Paratenic hosts of the swimbladder nematode <i>Anguillicola crassus</i>

The host specificity and population dynamics of Anguillicola crassus in a number of paratenic hosts were investigated. Various freshwater fish species were sampled monthly (March 1990 to March 1991) from the Kolenhaven (Albertcanal, Genk, Belgium) and examined for L3-larvae of Anguillicola crassus. Sixteen species were found to be infected: all the physoclist species examined (Gymnocephalus cernua, Lepomis gibbosus, Ictalurus nebulosus, Stizostedion lucioperca, Gasterosteus aculeatus, Oreochromis niloticus and Perca fluviatilis) and those physostome species of which a sufficient number could be examined to detect the infection (Gobio gobio, Leuciscus cephalus, Chondrostoma nasus, Leuciscus leuciscus, Alburnus alburnus, Leuciscus idus, Scardinius erytrophthalmus, Rutilus rutilus and Tinca tinca). There were large differences in prevalence among the fish species examined but generally the prevalence was higher in physoclist fishes and was highest in G. cernua (96%). In 4 species there was a significant positive correlation between the fish length and parasite abundance. The percentage of grown larvae varied among fish species, being lowest in G. cernua and highest in P. fluviatilis. No clear seasonal incidence cycle was observed

Population biology of <i>Anguillicola crassus</i> in the final host <i>Anguilla anguilla</i>

Eels were sampled twice a month (March 1990 though March 1991) at the Kolenhaven (Albertcanal, Genk, Belgium) and examined for Anguillicola crassus nematodes in the swimbladder. Averaged over all samples, 90.2% of the 345 eels were infected with a mean intensity (averaged for all stages) of 17 nematodes. Thirty-one percent of all nematodes, which were adult worms or their remnants, were present in the swimbladder lumen, while the other 69% which consisted of the third or fourth larval stage and preadults were found in the swimbladder wall. No clear seasonal fluctuations in parasite prevalence or mean intensity were observed. Both L3-larvae and mature adult worms were present throughout the year. Eggs or L2-larvae were observed in 41% of the infected eels. This presence showed no seasonal pattern, though it was positively correlated with eel length: massive concentrations of L2-larvae were only found in eels bigger than 40 cm. A positive but weak correlation between eel length and number of adult nematodes in the swimbladder was significant, while no correlation was found between eel length and total parasite burden

Efficacy of some anthelmintics against the swimbladder nematode <i>Anguillicola crassus</i> of eel <i>Anguilla anguilla</i> under saltwater conditions

Six anthelmintic drugs, Ivermectin, Closantel, Safewormer, Masoten, Oxfendazole and L-Levamisol·HCL were screened for their efficacy against Anguillicola crassus, a swimbladder nematode of European eel Anguilla anguilla. All experiments were carried out under saltwater conditions. Of the 6 drugs, L-Levamisole·HCL used in a bath treatment gave the most promising results. Evaluation of the curative dose of L-Levamisole·HCL showed that a long-term treatment (24h). Dosages lower than 20 mg 1-1 showed a lower efficacy against Anguillicola crassus. However, even with a long-term, high-dosage treatment, some recovery of adult nematodes with sublethal damage and/or migration of viable juveniles into the swimbladder lumen could not be excluded. When examined 35d after treatment, the percentage of mobile adults had increased, while a decrease in percentage of dead worms was noticed compared to values obtained immediately after treatment

Mode of infection and spread of <i>Vibrio anguillarum</i> in turbot <i>Scophthalmus maximus</i> larvae after oral challenge through life feed

The infection route of the marine fish pathogen Vibrio anguillarum was studied after oral challenge of the juvenile turbot Scophthalmus maximus L. through a life feed. Artemia nauplii were incubated in a suspension of V. anguillarum cells, and subsequently fed twice to the fish. All challenged fish died within 4d after the first challenge, while no mortality occurred in the non-challenged controls. The results of an immunohistochemical examination of the sectioned fish samples clearly demonstrated that V. anguillarum cells were ingested by the Artemia and that the latter were ingested by the fish. Bacteria were released from the Artemia mainly in the anterior part of the intestine. Most challenged fish started to show disease signs 24h after the second challenge and died 2d later. A histopathological analysis of moribund fish showed the development of septicaemia. Moreover, the sequential sampling, allowed the reconstruction of the infection route after oral challenge. Our results show that V. anguillarum was transported through the intestinal epithelium by endocytosis, after which the bacterium was released in the lamina propria. From there the bacterium was transported by the blood to the different organs, eventually leading the septicaemia and mortality

Estimating turnover rates of d¹³C and d¹⁵N in muscle, heart and liver tissue of juvenile sand gobies (<i>Pomatoschistus minutus</i>)

Large numbers of marine fish typically enter and remain within estuaries during their juvenile life stage. Stable isotopes of carbon and nitrogen can serve to trace these individual movements due to the food web differences among marine and estuarine habitats. Here, we present the background for the utilization of d13C and d15N to analyze the migration dynamics of juvenile sand gobies (Pomatoschistus minutes), between the North Sea and the Scheldt estuary. The isotopic turnover of P. minutes, defined as the change in isotopic composition due to growth and metabolic tissue replacement, was examined for muscle, liver and heart tissue. A diet switch experiment simulating natural conditions for the Scheldt estuary was conducted during 90 days. Fish were fed a commercial pellet diet, which was isotopically different from the initial goby tissue d13C and d15N. Chopped polychaetes (Arenicola sp.) and mussels (Mytilus edulis) were used as control diets to test for effects other than diet. Fish were sacrificed for stable isotope analysis (CF-IRMS) on regular time intervals depending on the diet. Trophic fractionation was estimated for the different tissues and the effect of food deprivation on stable isotope composition was also evaluated. Heart and liver tissues had a faster isotopic turnover than muscle tissue as a result of their higher metabolism. However, growth was found to explain most of the variation in isotopic composition within a single tissue. There was no significant effect of 20 days of food deprivation on d13C and d15N for the tissues. The isotopic assay of muscle, liver and heart tissue within and among individuals will allow a better delineation of those individuals not in equilibrium with their isotopic environment. Therefore new arrivals in the estuary will be identified on a finer temporal resolution than feasible with muscle tissue alone

Phenotypic characterization of the marine pathogen <i>Photobacterium damselae</i> subsp. <i>piscicida</i>

The taxonomic position of Photobacterium damselae subsp. piscicida, the causative agent of fish pasteurellosis, is controversial as this organism has also been described as 'Pasteurella piscicida'. To clarify the taxonomic position of the pathogen, a total of 113 P. damselae subsp. piscicida strains and 20 P. damselae subsp. damselae strains, isolated from different geographical areas and from the main affected fish species, were analysed using 129 morphological and biochemical tests, including the commercial API 20E and API CH50 test systems. For comparison, the type strains of other Photobacterium species (i.e. Photobacterium leiognathi and Photobacterium angustum) were included in the analyses. The results were statistically analysed by unweighted pair group average clustering and the distance between the different clusters was expressed as the percentage disagreement. The analyses showed that, based on morphological and biochemical identification tests, P. damselae subsp. piscicida is related to other Photobacterium species. However, it is clearly distinguishable from P. damselae subsp. damselae and no phenotypic evidence was found to include P. damselae subsp. piscicida as a subspecies in the species P. damselae