Gastrointestinal myxosporean parasites from the genus Enteromyxum are known to cause severe disease, resulting in high mortalities in numerous species of cultured marine fishes globally.Originally described as Myxidium spp., they were transferred to a new genus, Enteromyxum, to emphasize their novel characteristics.Their retention in the family Myxidiidae at the time was warranted, but more comprehensive phylogenetic analyses have since demonstrated the need for a new family for these parasites.We discovered a novel Enteromyxum in wild fish from Malaysia and herein describe the fourth species in the genus and erect a new family, the Enteromyxidae n. fam., to accommodate them.Enteromyxum caesio n. sp. is described infecting the tissues of the stomach in the redbelly yellowtail fusilier, Caesio cuning, from Malaysia.The new species is distinct from all others in the genus, as the myxospores although morphologically similar, are significantly smaller in size.Furthermore, small subunit ribosomal DNA sequence data reveal that E. caesio is <84% similar to others in the genus, but collectively they form a robust and discrete clade, the Enteromyxidae n. fam., which is placed as a sister taxon to other histozoic marine myxosporeans.In addition, we describe, using transmission electron microscopy, the epicellular stages of Enteromyxum fugu and show a scanning electron micrograph of a mature myxospore of E. caesio detailing the otherwise indistinct sutural line, features of the polar capsules and spore valve ridges.The Enteromyxidae n. fam. is a commercially important group of parasites infecting the gastrointestinal tract of marine fishes and the histozoic species can cause the disease enteromyxosis in intensive finfish aquaculture facilities.Epicellular and sloughed histozoic stages are responsible for fish-to-fish transmission in net pen aquaculture systems but actinospores from an annelid host are thought to be necessary for transmission to fish in the wild.
Ninety-five eels from one marine and three freshwater localities in Iceland were examined for parasites.Twenty species were found, 12 from marine habitat, 12 from freshwater and 4 species were found in both habitats.These are: Eimeria anguillae, Chilodonella hexasticha, Trichodina fultoni, T. jadranica, Myxidium giardi, Myxobolus kotlani, two Zschokkella spp., Derogenes varicus, Deropristis inflata, Diplostomum sp., Plagioporus angulatus, Podocotyle atomon, Anisakis simplex (larva), Eustrongylides sp.(larva), Hysterothylacium aduncum (larva), Raphidascaris acus (larval and adult stages), Bothriocephalus claviceps, Proteocephalus macrocephalus, and a pseudophyllidean larva.Thirteen of these species are new parasite records from Icelandic waters.The component community of marine eels was characterized by low diversity and a high dominance of a single species.Overall, seven species of helminths were observed, up to five different species occurring in an individual fish.The component community of the freshwater eels was species-poor with low diversity and relatively high dominance of single species.A between-sites difference in the freshwater eels was considerable; only Diplostomum sp. was found at more then one sampling site.Similar to previous studies, there is a total replacement of freshwater macroparasite species by marine ones in saline waters.But unlike research abroad in which species richness decreases with higher salinity, the marine eels in Iceland have considerably higher richness than the freshwater ones.The parasite communities of freshwater eels in Iceland are, in general species-poorer, less diverse and having higher Berger Parker (BP) dominance than other eel communities in Europe.Marine eels have on the other hand comparable species richness, are less diverse and with a high BP dominance.
Commercial fisheries of lumpfish Cyclopterus lumpus have been carried out in Iceland for centuries. Traditionally the most valuable part is the eggs which are harvested for use as a caviar substitute. Previously reported parasitic infections from lumpfish include an undescribed intranuclear microsporidian associated with abnormal kidneys and mortalities in captive lumpfish in Canada. During Icelandic lumpfish fisheries in spring 2011, extensive enlargements to the kidneys were observed in some fish during processing. The aim of this study was to identify the pathogen responsible for these abnormalities. Lumpfish from the Icelandic coast were examined for the causative agent of kidney enlargement. Fish were dissected and used in histological and molecular studies. Lumpfish, with various grades of clinical signs, were observed at 12 of the 43 sites sampled around Iceland. From a total of 77 fish examined, 18 had clear clinical signs, the most prominent of which was an extensive enlargement and pallor of the kidneys. The histopathology of the most severely affected fish consisted of extensive degeneration and necrosis of kidney tubules and vacuolar degeneration of the haematopoietic tissue. Intranuclear microsporidians were detected in all organs examined in fish with prominent clinical signs and most organs of apparently healthy fish using the new PCR and histological examination. One or multiple uniformly oval shaped spores measuring 3.12 ± 0.15 × 1.30 ± 0.12 μm were observed in the nucleus of affected lymphocytes and lymphocyte precursor cells. DNA sequencing provided a ribosomal DNA sequence that was strongly supported in phylogenetic analyses in a clade containing other microsporidian parasites from the Enterocytozoonidae, showing highest similarity to the intranuclear microsporidian Nucleospora salmonis. Intranuclear microsporidian infections are common in wild caught lumpfish from around the Icelandic coast. Infections can cause severe clinical signs and extensive histopathological changes, but are also present, at lower levels, in fish that do not show clinical signs. Some common features exist with the intranuclear microsporidian previously reported from captive Canadian lumpfish, but DNA sequence data is required from Canadian fish to confirm conspecificity. Based on phylogenetic analysis and the intranuclear location of the parasite, the name Nucleospora cyclopteri n. sp. is proposed.
Tetracapsuloides bryosalmonae is a myxozoan parasite and the causative agent of proliferative kidney disease (PKD), a serious, temperature-dependent and emerging disease affecting salmonid fish. It was first identified in Iceland in 2008, from Arctic charr inhabiting a shallow lowland lake. The aim of this study was to investigate the distribution and prevalence of macroscopic and subclinical T. bryosalmonae infections in Icelandic salmonids and compare different time periods, in context with depths, volumes, altitudes and areas of the lakes and fish age. Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) from 34 lakes, sampled between 1994-1998 and 2009-2017, were examined for macroscopic signs of PKD (n = 2,151) and the presence of T. bryosalmonae infections (n = 1,424). In the earlier period, 43% of lakes (10/23) harboured T. bryosalmonae -infected fish. The mean prevalence in those lakes was 62.1%, being most common in shallow lowland lakes whilst deeper lakes at high altitudes were all free from infection. Only a single fish from one lake showed macroscopic signs of PKD, a shallow lowland lake in southwestern Iceland. In the latter period, T. bryosalmonae was found in 16/18 lakes studied (89%), with a mean prevalence of 78-79% (excluding T.b. free lakes), being most common in the smaller, shallower lakes at lower alttudes. Macroscopic signs of PKD were observed in 11 of 18 of the lakes studied (61%) with prevalences up to 67%, most common in younger fish inhabiting small shallow lowland lakes. The results indicate that the distribution of T. bryosalmonae and the presence of PKD in Iceland have increased over the last few decades. The disease was almost non-existent in the 1990s but has become very common during the last decade or two. With further water temperature increases, as predicted by climate models, PKD is likely to increasingly affect wild salmonid populations in Iceland.
A novel viral haemorrhagic septicaemia virus (VHSV) of genotype IV was isolated from wild lumpfish (Cyclopterus lumpus), brought to a land-based farm in Iceland, to serve as broodfish. Two groups of lumpfish juveniles, kept in tanks in the same facility, got infected. The virus isolated was identified as VHSV by ELISA and real-time RT-PCR. Phylogenetic analysis, based on the glycoprotein (G) gene sequences, may indicate a novel subgroup of VHSV genotype IV. In controlled laboratory exposure studies with this new isolate, there was 3% survival in the I.P. injection challenged group while there was 90% survival in the immersion group. VHSV was not re-isolated from fish challenged by immersion. In a cohabitation trial, lumpfish infected I.P. (shedders) were placed in tanks with naïve lumpfish as well as naïve Atlantic salmon (Salmo salar L.). 10% of the lumpfish shedders and 43%-50% of the cohabiting lumpfish survived after 4 weeks. 80%-92% of the Atlantic salmon survived, but no viral RNA was detected by real-time RT-PCR nor VHSV was isolated from Atlantic salmon. This is the first isolation of a notifiable virus in Iceland and the first report of VHSV of genotype IV in European waters.
Abstract This paper reports the adult stage of Prosorhynchoides borealis (Digenea) from Lophius piscatorius in Icelandic waters and infections with the larval stages (sporocysts and cercariae) found for the first time in the bivalve Abra prismatica (Semelidae). The previously known first intermediate host was Abra alba (Semelidae). Ribosomal DNA sequencing studies on all three life stages of the parasite (cercariae, metacercariae, adults) were performed to confirm their identites. Morphometric measurements confirmed that the adult worms belong to the newly described species P. borealis. Prosorhynchoides borealis sporocysts filled with cercariae were found in 16% of A. prismatica bivalves sampled at depths between 34 and 93 m off South Iceland. Prevalence ranged from 0 to 44% between different localities. The parasite was found only in the larger bivalves. Extensive sporocyst infection in the haemocoel of the foot caused mechanical muscle damage with subsequent degeneration and necrosis. Other tissues, including the digestive gland, nephridia, gills and intestine, were less heavily infected. Only focal necrosis was observed in the digestive gland, nephridia and gills, and local atrophy in the intestine. Cercariae were also observed in the lumen of both the stomach and intestine. This is the first report of A. prismatica as an alternative first intermediate host for P. borealis . Ribosomal DNA sequence data reveals 100% homology in the data between cercariae, metacercariae and adult digeneans, supporting the morphological data suggesting that all stages belong to the same species.
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