Evaluation of histopathology, PCR, and qPCR to detect Mikrocytos mackini in oysters Crassostrea gigas using Bayesian latent class analysis

Latent class analysis (LCA) is a common method to evaluate the diagnostic sensitivity (DSe) and specificity (DSp) for pathogen detection assays in the absence of a perfect reference standard. Here we used LCA to evaluate the diagnostic accuracy of 3 tests for the detection of Mikrocytos mackini in Pacific oysters Crassostrea gigas: conventional polymerase chain reaction (PCR), real-time quantitative PCR (qPCR), and histopathology. A total of 802 Pacific oysters collected over 12 sampling events from 9 locations were assessed. Preliminary investigations indicated that standard LCA assumptions of test independence and constant detection accuracy across locations were likely unrealistic. This was mitigated by restructuring the LCA in a Bayesian framework to include test-derived knowledge about pathogen prevalence and load for categorizing populations into 2 classes of infection severity (low or high) and assessing separate DSe and DSp estimates for each class. Median DSp estimates were high (>96%) for all 3 tests in both population classes. DSe estimates varied between tests and population classes but were consistently highest for qPCR (87-99%) and lowest for histopathology (21-51%). Acknowledging that detection of M. mackini may be fitted to multiple diagnostic and management purposes, qPCR had the highest DSe while maintaining similar DSp to both conventional PCR and histopathology and thus is generally well-suited to most applications.

Parallel studies confirm Francisella halioticida causes mortality in Yesso scallops Patinopecten yessoensis

Francisella halioticida is a marine bacterium originally described as the causative agent of mass mortality among giant abalone Haliotis gigantea. Recent field studies in Canada and Japan have suggested that this bacterium is also the cause of adductor muscle lesions and high mortality of Yesso scallops Patinopecten yessoensis, although a causal relationship has not been established. In the present study, the pathogenicity of F. halioticida in Yesso scallops was assessed in both Canada and Japan using bacteria isolated from diseased Yesso scallops in each respective country. Independent laboratory experiments revealed that scallops challenged with F. halioticida via bath exposure resulted in high mortality and histological lesions characterized by massive haemocyte infiltration. The presence of F. halioticida was confirmed using PCR, and F. halioticida was re-isolated from a portion of dead and surviving specimens. These results fulfill Koch's classic criteria for establishing disease causation and provide conclusive evidence that F. halioticida causes adductor muscle lesions and high mortality in Yesso scallops.

Seawater detection and biological assessments regarding transmission of the oyster parasite Mikrocytos mackini using qPCR

Mikrocytos mackini is an intracellular parasite of oysters and causative agent of Denman Island disease in Pacific oysters Crassostrea gigas. Although M. mackini has been investigated for decades, its natural mode of transmission, mechanism for host entry, and environmental stability are largely unknown. We explored these biological characteristics of M. mackini using a recently described quantitative PCR (qPCR) assay. We detected M. mackini in the flow-through tank water of experimentally infected oysters and during disease remission in host tissues following 6 wk of elevated water temperature. Waterborne exposure of oysters to M. mackini further confirmed the potential for extracellular seawater transmission of this parasite and also identified host gill to have the highest early and continued prevalence for M. mackini DNA compared to stomach, mantle, labial palps, or adductor muscle samples. However, infections following waterborne challenge were slow to develop despite a substantial exposure (>106 M. mackini l-1 for 24 h), and further investigation demonstrated that M. mackini occurrence and infectivity severely declined following extracellular seawater incubation of more than 24 h. This study demonstrates a potential for using qPCR to monitor M. mackini in wild or farmed oyster populations during periods of disease remission or from environmental seawater samples. This work also suggests that gill tissues may provide a primary site for waterborne entry and possibly shedding of M. mackini in oysters. Further, although extracellular seawater transmission of M. mackini was possible, poor environmental stability and infection efficiency likely restricts the geographic transmission of M. mackini between oysters in natural environs and may help to explain localized areas of infection.

Rare occurrence of heart lesions in Pacific oysters Crassostrea gigas caused by an unknown bacterial infection

On rare occasions, small cream-coloured cysts have been observed in the heart and pericardial cavity of Pacific oysters Crassostrea gigas from British Columbia, Canada. Histopathology revealed the presence of large colonies of bacteria (up to 800 µm in diameter) causing significant host response and hypertrophy of the heart epithelium. The causative bacteria were characterized as follows: Gram-negative, coccoid to small rod-shaped, typically <1.5 µm in size, cell walls highly endowed with surface fimbriae and division via binary fission. Although these bacteria shared some morphological characteristics with the order Rickettsiales, they did not require an intracellular existence for multiplication. Unfortunately, a cultured isolate was not available, and a retrospective attempt to further characterize the bacteria using DNA sequence analysis of a fragment from the 16S rDNA region proved to be uninformative.

Ameson metacarcini sp. nov. (Microsporidia) infecting the muscles of Dungeness crabs Metacarcinus magister from British Columbia, Canada

The Dungeness crab Metacarcinus magister supports a large and valuable fishery along the west coast of North America. Since 1998, Dungeness crabs exhibiting pink- to orange-colored joints and opaque white musculature have been sporadically observed in low prevalence from the Fraser River delta of British Columbia, Canada. We provide histological, ultrastructural, and molecular evidence that this condition is caused by a new microsporidian parasite. Crabs displaying gross symptoms were confirmed to have heavy infections of ovoid-shaped microsporidian spores (~1.8 × 1.4 µm in size) within muscle bundles of the skeletal musculature. The parasite apparently infected the outer periphery of each muscle bundle, and then proliferated into the muscle fibres near the centre of each infected bundle. Light infections were observed in heart tissues, and occasionally spores were observed within the fixed phagocytes lining the blood vessels of the hepatopancreas. Transmission electron microscopy (TEM) revealed multiple life stages of a monokaryotic microsporidian parasite within the sarcoplasm of muscle fibres. Molecular analysis of partial small subunit rRNA sequence data from the new species revealed an affinity to Ameson, a genus of Microsporidia infecting marine crustaceans. Based on morphological and molecular data, the new species is distinct from Nadelspora canceri, a related microsporidian that also infects the muscles of this host. At present, little is known about the distribution, seasonality, and transmission of A. metacarcini in M. magister.

Molecular taxonomy of Mikrocytos boweri sp. nov. from Olympia oysters Ostrea lurida in British Columbia, Canada

Mikrocytos mackini is a microcell parasite that usually infects Crassostrea gigas distributed along the Pacific Northwest coast of North America. For many years, M. mackini was the only known species in the genus, but there have been multiple recent findings of genetically divergent forms of Mikrocytos in different hosts and in distantly located geographic locations. This note describes M. boweri sp. nov. found in Olympia oysters Ostrea lurida collected from and native to British Columbia, Canada, primarily using a molecular taxonomic approach.

Review of Mikrocytos microcell parasites at the dawn of a new age of scientific discovery

The genus Mikrocytos is traditionally known for Mikrocytos mackini, the microcell parasite that typically infects Pacific oysters along the west coast of North America. Multiple factors have conspired to create difficulty for scientific research on Mikrocytos parasites. These include their tiny cell size, infections that are often of light intensity, lack of suitable cell lines and techniques for in vitro culture, and the seasonal nature of infections. The extreme rate of molecular evolution in Mikrocytos stymied new species discovery and confounded attempts to resolve its phylogenetic position for many years. Fortunately, 2 recent landmark studies have paved the way forward for future research by drastically changing our understanding of the evolution and diversity of these parasites. No longer an orphan eukaryotic lineage, the phylogenetic placement of Mikrocytos has been confidently resolved within Rhizaria and as sister taxon to Haplosporidia. The genus has also found a taxonomic home within the newly-discovered order, Mikrocytida - a globally distributed lineage of parasites infecting a wide range of invertebrate hosts. Here we review available scientific information on Mikrocytos parasites including their evolution and diversity, host and geographic ranges, epizootiology, and detection of the regulated pathogen, M. mackini. We also make recommendations towards a consistent taxonomic framework for this genus by minimally suggesting the use of 18S rDNA sequence, host species information, and histopathological presentation in new species descriptions. This is timely given that we are likely embarking on a new era of scientific advancements, including species discovery, in this genus and its relatives.

Detection of a parasitic amoeba (Order Dactylopodida) in the female gonads of oysters in Brazil

The impacts of oocyte parasites on the reproductive success of molluscs are largely unknown. In this study, we evaluated the presence of gonad parasites in 6 species of marine bivalve molluscs native to southern Brazil. Cultured bivalves included the mangrove oyster Crassostrea gasar (sometimes called C. brasiliana), the brown mussel Perna perna, the lion's paw scallop Nodipecten nodosus and the wing pearl oyster Pteria hirundo. Another species of mangrove oyster, C. rhizophorae, and the carib pointed venus clam Anomalocardia brasiliana (syn. A. flexuosa) were collected from the wild. Molluscs were collected in winter 2009 and summer 2010 for histopathological and molecular evaluation. An unknown ovarian parasite (UOP) was observed in histopathological sections of female gonads of C. gasar and C. rhizophorae. The UOP possessed features suggestive of amoebae, including an irregular outer membrane, frothy cytoplasm, a nucleus with a prominent central nucleolus and a closely associated basophilic parasome. PCR analysis was negative for Marteilioides chungmuensis, Perkinsus spp. and Paramoeba perurans. However, real-time PCR successfully amplified DNA from oyster gonads when using universal Paramoeba spp. primers. Also, conventional PCR amplified DNA using primers specific for Perkinsela amoebae-like organisms (syn. Perkinsiella), which are considered as endosymbionts of Parameoba spp., previously thought to be the parasome. Our results suggest that this UOP is a species of amoeba belonging to 1 of the 2 families of the order Dactylopodida, possibly related to Paramoeba spp. This study represents the first report of this type of organism in oysters. We found that C. gasar and C. rhizophorae were the most susceptible molluscs to these UOPs.

Rediscovery of the Yesso scallop pathogen Perkinsus qugwadi in Canada, and development of PCR tests

Perkinsus qugwadi, a pathogenic protozoan parasite of Yesso scallops Patinopecten yessoensis, is found only in cultured populations in British Columbia, Canada. This pathogen was first identified in 1988 and caused significant mortalities at some locations during the early 1990s. Prevalence of infection decreased dramatically following 1995, and the disease was last reported in 1997, leading to speculation that the Yesso scallop stocks in Canada had developed resistance to the disease, or that P. qugwadi had disappeared. However, the present study revealed that infection with P. qugwadi and associated mortality is still occurring in scallops from at least one location in British Columbia. One of the PCR tests developed for P. qugwadi detected the parasite in a 105-fold dilution of DNA extracted from a heavily infected sample and detected 52% more positive scallops than histology; however, the assay also cross-reacted with P. honshuensis and P. olseni. The other PCR test was less sensitive and detected 34% more positives, but did not react to any of the other Perkinsus species tested, suggesting that these PCR tests are powerful tools for screening for the presence of P. qugwadi. Phylogenetic analysis of 1796 bp of SSU rRNA gene sequence clearly indicated that P. qugwadi is positioned basally to other Perkinsus species.

First report of a Mikrocytos-like parasite in European oysters Ostrea edulis from Canada after transport and quarantine in France

As part of a disease resistance experiment, 112 apparently healthy European flat oysters Ostrea edulis L. were exported from Canada (Nova Scotia) into France to test their susceptibility to Bonamia ostreae infection. Twelve oysters died in transit and 17 others died within 2 wk of laboratory quarantine acclimation. All oysters were examined histologically, and the 17 that died during quarantine were assayed for microcells (Bonamia sp. and Mikrocytos mackini) using molecular techniques. A microcell parasite was detected in the connective tissue of 5 of the 112 oysters. Morphological appearance, tissue affinity and molecular characterization through PCR, in situ hybridization (ISH), fluorescence in situ hybridization (FISH) and sequencing revealed a protist related to M. mackini. This is the first report of a parasite of the genus Mikrocytos in a species belonging to the genus Ostrea from the Atlantic Ocean.

Susceptibility of juvenile Crassostrea gigas and resistance of Panope abrupta to Mikrocytos mackini

Samples from the field and laboratory exposure to Mikrocytos mackini (a tiny protistan parasite of unknown taxonomic affiliation) confirmed that juvenile Pacific oysters (Crassostrea gigas) are susceptible to infection and the resulting disease. In the laboratory bath exposure experiment, a prevalence of infection approaching 100% and mortalities were observed in the small oysters (about 18 mm in shell length). However, in the same laboratory exposure experiment, similar aged geoduck clams (Panope abrupta, about 8mm in shell length) were resistant to infection. The main route of infection in the oysters appeared to be via the digestive tract and possibly the gills where the parasite multiplied within host cells. Other tissues such as the adductor muscle and vesicular connective tissue were subsequently colonized. Although the infection resulted in the mortality of some oysters, others appeared to overcome the disease.

Sensitivity of a digoxigenin-labelled DNA probe in detecting Mikrocytos mackini, causative agent of Denman Island disease (mikrocytosis), in oysters

The protistan parasite Mikrocytos mackini, causative agent of Denman Island disease (mikrocytosis), induces mortality and reduces marketability in the Pacific oyster, Crassostrea gigas, in British Columbia, Canada. This parasite is a pathogen of international concern because it infects a range of oyster species, and because its life cycle and mode of transmission are unknown. A digoxigenin-labelled DNA probe in situ hybridisation technique (DIG-ISH) was developed, and its detection sensitivity was compared to standard histological sections stained with haematoxylin and eosin stain (H&E-histo). In H&E-histo preparations, the detection of M. mackini was certain only when the parasite occurred within the vesicular connective tissue of adult oysters. However, the DIG-ISH technique clearly demonstrated the presence of infection in all other host tissues as well as in juvenile oysters with poorly developed vesicular connective tissue. The probe hybridised strongly to M. mackini, did not hybridise to oyster tissues or with the other shellfish parasites tested, and was more sensitive for detecting infections when compared to H&E-histo.

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.

Morphology and ultrastructure of a protistan pathogen in the haemolymph of shrimp (Pandalusspp.) in the northeastern Pacific Ocean

A eukaryotic parasite of uncertain taxonomic affiliation, with superficial similarity to parasitic dinoflagellates (large plasmodia and numerous trophonts) but a different mechanism of nuclear division and a lack of organelles characteristic of parasitic dinoflagellates, is described from spot prawns (Pandalus platyceros). Up to 20% of the spot prawns examined from Malaspina Strait, British Columbia, were infected. Infections in the majority of the prawns were cryptic (asymptomatic) but of sufficient duration to affect secondary sexual characteristics and castrate the host. Cryptic infections consisted of plasmodia containing numerous nuclei. The plasmodia were observed in the haemal sinuses of all tissues. In mature plasmodia the nuclei stopped dividing and showed a peripheral chromatin ring, an internal chromatin web, and up to three tiny nucleoli. Mature plasmodia divided into numerous uninucleate trophonts, resulting in symptoms of lethargy, orange discoloration, and milky haemolymph caused by a plethora of either spherical or discoid trophonts. Symptomatic infections of the prawns fished with traps rarely exceeded 2%. In 3 of the 156 symptomatic prawns examined, about 30% of the trophonts were in the process of binary fission. During mitosis the nuclear membrane was persistent, but openings (about 0.8 µm in width) at either pole accommodated emergent spindle-pole bodies to which the few chromosomes were attached by microtubules. Attempts to transmit the infection between prawns in the laboratory were unsuccessful.

Ultrastructure of Mikrocytos mackini, the cause of Denman Island disease in oysters Crassostrea spp. and Ostrea spp. in British Columbia, Canada

An ultrastructural study was carried out on Mikrocytos mackini, the cause of Denman Island disease in Pacific oysters Crassostrea gigas in western Canada. Three forms were identified, quiescent cells (QC), vesicular cells (VC) and endosomal cells (EC). QC occurred in the vesicular connective tissue (VCT), haemocytes (hyalinocytes), adductor and heart myocytes, and extracellularly. They had a central round to ovoid nucleus, < 7 cisternae of inactive nuclear membrane-bound Golgi, few vesicles and lysosome-like bodies. VC were rarely extracellular and usually occurred in adductor and heart myocytes, in close association with host cell mitochondria. The contents of the host cell mitochondria appeared to pass through a tubular extension into the cytoplasm of the parasite. Cytoplasmic vesicles resembled the tubular structure in appearance and size. EC occurred in the VCT, in haemocytes and extracellularly. They had a dilated nuclear membrane, sometimes containing a looped membranous structure that appeared to derive from the nucleus, and pass into the cytoplasm. A well-developed anastomosing endoplasmic reticulum connected the nuclear and plasma membranes, and endosomes were present in the cytoplasm. QC and EC cells were frequently observed tightly against, or between, the nuclear membranes of the host cell. Few organelles occurred in all forms of M. mackini, especially QC. The lack of organelles found in most eukaryotic cells, including mitochondria or their equivalents, may be due to obligate parasitism and the utilization of host cell organelles reducing the need for parasite organelles. Alternatively, perhaps M. mackini is a primitive eukaryote. Although phylogenetic affinities could not be determined, it is not a haplosporidian. A developmental cycle is proposed from these findings.

A simple technique to concentrate the protozoan Mikrocytos mackini, causative agent of Denman Island Disease in oysters

This report describes a simple filtration technique to isolate the parasite Mikrocytos mackini from oyster tissue. The technique is based on successive filtration through filter papers and polycarbonate membrane filters of decreasing mesh using a low vacuum (<8 cm Hg). This technique allows for the recovery of about 1 x 10(8) parasites (microcells) from about 2 g of heavily infected oyster tissue. About 99% of the particulate material in the final preparation consisted of intact M. mackini.

Distribution, prevalence, and pathogenicity of the protozoan Perkinsus qugwadi in Japanese scallops, Patinopecten yessoensis, cultured in British Columbia, Canada

Perkinsus qugwadi was detected in cultured Japanese scallops, Patinopecten yessoensis, held for at least 3 months at 5 of 14 locations along the coast of British Columbia. At four of the locations, infected scallops were observed in over half of the samples examined and the prevalence of infection reached 98%. At one location (Cypress Bay), infected scallops were rarely encountered and when infection was present, the intensity was light. Field studies, laboratory experiments, and commercial grow-out operations indicate that high mortalities among Japanese scallops are associated with P. qugwadi infection. Although some scallops responded to infection with an accumulation of numerous haemocytes in the vicinity of the parasite, other scallops appeared not to respond and were overwhelmed by numerous parasites that multiplied in the connective tissues of all organs. Epizootiology of this pathogen in Japanese scallops suggests that P. qugwadi is enzootic in an unknown host in British Columbia, with the introduced Japanese scallop serving as an aberrant host.

Perkinsus qugwadi sp.nov. (incertae sedis), a pathogenic protozoan parasite of Japanese scallops, Patinopecten yessoensis, cultured in British Columbia, Canada

The pathogenic protozoan that has caused high mortalities among cultured Japanese scallops, Patinopecten yessoensis (introduced into British Columbia in 1983), is described as a new species, Perkinsus qugwadi. As in other species in the genus Perkinsus, vegetative multiplication occurred in the intercellular spaces and haemolymph sinuses of all organs of the host. Although the morphology of all developmental stages was similar to that of other Perkinsus spp., differences were observed in the ultrastructural morphology of some organelles in the biflagellate zoospore of P. qugwadi. Also, unlike all other Perkinsus spp., P. qugwadi multiplied and was pathogenic at low temperatures (8-15°C), the development of zoospores was found to occur only within the interstitial spaces of living hosts (and only in some heavily infected juvenile scallops that were less than 50 mm in valve height), and the thioglycollate culture technique that is a diagnostic test for all previously described Perkinsus spp. was negative for P. qugwadi.

Detection, isolation, and experimental transmission of Mikrocytos mackini, a microcell parasite of Pacific oysters Crassostrea gigas (Thunberg)

Denman Island disease, characterized by clinical signs of focal green lesions on the body surface or within the mantle, palps, and adductor muscle of Pacific oysters (Crassostrea gigas), is caused by a protozoan parasite of unknown taxonomic affiliations, Mikrocytos mackini. Detection of M. mackini was more sensitive and rapid by use of tissue imprints than histological sections. Of several isolation procedures investigated, centrifugation of homogenized infected tissues through a 15% sucrose solution enabled the isolation of the highest number of M. mackini with the lowest amount of oyster debris. Experimental transmission showed that oysters exposed to M. mackini by inoculation with isolated parasites had shorter prepatent periods and higher prevalences and intensities of infection than those incubated in homogenates from infected oysters (bath exposure) or those naturally exposed in the field. Experimental transmission was also used to propagate M. mackini in vivo in the laboratory year round. For the development of the disease, exposed oysters required prolonged incubation at low temperatures (about 10 degrees C).

Evaluation and comparison of the nutrition care process for persons with diabetes among inpatient and outpatient dietitians

The purpose of this study was to compare the problem-solving skills used by dietitians when planning care for inpatient and outpatient persons with type II diabetes. Telephone interviews were conducted with 44 inpatient dietitians and 45 outpatient dietitians. Inpatient dietitians used more information from the medical record to make clinical judgments than outpatient dietitians. Inpatient dietitians reported condensing their assessment more frequently due to time pressure than outpatient dietitians. Inpatient dietitians were more likely to identify nutrition-related problems via information from the medical record while outpatient dietitians reported using diet history information. Outpatient dietitians more frequently identified specific behavioral goals whereas inpatient dietitians recommended general goals. The increased availability of objective, detailed information necessary for a thorough nutritional assessment is an advantage of inpatient care planning. However, outpatient diabetes education may be a preferred setting because of more time available for education and better learning effectiveness.