Csaba Székely

RELATIONSHIPS AMONG MEMBERS OF THE GENUS MYXOBOLUS (MYXOZOA: BILVALVIDAE) BASED ON SMALL SUBUNIT RIBOSOMAL DNA SEQUENCES

Sequences representing approximately 1,700 base pairs of the 18S rRNA gene from 10 different species in the genus Myxobolus were found to group them into 3 clusters that showed little correlation with spore morphology and size or host specificity, criteria currently used for both higher and lower taxonomic placements in the Myxozoa. Of the phenotypic criteria examined, tissue tropism was most correlated with the rRNA groupings observed. Spores of similar size and shape (Myxobolus cerebralis vs. Myxobolus squamalis) were distantly related in some instances, whereas spores with divergent morphology and size were sometimes found to be closely related (M. cerebralis and Myxobolus insidiosus). These initial investigations into the phylogenetic relationships of putative members of the genus Myxobolus clearly indicate the potential limitations of groupings based on size and morphological properties of the spores and host species infected. We propose that 18S rRNA gene sequences, combined with information on tissue tropism, host species infected, and developmental cycles in the fish and alternate host (when and if known) be given greater consideration in taxonomic placements of myxosporeans.

First detection and analysis of a fish circovirus

Circoviruses are present worldwide in birds and pigs but their occurrence in fish has not yet been reported. Recently, increased mortality was observed in barbel fry (Barbus barbus) in Hungary. This paper reports the detection of previously unknown circular viral DNA genomes in barbels by the use of a circovirus-specific wide-range nested PCR. The analysis of two complete genomes (Barbel circovirus, BaCV1 and BaCV2) indicated that they belonged into a new genetic group within the family Circoviridae, distinct from known circoviruses and circovirus-like genomes. Their genome size was 1957 bases and contained two major ORFs similar to the capsid and replication-associated protein genes of circoviruses. A connection between the presence of the virus and clinical manifestations of the infection could not be proved.

Differentiation of Myxobolus spp. (Myxozoa: Myxobolidae) infecting roach (Rutilus rutilus) in Hungary

During a survey on fishes from Hungarian freshwaters, the occurrence of eight Myxobolus species was registered in roach (Rutilus rutilus L.). Most species had a specific location within the fish host. Of the known species infecting roach, the intramuscularly developing species Myxobolus pseudodispar was found to be the most common. Besides Myxobolus intimus and Myxobolus diversicapsularis infecting the capillary network of gill lamellae, Myxobolus feisti infecting the cartilaginous gill rays were observed. Of the “muelleri-type” Myxobolus spp, Myxobolus rutili and Myxobolus sommervillae sp. n. formed elongated plasmodia in the gill filaments outside and inside the arteria efferens, respectively, while Myxobolus wootteni sp. n. developed cysts in fins. Plasmodia and spores of Myxobolus fundamentalis sp. n. were detected in the cartilaginous gill arch under the basis of gill filaments. Despite similarities of some species in spore morphology, 18S rDNA sequences and phylogenetic analyses showed clear differences among all species examined. The findings of the study demonstrate that morphologically similar spores could only be correctly identified by considering the location of plasmodia and the genetic characters of the myxozoan species.

Morphology, Molecular Data, and Development of Zschokkella mugilis (Myxosporea, Bivalvulida) in a Polychaete Alternate Host, Nereis diversicolor

Abstract The morphology of Zschokkella mugilis Sitjà-Bobadilla and Alvarez-Pellitero, 1993 (Myxosporea, Bivalvulida) in Nereis diversicolor O. F. Müller, 1776 is described for the first time. The molecular data show that the actinospore has 100% similarity to the myxospore of Z. mugilis. Fully mature actinospores are tri-radiate, the spore body has a small process, and the sporoplasm has 2 inner daughter cells. In the polychaete, the spores of the parasite develop in groups of 8 inside pansporocysts. The schizogony phase takes place in the intestinal epithelium, while gametogony and sporogony occur in the coelom of the polychaete. Observations indicate that mature spores are released only during the polychaete reproductive season. Infection was detected only in the winter and spring. In the Aveiro estuary (Portugal), the overall prevalence of infection of the polychaete was 0.5%.

Myxozoan pathogens of Malaysian fishes cultured in ponds and net-cages

We describe new myxosporean species from Malaysian fishes cultured in pond farms and net-cages. Myxobolus omari sp. nov. and M. leptobarbi sp. nov. were found in the muscles of Pangasianodon hypophthalmus and Leptobarbus hoevenii, respectively, while plasmodia and spores of Thelohanellus zahrahae sp. nov. and Henneguya daoudi sp. nov. were detected in the gills of Barbonymus gonionotus and Trichogaster trichopterus, respectively. Plasmodia and spores found in these fishes differed from the known myxosporean species in respect of their morphology, tissue tropism and 18S rDNA structure. No major pathological changes were found, but in the future these species might pose a potential threat to more intensified fish culture.

Some remarks on the occurrence, host-specificity and validity of Myxobolus rotundus Nemeczek, 1911 (Myxozoa: Myxosporea)

Myxobolus rotundus Nemeczek, 1911 is a common and specific parasite of the common bream Abramis brama (L.). Small, round or ellipsoidal shaped plasmodia of this species develop in the gill and exhibit strong histotropism to the secondary gill lamellae with plasmodial development in their capillary network. M. rotundus is frequently found in mixed infection with M. bramae Reuss, 1906, a parasite of the afferent arteries of gill filaments. The round spores of M. rotundus resemble several other Myxobolus spp., but can be distinguished from these by their small subunit ribosomal RNA gene sequence (GenBank accession no. EU710583), which also differs from the sequence for ‘M. rotundus’ from the skin of Chinese goldfish Carassius auratus auratus (L.), which we suggest has been misidentified. The SSU rRNA gene sequence of M. rotundus myxospores from bream corresponded to Triactinomyxon type 4 actinospores (AY495707) isolated from Tubifex tubifex (Müller) by Hallett et al. (2005), and we infer from this that these are alternate life stages.

Impact of the Swim-Bladder Parasite on the Health and Performance of European Eels

A growing number of diseases including infections by parasites are thought to play an important role in the drastic reduction of eel in European rivers and lakes. Until now, the occurrence of about forty parasite species has been reported for the European eel. Bykhovskaya-Pavlovskaya et al. (1962) listed 30 species in this fish. Since then, several new species were recorded and among them highly pathogenic ones. Several of these species arrived by Pacific eels (Anguilla japonica, A. australis) that were introduced into Europe for experimental purposes. Parasitic infections leading to severe symptoms and eventually death of the fish are caused primarily by the highly pathogenic species. The common occurrence of some helminths like Proteocephalus macrocephalus, Bothriocephalus claviceps, Acanthocephalus anguillae, Paraquimperia tenerrima has been known for a long time (Murai 1971; Moravec 1994). There were also data on the pathogenic effect of some well-known myxosporeans as Myxidium giardi, Hoferellus gilsoni (Copland 1983; Lom et al. 1986), but these parasites rarely caused any diseases. The first real pathogenic disease of parasitic origin in Europe was reported by Molnar (1983) who discovered Pseudodactylogyrus anguillae and P. bini in cultured eels. These monogeneans caused heavy losses in intensively cultured eel stocks (Buchmann et al. 1987; Buchmann 1993) but no losses were recorded in natural waters. From time to time reports appeared about severe infections caused by unknown or less known parasites affecting eels in natural water, but none of these parasites caused measurable losses in eel populations. Besides monogeneans, Moravec and Koie (1987) described Daniconema anguillae, a skrjabillanid nematode from the abdominal cavity of the European eel. Molnar and Moravec (1994) found a heavy infection by its larval stages in the fins of eels of Lake Balaton. Myxobolus portucalensis as

Myxozoan infections in fishes of the Tasik Kenyir Water Reservoir, Terengganu, Malaysia

During a survey on fishes of the Tasik Kenyir Reservoir, Malaysia, 5 new Myxobolus spp. and 2 known Henneguya spp. were found. The specific locations for 2 Myxobolus spp. were the hosts muscles, while 2 other Myxobolus spp. were found to develop in the hosts kidney and gills, respectively. Of the species developing intracellularly in muscle cells, M. terengganuensis sp. nov. was described from Osteochilus hasselti and M. tasikkenyirensis sp. nov. from Osteochilus vittatus. M. csabai sp. nov. and M. osteochili sp. nov. were isolated from the kidney of Osteochilus hasselti, while M. dykovae sp. nov. was found in the gill lamellae of Barbonymus schwanenfeldii. Henneguya shaharini and Henneguya hemibagri plasmodia were found on the gills of Oxyeleotris marmoratus and Hemibagrus nemurus, respectively. Description of the new and known species was based on morphological characterization of spores, histological findings on locations of plasmodia and DNA sequence data.

Complete life cycle of Myxobolus rotundus (Myxosporea: Myxobolidae), a gill myxozoan of common bream Abramis brama.

The life cycle of Myxobolus rotundus Nemeczek, 1911, a myxosporean parasite of the gills of common bream Abramis brama L., was studied under laboratory conditions. Mature Myxobolus spp. spores from plasmodia in the gills of wild bream were used to infect naïve oligochaete worms in a flow-through system of aquaria. Triactinomyxon-type actinospores were released from the oligochaetes 1 yr later and allowed to continually flow into a tank containing uninfected bream fry. The gills of the fry were checked for development of plasmodia in squash preparations 3 d postexposure, and then at weekly intervals for 8 wk. Tissue samples were fixed at each time point. Developing plasmodia were first observed 17 d post-exposure (Day 17). Mature spores were collected from plasmodia on Day 56 and were added to plastic dishes containing parasite-free Tubifex tubifex oligochaetes. Second-generation actinospores were released from these worms 8 mo post-exposure, and were morphologically identical to first-generation spores. Myxospores obtained from the bream fry were morphologically identical to those identified in wild bream as M. rotundus. Small subunit ribosomal RNA gene sequences obtained from first- and second-generation actinospores and the bream fry myxospores were 100% similar to M. rotundus spores from the original wild fish.

Novel circovirus in European catfish (Silurus glanis)

Circular single-stranded DNA viral genomes had been identified worldwide in different species and in environmental samples. Among them, viruses belonging to the genus Circovirus of the family Circoviridae are present in birds and pigs, and recently, they were detected in barbels. The present study reports the identification of a new circovirus in fish. PCR amplification and sequencing were used to identify the novel circular DNA virus in European catfish (Silurus glanis). Full genome characterization and phylogenetic analysis showed that the virus belonged to the family Circoviridae and that it was distantly related to the previously described barbel circovirus.