cells showed reactivity for a number of enzymes, complement factor C3, interleukin (Il-1) and reactive oxygen metabolites. Chemotaxis assays with pronephric leucocytes showed chemoattractants in G. derjavini, and the respiratory burst level of macrophages was slightly elevated due to parasite exposure. It is suggested that skin leucocytes contribute to an increased level of complement factors in the trout skin during the host response, whereby a hostile microenvironment for the parasites is created. In addition, the IL-1 production could affect mucous cell secretion and hyperplasia and add to the antiparasitic action of the epithelium. Likewise, reactive oxygen metabolites and various enzymes are likely to be involved in the skin response. (+info
Pathogenicity of Ichthyophonus hoferi for laboratory-reared Pacific herring Clupea pallasi and its early appearance in wild Puget Sound herring.
Laboratory-reared pathogen-free Pacific herring were exposed to pure cultures of Ichthyophonus hoferi, and reproduced the disease seen in naturally infected fish--thus fulfilling Koch's Postulates. Pathogen-free herring used in this study were reared from artificially spawned eggs incubated in filtered, UV-sterilized seawater, eliminating the variables associated with multiple infections, which are common in wild herring. Wild free-ranging herring were captured monthly from June through October by dip net from 'herring balls' located in the northern Puget Sound. I. hoferi infections were identified in these fish soon after metamorphoses, about 4 mo post-hatch. The prevalence increased from 5 to 6% in 0-yr fish to 24% in 1-yr-old fish to 50 to 70% in fish over 2 yr old, with no associated increase in mortality. The route of natural transmission to wild herring was not determined, but carnivorous fish became infected and died when they were experimentally fed tissues infected with the organism. In vitro culture of tissues was the most sensitive method for identifying both clinical and subclinical infections. (+info
Natural mass infection by heterophyid metacercariae in aquacultured Japanese eel in Taiwan.
A natural mass infection of heterophyid metacercariae in aquacultured Japanese eel Anguilla japonica in Taiwan was observed. Of the 28,000 adult eels in 2 ponds, about 25,000 (90%) showed swollen, cloudy and white eyes. Although morbidity was about 90%, there was no mortality among the affected eels. Histopathological sections showed edema and hemorrhage of the eye. Numerous metacercariae were observed in the muscle tissues around the eyeball, the subcutaneous tissue and even in the cartilage. Of the 6 eels digested with artificial gastric juice, all were found to contain metacercariae in their muscle tissues. The average number of metacercariae recovered from the 6 eels was 1219, with a range of 50 to 3762. These metacercariae, when fed orally to immunodeficient (scid) mice, developed into adult worms which were identified as Procerovum cheni Hsu 1950. The naturally infected eels were transferred to a new pond without snails and their eye lesions were not apparent anymore after 2 wk. In a follow-up investigation, 19 of 20 apparently healthy eels in a nearby aquaculture farm were found to harbour metacercariae in their muscles. However, the number of the metacercariae ranged from 1 to 14, with an average of 4.21. This is the first report of heterophyid metacercariae causing mass morbidity in aquacultured eels. (+info