Phenoloxidase and QX disease resistance in Sydney rock oysters (Saccostrea glomerata)

QX is a fatal disease in Sydney rock oysters (Saccostrea glomerata) that results from infection by the protistan parasite, Marteilia sydneyi. Since 1997, the New South Wales Fisheries Service has bred S. glomerata for resistance to QX disease. The current study shows that the QX resistance breeding program has selected oysters with enhanced phenoloxidase (PO) activities. The third generation of QX-selected oysters was compared to S. glomerata that had never been selected for disease resistance. PO enzyme assays showed that oysters bred for resistance had significantly higher PO activities than the non-selected population. There was no difference between populations in the activities of a variety of other enzymes. Native polyacrylamide gel electrophoresis identified a novel form of PO in QX-selected oysters that contributes to their enhanced PO activities. This novel form of PO may represent a specific QX disease resistance factor.

Application of a multiplex PCR for the detection of protozoan pathogens of the eastern oyster Crassostrea virginica in field samples

Populations of eastern oysters Crassostrea virginica along the east coast of North America have repeatedly experienced epizootic mass mortality due to infections by protozoan parasites, and molecular diagnostic methodologies are fast becoming more widely available for the diagnosis of protozoan diseases of oysters. In this study we applied a modified version of an existing multiplex polymerase chain reaction (PCR) for detection of the eastern oyster parasites Haplosporidium nelsoni, H. costale and Perkinsus marinus from field-collected samples. We incorporated primers for DNA quality control based on the large subunit ribosomal RNA (LSU rRNA) gene of C. virginica. The multiplex PCR (MPCR) simultaneously amplified genomic DNA of C. virginica, and cloned DNA of H. nelsoni, P. marinus and H. costale. In field trial applications, we compared the performance of the MPCR to that of the conventional diagnostic techniques of histopathological tissue examination and the Ray/Mackin fluid thioglycollate medium (RMFT) assay. A total of 530 oysters were sampled from 18 sites at 12 locations along the east coast of the United States from the Gulf of Mexico to southern New England. The modified MPCR detected 21% oysters with H. nelsoni, 2% oysters with H. costale, and 40% oysters with P. marinus infections. In comparison, histopathological examination detected H. nelsoni and H. costale infections in 6 and 0.8% oysters, respectively, and the RMFT assay detected P. marinus infection in 31% oysters. The MPCR is a more sensitive diagnostic assay for detection of H. nelsoni, H. costale, and P. marinus, and incorporation of an oyster quality control product limits false negative results.

Supplementation of Perkinsus marinus cultures with host plasma or tissue homogenate enhances their infectivity

The protozoan oyster parasite Perkinsus marinus can be cultured in vitro in a variety of media; however, this has been associated with a rapid attenuation of infectivity. Supplementation of defined media with products of P. marinus-susceptible (Crassostrea virginica) and -tolerant (Crassostrea gigas, Crassostrea ariakensis) oysters alters proliferation and protease expression profiles and induces differentiation into morphological forms typically seen in vivo. It was not known if attenuation could be reversed by host extract supplementation. To investigate correlations among these changes as well as their association with infectivity, the effects of medium supplementation with tissue homogenates from both susceptible and tolerant oyster species were examined. The supplements markedly altered both cell size and proliferation, regardless of species; however, upregulation of low-molecular-weight protease expression was most prominent with susceptible oysters extracts. Increased infectivity occurred with the use of oyster product-supplemented media, but it was not consistently associated with changes in cell size, cell morphology, or protease secretion and was not related to the susceptibility of the oyster species used as the supplement source.

Development of a PCR-ELISA assay for diagnosis of Perkinsus marinus and Perkinsus atlanticus infections in bivalve molluscs

Perkinsus atlanticus and P. marinus have been associated with mass mortality of bivalve molluscs. Perkinsus infections are routinely diagnosed by histology or the fluid thioglycollate medium (FTM) assay. In this study, we describe the development of a PCR-enzyme-linked immunosorbent assay (ELISA) for amplification and rapid detection of Perkinsus species. The PCR reactions were selected to either amplify an IGS sequence region shared by currently accepted Perkinsus species or to simultaneously amplify IGS regions specific to either P. atlanticus or P. marinus. The specific hybridisation of DIG-labelled amplified products to species-specific capture probes was detected colorimetrically. This assay is able to specifically detect P. atlanticus and P. marinus, and the intensity of the colorimetric signal is dependent upon the amount of amplified product. The PCR-ELISA assay format is 100-fold more sensitive than visualisation of PCR products on ethidium bromide (EtdBr)-stained agarose gels, and as sensitive as Southern hybridisation. The sensitivity limit of PCR-ELISA was 1 pg of DNA from P. atlanticus. No cross-reactivity of the assay was observed against the host DNA. When applied to the detection of P. atlanticus in clams, 39 samples out of 45 yielded concordant results for FTM assay and PCR-ELISA detection.

The protistan parasite Perkinsus marinus is resistant to selected reactive oxygen species

The parasite Perkinsus marinus has devastated natural and farmed oyster populations along the Atlantic and Gulf coasts of North America. When viable P. marinus trophozoites are engulfed by oyster hemocytes, the typical accumulation of reactive oxygen species (ROS) normally associated with phagocyte activity is not observed. One hypothesis to explain this is that the parasite rapidly removes ROS. A manifestation of efficient ROS removal should be a high level of resistance to exogenous ROS. We investigated the in vitro susceptibility of P. marinus to ROS as compared to the estuarine bacterium Vibrio splendidus. We find that P. marinus is markedly less susceptible than V. splendidus to superoxide and hydrogen peroxide (H(2)O(2)), but equally sensitive to hypochlorite. Viable P. marinus trophozoites degrade H(2)O(2) in vitro, but lack detectable catalase activity. However, extracts contain an ascorbate dependent peroxidase activity that may contribute to H(2)O(2) removal in vitro and in vivo.

Mutation scanning analysis of Marteilia sydneyi populations from different geographical locations in eastern Australia

Marteilia sydneyi (Paramyxea) is the causative agent of QX disease in oysters. In spite of the economic impact of this disease, its origin and the precise reason(s) for its apparent spread in Australian waters are not yet known. Given such knowledge gaps, investigating the population genetic structure(s) of M. sydneyi populations could provide insights into the epidemiology and ecology of the parasite and could assist in its prevention and control. In this study, single strand conformation polymorphism (SSCP)-based analysis of a region (195 bp) of the first internal transcribed spacer (ITS-1) of ribosomal DNA was employed to investigate genetic variation within and among five populations of M. sydneyi from oysters from five different locations in eastern Australia. The analysis showed the existence of a genetic variant of M. sydneyi common to the Great Sandy Strait, and the Richmond and Georges Rivers, as distinct from variants at the Pimpama and Clarence Rivers. Together with historical and other information relating to the QX disease outbreaks in eastern Australia, the molecular findings support the proposal that the parasite originated in the Great Sandy Strait and/or Richmond River and then extended southward along the coast. From a technical perspective, the study demonstrated the usefulness of SSCP as a tool to study the population genetics and epidemiology of M. sydneyi.

Flow cytometry as a tool to quantify oyster defence mechanisms

The fast growing oyster aquaculture industry is greatly hindered by Perkinsus marinus and Haplosporidium nelsoni which can kill up to 80% of the production. The relationship between parasites and oyster defence mechanisms is unclear. Two defence mechanisms of the Eastern Oyster (Crassostrea virginica) were quantified at the single cell level utilising flow cytometry. Phagocytosis was measured using fluorescent beads. Respiratory burst activity was quantified as the H2O2-specific increase in dichlorofluorescein-associated fluorescence upon stimulation. These two assays distinguished three populations of haemocytes (granulocytes, hyalinocytes and intermediate cells) with unique functional characteristics. Granulocytes were most active at phagocytosis and H2O2 production while hyalinocytes were relatively inactive. The intermediate cells had moderate phagocytic and respiratory burst activity. Flow cytometry can rapidly, accurately and directly quantify the morphology and function of a large number of individual cells, and will lead to a better understanding of the bivalve immune system.

Perkinsus mediterraneus n. sp., a protistan parasite of the European flat oyster Ostrea edulis from the Balearic Islands, Mediterranean Sea

A new species, Perkinsus mediterraneus, a protistan parasite of the European oyster Ostrea edulis (L.), farmed along the coast of the Balearic Islands, Mediterranean Sea, is described. Morphological examinations with light and transmission electron microscopy, DNA sequence-analysis and enlargement in Ray's fluid thioglycollate medium (RFTM) confirmed that this parasite belongs to the genus Perkinsus. Specific morphological and genetic characteristics indicated that it should be considered a new species in the genus. Sequencing of the small subunit ribosomal (ssu rRNA) gene confirmed that the parasite belongs to the genus Perkinsus, and sequences of the internal transcribed spacer (ITS) were distinct from any Perkinsus ITS sequences previously published and/or deposited in the GenBank. Phylogenetic analysis revealed that the ITS sequences of the new species formed a monophyletic group comprising a sister clade to the P. atlanticus/olseni group. In addition, morphological differences were observed between the new species and the other described Perkinsus spp.. After incubation in RFTM for 1 wk, the prezoosporangium had reached an extremely large size (97.4 +/- 1.99 microm) (mean +/- SE), and after 2 wk incubation had again almost doubled in size (167.1 +/- 8.09 microm). The discharge-tube length was one sixth the diameter of the zoosporangium, i.e. a ratio of 17.36:97.38, the lowest ratio observed for any Perkinsus species. At the ultrastructural level, zoosporangia and zoospores exhibited some differences compared to other Perkinsus species.

Comparison of light microscopic techniques for the diagnosis of the infection of the European flat oyster Ostrea edulis by the protozoan Bonamia ostreae

A comparison among various histological techniques for the detection of the parasite Bonamia ostreae in oysters Ostrea edulis was performed to evaluate their sensitivity and suitability for different purposes. The comparison involved examination of histological sections, tissue imprints from gills, digestive gland, gonad and heart, and haemolymph cell monolayers, prepared through various protocols. Every technique produced some false negative. The haemolymph cell monolayers were more sensitive than tissue imprints and histological sections. Heart imprints provided the highest sensitivity among tissue imprints. Examination of histological sections was among the least sensitive techniques. Four procedures for estimation of infection intensity were compared. Some differences in accuracy for the estimation of infection intensity between haemolymph cell monolayers and histological sections (HS) were detected: there was a very good agreement when the infection appeared low or heavy in HS but it was not so good in the remaining cases. The results suggest the need for a critical review of the recommendations of the "Office Internationale des Epizooties" and the European Union for diagnosis of bonamiosis.

Arachidonic acid synthetic pathways of the oyster protozoan parasite, Perkinsus marinus: evidence for usage of a delta-8 pathway

The meront stage of the oyster protozoan parasite, Perkinsus marinus, is capable of synthesizing saturated and unsaturated fatty acids including the essential fatty acid, arachidonic acid [20:4(n-6)]. Eukaryotes employ either delta-6 (Delta-6) or delta-8 (Delta-8) desaturase pathway or both to synthesize arachidonic acid. To elucidate the arachidonic acid synthetic pathways in P. marinus, meronts were incubated with deuterium-labeled precursors [18:1(n-9)-d6, 18:2(n-6)-d4, 18:3(n-3)-d4, and 20:3(n-3)-d8]. The lipids were extracted, converted to fatty acid methyl esters, and analyzed using gas chromatography/mass spectrometry and gas chromatography/flame ionization detection. Deuterium-labeled 18:2(n-6), 20:2(n-6), 20:3(n-6), and 20:4(n-6) were detected in meront lipids after 1-, 3-, 5-, and 10-day incubation with 18:1(n-9)-d6. Deuterium-labeled 20:2(n-6), 20:3(n-6) and 20:4(n-6) were found in lipids from meronts after incubation with 18:2(n-6)-d4 methyl ester. No labeled 18:3(n-6) was detected in either incubation. Apparently, when incubated with 18:1(n-9)-d6, the parasite first desaturated 18:1(n-9)-d6 to 18:2(n-6)-d6 by Delta-12 desaturase, then to 20:2(n-6)-d6 by elongation, and ultimately desaturated to 20:3(n-6)-d6 and 20:4(n-6)-d6 using the sequential Delta-8 and Delta-5 desaturation. Similarly, when incubated with 18:2(n-6)-d4, P. marinus converted the 18:2(n-6)-d4 to 20:2(n-6)-d4 by elongation and 20:2(n-6)-d4 to 20:3(n-6)-d4 by Delta-8 desaturase then by Delta-5 desaturase to 20:4(n-6)-d4. These results provide evidence that P. marinus employed the Delta-8 rather Delta-6 pathway for arachidonic acid synthesis. Additional support for the presence of a Delta-8 pathway was the demonstrated ability of the parasite to metabolize 18:3(n-3)-d4 to 20:3(n-3)-d4 and 20:4(n-3)-d4, and 20:3(n-3)-d8 to 20:4(n-3)-d6 and 20:5(n-3)-d6 using the sequential position-specific Delta-8 and Delta-5 desaturases.

Protease activity in the plasma of American oysters, Crassostrea virginica, experimentally infected with the protozoan parasite Perkinsus marinus

Perkinsus marinus is responsible for disease and mortality of the American oyster, Crassostrea virginica. To investigate the interactions between P. marinus and oyster hemocytes, protease activity was measured in plasma of oysters collected 4 hr, 24 hr, 4 days, and 2 mo after experimental infection with P. marinus. A significant increase in protease activity was observed in oyster plasma 4 hr after injection with P. marinus, followed by a sharp decrease within 24 hr. Gelatin-impregnated gel electrophoresis showed the presence of 2 major bands (60 and 112 kDa) and 3 less prevalent bands (35, 92, and 200 kDa) with metalloproteinaselike activity in the plasma of noninfected oysters. Additional bands in the 40- to 60-kDa range, corresponding to P. marinus serine proteases, were observed in oyster plasma at early time points after infection. A transient, but significant, decrease in the activity of oyster metalloproteinases was observed at early time points after infection. Coincubation of oyster plasma with P. marinus extracellular products resulted in a decrease in oyster metalloproteinases and several P. marinus proteases. This study provides insights into the role of proteases in the pathogenesis of Dermo disease.

Electron and light microscopic study of various glands and the secretions released into the environment by the turbellarian Urastoma cyprinae

The turbellarian Urastoma cyprinae (Graff) occurs on the gills of various bivalve species including the mussel Mytilus galloprovinciallis, where it is known to cause serious damage. More recently, it has been shown that the worms are strongly attracted to the gill of the American oyster (Crassostrea virginica) and are capable of inducing changes to the composition of proteolytic enzymes of the host mucus. Such changes may be attributable to secretory products released by the worms. Mucous glands (11-18 mum in diameter) produce minute spherules (0.7-0.9 mum in diameter) tightly bound together. The glands occupy approximately 20% of the body volume and are the most voluminous secretory organs in the worm. The smaller rhabdoid glands are unevenly distributed throughout the peripheral parenchyma and contain secretory granules of 0.35-1.2 mum in diameter. The latter occur most prominently along the distal margins of the epidermis. The frontal pole of U. cyprinae consists of a complex assembly of mucous and rhabdoid gland cells as well as other glandular structures. Collectively, these bodies release their contents to the outside via narrow gland necks. The overall organization is consistent with the frontal gland previously described for other free-living turbellarians, including other rhabdocoels. A variety of secretory products, displaying variations in staining properties, have likewise been identified in association with the body wall from other regions of the worm. This work attempts to gain a better appreciation of the secretory structures associated with the worm tegument, focusing primarily on the widespread mucous and rhabdoid glands. The secretions play a role in host-parasite interactions.

Prevalence and infection intensity of the ovarian parasite Marteilioides chungmuensis during an annual reproductive cycle of the oyster Crassostrea gigas

The occurrence of Marteilioides chungmuensis, a protozoan paramyxean parasite in the reproductive system of the Pacific oyster Crassostrea gigas, was observed at Gosung Bay, Korea. Seasonal variation in gonad development was investigated in a suspended cultured oyster population. Gametogenesis began in February and first-spawning was observed between mid and late June when surface water temperature reached 22 to 25 degrees C. Spawning activity extended from mid June to late September, with 2 marked spawning peaks in June and August. Histological examination indicated that gonad development paralleled seasonal fluctuations in water temperature. Spawning in late June was partly associated with a sudden drop in salinity due to large freshwater inputs to the Bay with the summer monsoon. M. chungmuensis occurred in developing and fully mature eggs of spawning oysters in late June to January, but were not observed from February to May. Monthly mean infection intensity was high in late June when most oysters had their first spawning period. The infection level was also relatively high in late August and November, when oysters were spawning or had completed spawning. Several oysters collected in November (11.4%) and December (16.3%) carried a large quantity of ripe but M. chungmuensis-infected eggs, suggesting that infection also causes spawning failure by delaying spawning and destroying ripe oocytes.

Perkinsus marinus, a protozoan parasite of the Eastern oyster (Crassostrea virginica): effects of temperature on the uptake and metabolism of fluorescent lipid analogs and lipase activities

The effects of temperature on the uptake and metabolism of fluorescent labeled palmitic acid (FLC16) and phosphatidylcholine (FLPC) and lipase activities in the oyster protozoan parasite, Perkinsus marinus, meront stage were tested at 10, 18, and 28 degrees C. Temperature significantly affected not only the uptake, assimilation, and metabolism of both FLC16 and FLPC in P. marinus, but also its triacylglycerol (TAG) lipase activities. The incorporation of both FLC16 and FLPC increased with temperature and paralleled the increase in the amount of total fatty acids in P. marinus meront cultures. The incorporation of FLC16 was higher than FLPC at all temperatures. The percentage of FLC16 metabolized to TAG was significantly higher at higher temperatures. Trace amounts of incorporated FLC16 were detected in monoacylglycerol (MAG) and PC at 18 and 28 degrees C. P. marinus meronts metabolized FLPC to TAG, diacylglycerol (DAG), monoacylglycerol (MAG), free fatty acids (FFA), phosphatidylethanolamine (PE), and cardiolipin (CL). The conversion of FLPC to TAG and PE was highest at 28 degrees C. The relative proportions of individual fatty acids and total saturated, monounsaturated and polyunsaturated fatty acids changed with temperatures. While total saturated fatty acids (SAFAs) increased with temperature, total monounsaturated fatty acids (MUFAs) decreased with temperature. Total polyunsaturated fatty acids (PUFAs) increased from 28 to 18 degrees C. The findings of increase of total SAFAs and decrease of total MUFAs with the increase of temperatures and upward shift of total PUFAs from 28 to 18 degrees C suggest that, as in other organisms, P. marinus is capable of adapting to changes in environmental temperatures by modifying its lipid metabolism. Generally, higher lipase activities were noted at higher cultivation temperatures. Both TAG lipase and phospholipase activities were detected in P. marinus cells and their extra cellular products (ECP), but phospholipase activities in both the cell pellets and ECP were very low. Also, lipase activities were much lower in ECP than in the cells. The observations of low metabolism, bioconversion of incorporated fluorescent lipid analogs and lipase activities at low temperatures are consistent with the low in vitro growth rate and low infectivity of P. marinus at low temperatures.

Apoptosis in the pathogenesis of infectious diseases of the eastern oyster Crassostrea virginica

Apoptosis, or programmed cell death, has been reported as being pivotal in infectious diseases of different organisms. The effects of apoptosis on the progression and transmission of the protistan parasites Perkinsus marinus and Haplosporidium nelsoni in the eastern oyster Crassostrea virginica were studied. Oysters were diagnosed for their respective infections by standard methods, and apoptosis was detected using in situ hybridization to detect DNA fragments by end labeling on paraffin sections. A digoxigenin nucleotide probe was used to label the 200 bp fragment produced by apoptosis and detected immunohistochemically using an antidigoxigenin peroxidase conjugate. The probe/DNA fragment complex was stained with a peroxidase substrate and tissues were counterstained with methyl green. Uninfected oysters had large numbers of apoptotic hemocytes present in the connective tissue underlying the stomach, gill, and mantle epithelia, whereas oysters infected with P. marinus had a reduced number of apoptotic hemocytes. The parasite may prevent hemocyte apoptosis in order to yield a greater number of hemocytes in which to house itself. Large numbers of P. marinus cells in some infected oysters were eliminated via apoptosis in the stomach epithelia, disabling the spread of infectious particles through seawater. The oysters infected with H. nelsoni also had reduced numbers of apoptotic hemocytes, while part of the vesicular connective tissue cells were apoptotic. H. nelsoni plasmodia were eliminated via apoptosis in some oysters. Apoptosis may enhance progression and prevent transmission of infectious oyster diseases.

Systematic evaluation of factors controlling Perkinsus marinus transmission dynamics in lower Chesapeake Bay

The transmission of Perkinsus marinus in eastern oysters Crassostrea virginica in relation to water temperature, host oyster mortality, and water-column abundance of anti-P. marinus antibody-labeled cells was systematically examined for 20 mo at a site in the lower York River, Virginia, USA. Uninfected sentinel oysters were naturally exposed to the parasite at 2 wk intervals throughout the course of the study to determine the periodicity and rates of parasite transmission. The timing and magnitude of disease-associated oyster mortalities in a local P. marinus-infected oyster population were estimated by monitoring a captive subset of the local oyster population. Flow cytometric immunodetection methods were employed to estimate the abundance of P. marinus cells in water samples collected 3 times each week. The acquisition of P. marinus infections by naïve sentinel oysters occurred sporadically at all times of the year; however, the highest incidence of infection occurred during the months of August and September. This window of maximum parasite transmission coincided with the death of infected hosts within the captive local oyster population. Counts of antibody-labeled cells ranged from 10 to 11900 cells l(-1), with the highest abundances in July and August coincident with maximum summer temperatures. A statistically significant relationship between water-column parasite abundance and infection-acquisition rate was not observed; however, highest parasite-transmission rates in both years occurred during periods of elevated water-column abundance of parasite cells. These results support the prevailing model of P. marinus transmission dynamics by which maximum transmission rates are observed during periods of maximum P. marinus-associated host mortality. However, our results also indicate that transmission can occur when host mortality is low or absent, so alternative mortality-independent dissemination mechanisms are likely. The results also suggest that atypically early-summer oyster mortality from Haplosporidium nelsoni infection, at a time when infections of P. marinus are light, has a significant indirect influence on P. marinus transmission dynamics. Elimination of these hosts prior to late-summer P. marinus infection-intensification effectively reduces the overall number of P. marinus cells disseminated.

Detection of the oyster parasite Bonamia ostreae by fluorescent in situ hybridization

Bonamia ostreae is an economically significant protistan parasite of the flat oyster Ostrea edulis in Europe and North America. Management of this parasite depends partly upon its reliable identification in wild and aquacultured oyster populations, but B. ostreae is small and difficult to detect by traditional microscopic methods. We designed a fluorescent in situ hybridization (FISH) assay to sensitively detect B. ostreae in standard histopathological sections of B. ostreae-infected oysters using fluorescently labeled DNA oligonucleotide probes. Hybridization using a cocktail of 3 presumptively B. ostreae-specific, fluorescein iso(thio)cyanate (FITC)-labeled oligonucleotides produced an unambiguous staining pattern of small green rings inside infected oyster hemocytes that was easily distinguished from host tissue background. This pattern is diagnostic for B. ostreae. A negative control cocktail of oligonucleotides containing 2 mismatches relative to target sequences, on the other hand, failed to hybridize at all. B. ostreae-specific probes did not cross-react with a related protist, Haplosporidium nelsoni.

Superoxide dismutases from the oyster parasite Perkinsus marinus: purification, biochemical characterization, and development of a plate microassay for activity

We have isolated and biochemically characterized superoxide dismutase (SOD) activity in cell extracts of clonally cultured Perkinsus marinus, a facultative intracellular parasite of the Eastern oyster, Crassostrea virginica. In order to assess the SOD activity throughout the purification, we developed and optimized a 96-well-plate microassay based on the inhibition of pyrogallol oxidation. The assay was also adapted to identify SOD activity type (Cu/Zn-, Mn-, or FeSOD), even in mixtures of more than one type of SOD. All SOD activity detected in the cell extracts was of the FeSOD type. Most of the SOD activity in P. marinus trophozoites resides in a major component of subunit molecular weight 24 kDa. The protein was purified by affinity chromatography on an anti-SOD antibody-Sepharose column. Amino-terminal peptide sequence of the affinity-purified protein corresponds to the predicted product of the PmSOD1 gene and indicates that amino-terminal processing has taken place. The results are discussed in the context of processing of mitochondrially targeted SODs.

Changes in circulating and tissue-infiltrating hemocyte parameters of European flat oysters, Ostrea edulis, naturally infected with Bonamia ostreae

We assayed European flat oyster, Ostrea edulis, hemocyte parameters, circulating and tissue-infiltrating hemocyte densities, circulating hemocyte type distribution and lysosomal enzyme contents, to possibly relate these hematological parameters to Bonamia ostreae infection. Circulating hemocyte densities were not statistically different between infected and uninfected oysters. In contrast, the number of tissue-infiltrating hemocytes increased with infection intensity suggesting a recruitment process at the site of infection and a possibility for cells to migrate from circulatory system to connective tissues. Lysosomal enzymes were localized mainly in granulocytes both infected and uninfected, and mean of alpha-naphtyl butyrate esterase activity decreased with increasing B. ostreae infection level. The main response observed was a change in hemocyte type distribution between uninfected and infected oysters and greater tissue-infiltrating hemocytes with increased infections. These results suggest that the decrease of circulating granulocytes, and, consequently of some cell enzyme activities may be related with B. ostreae infection.

Mikrocytos roughleyi taxonomic affiliation leads to the genus Bonamia (Haplosporidia)

Microcell-type parasites of oysters are associated with a complex of diseases in different oyster species around the world. The etiological agents are protists of very small size that are very difficult to characterize taxonomically. Associated lesions may vary according to the host species, and their occurrence may be related to variations in tissue structure. Lesion morphology cannot be used to distinguish the different agents involved. Ultrastructural observations on Mikrocytos roughleyi revealed similarities with Bonamia spp., particularly in regard to the presence of electron-dense haplosporosomes and mitochondria, whose absence from M. mackini also indicate that M. roughleyi and M. mackini are not congeneric. A partial small subunit (ssu) rRNA gene sequence of M. roughleyi was determined. This partial sequence, 951 nucleotides in length, has 95.2 and 98.4% sequence similarities with B. ostreae and B. exitiosus ssu rDNA sequences, respectively. Polymorphisms among the ssu rDNA sequences of B. ostreae, B. exitiosus and M. roughleyi allowed identification of restriction enzyme digestion patterns diagnostic for each species. Phylogenetic analysis based on the ssu rDNA data suggested that M. roughleyi belongs in the phylum Haplosporidia and that it is closely related to Bonamia spp. On the basis of ultrastructural and molecular considerations, M. roughleyi should be considered a putative member of the genus Bonamia.

Molecular detection of the oyster parasite Mikrocytos mackini, and a preliminary phylogenetic analysis

The protistan parasite Mikrocytos mackini, the causative agent of Denman Island disease in the oyster Crassostrea gigas in British Columbia, Canada, is of wide concern because it can infect other oyster species and because its life cycle, mode of transmission, and origins are unknown. PCR and fluorescent in situ hybridization (FISH) assays were developed for M. mackini, the PCR assay was validated against standard histopathological diagnosis, and a preliminary phylogenetic analysis of the M. mackini small-subunit ribosomal RNA gene (SSU rDNA) was undertaken. A PCR designed specifically not to amplify host DNA generated a 544 bp SSU rDNA fragment from M. mackini-infected oysters and enriched M. mackini cell isolates, but not from uninfected control oysters. This fragment was confirmed by FISH to be M. mackini SSU rDNA. A M. mackini-specific PCR was then designed which detected 3 to 4x more M. mackini infections in 1056 wild oysters from Denman Island, British Columbia, than standard histopathology. Mikrocytos mackini prevalence estimates based on both PCR and histopathology increased (PCR from 4.4 to 7.4%, histopathology from 1.2 to 2.1%) when gross lesions were processed in addition to standard samples (i.e. transverse sections for histopathology, left outer palp DNA for PCR). The use of histopathology and tissue imprints plus PCR, and standard samples plus observed gross lesions, represented a 'total evidence' approach that provided the most realistic estimates of the true prevalence of M. mackini. Maximum parsimony and evolutionary distance phylogenetic analyses suggested that M. mackini may be a basal eukaryote, although it is not closely related to other known protistan taxa.

Passage of a coccidial parasite (Eimeria acervulina) through the Eastern oyster (Crassostrea virginica)

Human illness resulting from the consumption of raw oysters is well documented for bacterial and viral pathogens but not for coccidial parasites. This study explores the passage of coccidial parasites through and the viability of these parasites in the Eastern oyster, Crassostrea virginica. Because both Cryptosporidium and Toxoplasma are human parasites and are not safe to handle, we chose to work with a close relative, Eimeria acervulina, as a surrogate. This parasite was analyzed in chickens. Oysters were found to concentrate coccidial oocysts within 6 h of exposure in a seawater tank. After 24 h, oysters still contained viable oocysts, but by 48 h, few oysters contained viable oocysts. No oysters were found to harbor oocysts 72 or 96 h after exposure to oocysts. After oysters had been exposed to oocysts for 24 h in one saltwater tank and then transferred to a clean saltwater tank for 48 h, their feces tested positive for viable oocysts. We conclude that coccidial parasites are not pathogenic to oysters, but move through oysters in just 1 day. Unless contaminated waters continuously carry oocysts, raw oysters are unlikely to pose a threat to human health through the carriage of coccidial parasites.

Host oyster tissue extracts modulate in vitro protease expression and cellular differentiation in the protozoan parasite, Perkinsus marinus

Perkinsus marinus is responsible for a chronic disease (Dermo) of the Eastern oyster, Crassostrea virginica. In order to simulate the in vivo environment more closely, a chemically defined medium (JL-ODRP-3) was supplemented with tissue homogenate extracts or plasma from oysters possessing varying degrees of susceptibility to P. marinus infection. In media supplemented with extracts from highly susceptible oysters (C. virginica), P. marinus cells secreted elevated amounts of a set of low molecular weight serine proteases (LMP: 30-45 kDa) as assessed by enhanced digestion within gelatin-substrate SDS-PAGE gels. Oyster species of low susceptibility (C. gigas and C. ariakensis) did not exhibit this ability to upregulate P. marinus LMP expression. Oyster extract supplementation also led to pronounced changes in P. marinus cellular morphology, such that the cells were comparable to those observed within naturally infected oysters.

Isolation and 18S ribosomal DNA gene sequences of Marteilioides chungmuensis (Paramyxea), an ovarian parasite of the Pacific oyster Crassostrea gigas

To develop sensitive detection techniques with the aim of elucidating the life cycle of Marteilioides chungmuensis, an intracellular paramyxean infecting the ovary of the Pacific oyster Crassostrea gigas, we isolated the parasite at the sporont stage from infected oysters using a freeze-thaw procedure at -20 degrees C and differential centrifugations in discontinuous sucrose and Percoll gradients. DNA was extracted from the isolated sporonts, and a PCR amplicon of 18S small subunit ribosomal RNA gene DNA was partially sequenced. In situ hybridization using 3 parasite-specific probes designed from the obtained sequence successfully detected parasite cells in infected oysters, and confirmed that the sequenced DNA was derived from M. chungmuensis.