Complete genome sequences of α-glucosidase-positive and -negative strains of Nocardia seriolae from Seriola species in Japan.

We report complete genome sequences of two strains of Nocardia seriolae, the causative agent of nocardiosis in fish. Strains KGN1266 (α-glucosidase-positive) and 024013 (α-glucosidase-negative) were isolated from Seriola dumerili and Seriola quinqueradiata, respectively. Whole genome sequences were hybrid-assembled using Oxford Nanopore long-read and BGI DNBseq short-read sequencing.

Pathogenicity, genomic analysis and structure of abalone asfa-like virus: evidence for classification in the family Asfarviridae.

This paper presents the rationale for classifying abalone asfa-like virus (AbALV) in the family Asfarviridae based on analyses of the host, whole genome and electron microscopic observations. AbALV caused >80 % cumulative mortality in an experimentally infected mollusc, Haliotis madaka. The AbALV genome was found to be linear, approximately 281 kb in length, with a G+C content of 31.32 %. Of the 309 predicted ORFs, 48 of the top hits with African swine fever virus (ASFV) genes in homology analysis were found to be in the central region of the genome. Synteny in the central region of the genome was conserved with ASFV. Similar to ASFV, paralogous genes were present at both ends of the genome. The pairwise average amino acid identity (AAI) between the AbALV and ASFV genomes was 33.97 %, within the range of intra-family AAI values for Nucleocytoviricota. Electron microscopy analysis of the gills revealed ~200 nm icosahedral virus particles in the cytoplasm of epithelial cells, and the size and morphology resembled ASFV. In addition to swine, ASFV also infects ticks, which are protostomes like abalone. The overall genome structure and virion morphology of AbALV and ASFV are similar, and both viruses infect protostomes, suggesting that AbALV is a new member of the family Asfarviridae.

Transcriptomic Analysis of the Effects of Chemokine Receptor CXCR3 Deficiency on Immune Responses in the Mouse Brain during Toxoplasma gondii Infection.

The obligate intracellular parasite Toxoplasma gondii infects warm-blooded animals, including humans. We previously revealed through a whole-brain transcriptome analysis that infection with T. gondii in mice causes immune response-associated genes to be upregulated, for instance, chemokines and chemokine receptors such as CXC chemokine receptor 3 (CXCR3) and its ligand CXC chemokine ligand 10 (CXCL10). Here, we describe the effect of CXCR3 on responses against T. gondii infection in the mouse brain. In vivo assays using CXCR3-deficient mice showed that the absence of CXCR3 delayed the normal recovery of body weight and increased the brain parasite burden, suggesting that CXCR3 plays a role in the control of pathology in the brain, the site where chronic infection occurs. Therefore, to further analyze the function of CXCR3 in the brain, we profiled the gene expression patterns of primary astrocytes and microglia by RNA sequencing and subsequent analyses. CXCR3 deficiency impaired the normal upregulation of immune-related genes during T. gondii infection, in astrocytes and microglia alike. Collectively, our results suggest that the immune-related genes upregulated by CXCR3 perform a particular role in controlling pathology when the host is chronically infected with T. gondii in the brain.

Biochemical Studies of Mitochondrial Malate: Quinone Oxidoreductase from Toxoplasma gondii.

Toxoplasma gondii is a protozoan parasite that causes toxoplasmosis and infects almost one-third of the global human population. A lack of effective drugs and vaccines and the emergence of drug resistant parasites highlight the need for the development of new drugs. The mitochondrial electron transport chain (ETC) is an essential pathway for energy metabolism and the survival of T. gondii. In apicomplexan parasites, malate:quinone oxidoreductase (MQO) is a monotopic membrane protein belonging to the ETC and a key member of the tricarboxylic acid cycle, and has recently been suggested to play a role in the fumarate cycle, which is required for the cytosolic purine salvage pathway. In T. gondii, a putative MQO (TgMQO) is expressed in tachyzoite and bradyzoite stages and is considered to be a potential drug target since its orthologue is not conserved in mammalian hosts. As a first step towards the evaluation of TgMQO as a drug target candidate, in this study, we developed a new expression system for TgMQO in FN102(DE3)TAO, a strain deficient in respiratory cytochromes and dependent on an alternative oxidase. This system allowed, for the first time, the expression and purification of a mitochondrial MQO family enzyme, which was used for steady-state kinetics and substrate specificity analyses. Ferulenol, the only known MQO inhibitor, also inhibited TgMQO at IC50 of 0.822 µM, and displayed different inhibition kinetics compared to Plasmodium falciparum MQO. Furthermore, our analysis indicated the presence of a third binding site for ferulenol that is distinct from the ubiquinone and malate sites.

CRISPR/Cas9 Ribonucleoprotein-Based Genome Editing Methodology in the Marine Protozoan Parasite Perkinsus marinus.

Perkinsus marinus (Perkinsozoa), a close relative of apicomplexans, is an osmotrophic facultative intracellular marine protozoan parasite responsible for "Dermo" disease in oysters and clams. Although there is no clinical evidence of this parasite infecting humans, HLA-DR40 transgenic mice studies strongly suggest the parasite as a natural adjuvant in oral vaccines. P. marinus is being developed as a heterologous gene expression platform for pathogens of medical and veterinary relevance and a novel platform for delivering vaccines. We previously reported the transient expression of two rodent malaria genes Plasmodium berghei HAP2 and MSP8. In this study, we optimized the original electroporation-based protocol to establish a stable heterologous expression method. Using 20 µg of pPmMOE[MOE1]:GFP and 25.0 × 106 P. marinus cells resulted in 98% GFP-positive cells. Furthermore, using the optimized protocol, we report for the first time the successful knock-in of GFP at the C-terminus of the PmMOE1 using ribonucleoprotein (RNP)-based CRISPR/Cas9 gene editing methodology. The GFP was expressed 18 h post-transfection, and expression was observed for 8 months post-transfection, making it a robust and stable knock-in system.

Natural history study on Riccardoella tokyoensis: life history with ontogeny and host distribution records.

Riccardoella tokyoensis is a snail parasite recently recorded in Japan. To understand the basic ecology of this mite, we surveyed the life history of a population, including seasonal dynamics, host range, and geographical distribution in East Japan. Seasonal dynamics of the mite were studied in Rinshi-no-mori Park, Tokyo, Japan, from June 2016 to May 2018; thereafter, the morphology of each stage was described. In this park, the host population was infected with the mite in all seasons; however, the prevalence and infection intensities were highest during the warm seasons. Larvae and nymphs were found in the warm seasons. Proto- and tritonymphs were uncommon. To study the host range and geographical distribution, we sampled 1135 terrestrial mollusks from 16 families at 21 locations in Kanto and east Tokai areas from June 2016 to May 2020. Mites were detected at 13 locations, and only clausiliid snails were infected with the mite. Through a laboratory experiment, our observations confirmed that the mite was transmitted to the host lung via the pneumostome. None of the artificially infected snails died, although we maintained the snails for 2 months. No inflammation was found in naturally infected host tissue, suggesting that their virulence against the host snails was low. Riccardoella tokyoensis was not found in the litter layer in the field surveys, and no mite was observed on the external surface of the hosts, suggesting that the major habitat of this mite was the host lung.

Babesia microti Confers Macrophage-Based Cross-Protective Immunity Against Murine Malaria.

Malaria and babesiosis, the two primary intraerythrocytic protozoan diseases of humans, have been reported in multiple cases of co-infection in endemic regions. As the geographic range and incidence of arthropod-borne infectious diseases is being affected by climate change, co-infection cases with Plasmodium and Babesia are likely to increase. The two parasites have been used in experimental settings, where prior infection with Babesia microti has been shown to protect against fatal malarial infections in mice and primates. However, the immunological mechanisms behind such phenomena of cross-protection remain unknown. Here, we investigated the effect of a primary B. microti infection on the outcome of a lethal P. chabaudi challenge infection using a murine model. Simultaneous infection with both pathogens led to high mortality rates in immunocompetent BALB/c mice, similar to control mice infected with P. chabaudi alone. On the other hand, mice with various stages of B. microti primary infection were thoroughly immune to a subsequent P. chabaudi challenge. Protected mice exhibited decreased levels of serum antibodies and pro-inflammatory cytokines during early stages of challenge infection. Mice repeatedly immunized with dead B. microti quickly succumbed to P. chabaudi infection, despite induction of high antibody responses. Notably, cross-protection was observed in mice lacking functional B and T lymphocytes. When the role of other innate immune effector cells was examined, NK cell-depleted mice with chronic B. microti infection were also found to be protected against P. chabaudi. Conversely, in vivo macrophage depletion rendered the mice vulnerable to P. chabaudi. The above results show that the mechanism of cross-protection conferred by B. microti against P. chabaudi is innate immunity-based, and suggest that it relies predominantly upon the function of macrophages. Further research is needed for elucidating the malaria-suppressing effects of babesiosis, with a vision toward development of novel tools to control malaria.

Metacytofilin Is a Potent Therapeutic Drug Candidate for Toxoplasmosis.

BACKGROUND: Toxoplasmosis, a parasitic disease caused by Toxoplasma gondii, is an important cause of miscarriage or adverse fetal effects, including neurological and ocular manifestations in humans. Current anti-Toxoplasma drugs have limited efficacy against toxoplasmosis and also have severe side effects. Therefore, novel efficacious drugs are urgently needed. Here, we identified metacytofilin (MCF) from a fungal Metarhizium species as a potential anti-Toxoplasma compound. METHODS: Anti-Toxoplasma activities of MCF and its derivatives were evaluated in vitro and in vivo using nonpregnant and pregnant mice. To understand the mode of action of MCF, the RNA expression of host and parasite genes was investigated by RNAseq. RESULTS: In vitro, MCF inhibited the viability of intracellular and extracellular T. gondii. Administering MCF intraperitoneally or orally to mice after infection with T. gondii tachyzoites increased mouse survival compared with the untreated animals. Remarkably, oral administration of MCF to pregnant mice prevented vertical transmission of the parasite. Interestingly, RNA sequencing of T. gondii-infected cells treated with MCF showed that MCF inhibited DNA replication and enhanced RNA degradation in the parasites. CONCLUSIONS: With its potent anti-T. gondii activity, MCF is a strong candidate for future drug development against toxoplasmosis.

The effects of environmental and nutritional conditions on the development of Perkinsus olseni prezoosporangia.

Perkinsus olseni is a widely distributed protozoan pathogen that infects a wide range of marine mollusks. Prezoosporulation of P. olseni trophozoites is easily observed in Ray's fluid thioglycollate medium, but in nature, trophozoites within host tissue should be able to develop into prezoosporangia without any additional artificial medium after the host dies. How this process might work in field conditions remains poorly understood, however, partly because of the lack of appropriate in vitro assays. In this study, we observed that trophozoites of P. olseni successfully developed into prezoosporangia when mixed with minced tissue of the Manila clam Ruditapes philippinarum and placed in seawater. We were thus able to establish a new method to examine the development of P. olseni to prezoosporangia under artificially simulated natural environmental conditions. Using this method, we found that low temperatures (5 °C, 15 °C) significantly suppressed prezoosporangia development. In addition, we found that prezoosporangia were developed in a wide range of salinities (10-50 practical salinity unit) and that P. olseni requires some nutrition factors from host tissue for prezoosporulation to occur. Because the transmission of P. olseni among a host population highly depends on the developmental process of prezoosporangia, which leads to production of the infective zoospore stage, these results will help further our understanding of the parasite's infection dynamics in nature.

Transcriptome analysis of the effect of C-C chemokine receptor 5 deficiency on cell response to Toxoplasma gondii in brain cells.

BACKGROUND: Infection with Toxoplasma gondii is thought to damage the brain and be a risk factor for neurological and psychotic disorders. The immune response-participating chemokine system has recently been considered vital for brain cell signaling and neural functioning. Here, we investigated the effect of the deficiency of C-C chemokine receptor 5 (CCR5), which is previously reported to be associated with T. gondii infection, on gene expression in the brain during T. gondii infection and the relationship between CCR5 and the inflammatory response against T. gondii infection in the brain. RESULTS: We performed a genome-wide comprehensive analysis of brain cells from wild-type and CCR5-deficient mice. Mouse primary brain cells infected with T. gondii were subjected to RNA sequencing. The expression levels of some genes, especially in astrocytes and microglia, were altered by CCR5-deficiency during T. gondii infection, and the gene ontology and Kyoto Encyclopedia of Genes and Genomes analysis revealed an enhanced immune response in the brain cells. The expression levels of genes which were highly differentially expressed in vitro were also investigated in the mouse brains during the T. gondii infections. Among the genes tested, only Saa3 (serum amyloid A3) showed partly CCR5-dependent upregulation during the acute infection phase. However, analysis of the subacute phase showed that in addition to Saa3, Hmox1 may also contribute to the protection and/or pathology partly via the CCR5 pathway. CONCLUSIONS: Our results indicate that CCR5 is involved in T. gondii infection in the brain where it contributes to inflammatory responses and parasite elimination. We suggest that the inflammatory response by glial cells through CCR5 might be associated with neurological injury during T. gondii infection to some extent.

Molecular screening approach to identify protozoan and trichostrongylid parasites infecting one-humped camels (Camelus dromedarius).

Little is known about the diversity of many parasites infecting camels, with most relying on morphological parameters. DNA extracted from different tissues (n = 90) and fecal samples (n = 101) from dromedary camels (Camelus dromedarius) in Egypt were screened for multiple parasites using different molecular markers. Screening of tissue samples (heart) for Toxoplasma gondii and Sarcocystis spp. was performed using B1 and 18S rRNA gene markers, respectively. T. gondii was further genotyped using multiplex multilocus nested PCR-RFLP (Mn-PCR-RFLP). Sarcocystis was analyzed using PCR-RFLP characterization (XbaI and MboI restriction enzymes). A taxonomically challenging but important group of nematodes (Trichostrongylidae family) were screened using the ITS-2 ribosomal DNA (rDNA) species-specific markers. Furthermore, nested PCR was used for the detection of Cryptosporidium spp. (SSU rRNA gene) and positive samples were genotyped after RFLP (SspI and VspI) and sequencing. Cryptosporidium parvum isolates were subtyped by sequence analysis of the 60-kDa glycoprotein gene. This study revealed that many parasites infect the investigated camels, including T. gondii (1.1%), Sarcocystis spp. (64.4%), Cryptosporidium spp. (5.9%) and Trichostrongylidae nematodes (22.7%). The species contribution for nematodes was as follows: Haemonchus spp. (95.6%), Trichostrongylus axei (26%), Trichostrongylus colubriformis (65.2%) and Cooperia oncophora (60.8%). Mn-PCR-RFLP typing for Toxoplasma was only successful for three markers: 5'-SAG2 (type II), 3'-SAG2 (type II) and alt. SAG2 (type II). PCR-RFLP using XbaI showed possible mixed Sarcocystis infection. Moreover, the Cryptosporidium genotypes detected were C. parvum (IIdA19G1 and IIaA15G1R1), Cryptosporidium rat genotype IV and a novel genotype (camel genotype). This approach revealed the unique Cryptosporidium genotypes infecting the investigated camels, and the high genetic diversity of the investigated parasites.

Angiostoma namekuji n. sp. (Nematoda: Angiostomatidae) from terrestrial slugs on Oshiba Island in the Seto Inland Sea, Japan.

A new species of nematode, Angiostoma namekuji n. sp. (Angiostomatidae: Rhabditida), is described from the intestinal lumen of the terrestrial slug Philomycidae gen. sp. collected from Oshiba Island in the Seto Inland Sea, Hiroshima Prefecture, Japan. The new species is recognized by the following characteristics: body length 2,782-3,599 (mean 3,240) µm (male); 4,666-5,532 (5,030) µm (female); lateral field present; pharyngeal corpus with valves in the bulb; male with short tail, c = 35-57 (48), with one denticle; and seven pairs of genital papillae arranged as 1+2/3+1; female with tail having small denticles on distal tip; uterus c.50% of the body size; each ovary long, starting near vulva, not coiled, reflexed and reaching uterus; ovaries not crossing each other. Our phylogenetic tree based on sequences of the nuclear 28S ribosomal RNA gene supported the generic allocation of the new species in Angiostoma Dujardin, 1845.

Neospora caninum Dense Granule Protein 7 Regulates the Pathogenesis of Neosporosis by Modulating Host Immune Response.

Neospora caninum is a protozoan parasite closely related to Toxoplasma gondii Neosporosis caused by N. caninum is considered one of the main causes of abortion in cattle and nervous-system dysfunction in dogs, and identification of the virulence factors of this parasite is important for the development of control measures. Here, we used a luciferase reporter assay to screen the dense granule proteins genes of N. caninum, and we found that NcGRA6, NcGRA7, and NcGRA14 are involved in the activation of the NF-κB, calcium/calcineurin, and cAMP/PKA signals. To analyze the functions of these proteins and Neospora cyclophilin, we successfully knocked out their genes in the Nc1 strain using plasmids containing the CRISPR/Cas9 components. Among the deficient lines, the NcGRA7-deficient parasites showed reduced virulence in mice. An RNA sequencing analysis of infected macrophage cultures showed that NcGRA7 mainly regulates the host cytokine and chemokine production. The levels of gamma interferon in the ascites fluid, CXCL10 expression in the peritoneal cells, and CCL2 expression in the spleen were lower 5 days after infection with the NcGRA7-deficient parasite than after infection with the parental strain. The parasite burden and the degree of necrosis in the brains of mice infected with the NcGRA7-deficient parasite were also lower than in those of the parental strain. Collectively, our data suggest that both the NcGRA7-dependent activation of the inflammatory response and the parasite burden are important in Neospora virulence.IMPORTANCENeospora caninum invades and replicates in a broad range of host species and cells within those hosts. The effector proteins exported by Neospora induce its pathogenesis by modulating the host immunity. We show that most of the transcriptomic effects in N. caninum-infected cells depend upon the activity of NcGRA7. A deficiency in NcGRA7 reduced the virulence of the parasite in mice. This study demonstrates the importance of NcGRA7 in the pathogenesis of neosporosis.

Impact of Perkinsus olseni infection on a wild population of Manila clam Ruditapes philippinarum in Ariake Bay, Japan.

Many studies have addressed the production decline of Manila clam, Ruditapes philippinarum, in Japan, but infection of clams with Perkinsus olseni has received scarce attention. To evaluate the impact of P. olseni, infection levels and host density of a wild, unexploited clam population were monitored monthly or bimonthly on a tidal flat from June 2009 to January 2013. Real-time PCR analysis discriminating P. olseni and Perkinsus honshuensis detected only P. olseni in tested clams. The prevalence of P. olseni was 100% or nearly 100% in 7 cohorts throughout the study period, except in newly recruited clams. Infection intensity remained low and seldom reached 106 cells/g wet tissue in newly recruited clams until the month of September. Infection intensity reached 106 cells/g in autumn and remained high at 104-106 cells/g until each cohort of clams disappeared. Clam density began to decrease in the autumn when the infection intensities reached ca. 106 cells/g. Density was relatively stable in winter, increased in spring and decreased again in clams aged 1 year or older during summer and autumn in the following years. Survival of clams experimentally infected with P. olseni at ca. 106 cells/g and placed in a cage in the tidal flat for 1 or 2 months was significantly lower than survival of uninfected control clams. Our results suggested that heavy infection with P. olseni was a major cause of the clam density decrease, although other environmental and biological factors also may have contributed to the decline in density. In addition, uninfected clams were deployed in cages for 1-2 months from June 2010 to January 2013 and prevalence and infection intensity were monitored. Parasite transmission and infection progression increased in summer and autumn.

Transcriptional profiling of Toll-like receptor 2-deficient primary murine brain cells during Toxoplasma gondii infection.

BACKGROUND: Toxoplasma gondii is capable of persisting in the brain, although it is efficiently eliminated by cellular immune responses in most other sites. While Toll-like receptor 2 (TLR2) reportedly plays important roles in protective immunity against the parasite, the relationship between neurological disorders induced by T. gondii infection and TLR2 function in the brain remains controversial with many unknowns. In this study, primary cultured astrocytes, microglia, neurons, and peritoneal macrophages obtained from wild-type and TLR2-deficient mice were exposed to T. gondii tachyzoites. To characterize TLR2-dependent functional pathways activated in response to T. gondii infection, gene expression of different cell types was profiled by RNA sequencing. RESULTS: During T. gondii infection, a total of 611, 777, 385, and 1105 genes were upregulated in astrocytes, microglia, neurons, and macrophages, respectively, while 163, 1207, 158, and 1274 genes were downregulated, respectively, in a TLR2-dependent manner. Overrepresented Gene Ontology (GO) terms for TLR2-dependently upregulated genes were associated with immune and stress responses in astrocytes, immune responses and developmental processes in microglia, metabolic processes and immune responses in neurons, and metabolic processes and gene expression in macrophages. Overrepresented GO terms for downregulated genes included ion transport and behavior in astrocytes, cell cycle and cell division in microglia, metabolic processes in neurons, and response to stimulus, signaling and cell motility in macrophages. CONCLUSIONS: To our knowledge, this is the first transcriptomic study of TLR2 function across different cell types during T. gondii infection. Results of RNA-sequencing demonstrated roles for TLR2 varied by cell type during T. gondii infection. Our findings facilitate understanding of the detailed relationship between TLR2 and T. gondii infection, and elucidate mechanisms underlying neurological changes during infection.

CCR5 Is Involved in Interruption of Pregnancy in Mice Infected with Toxoplasma gondii during Early Pregnancy.

Toxoplasmosis can cause abortion in pregnant humans and other animals; however, the mechanism of abortion remains unknown. C-C chemokine receptor type 5 (CCR5) is essential for host defense against Toxoplasma gondii infection. To investigate the relationship between CCR5 and abortion in toxoplasmosis, we inoculated wild-type and CCR5-deficient (CCR5-/-) mice with T. gondii tachyzoites intraperitoneally on day 3 of pregnancy (embryonic day 3 [E3]). The pregnancy rate decreased as pregnancy progressed in infected wild-type mice. Histopathologically, no inflammatory lesions were observed in the fetoplacental tissues. Although wild-type mice showed a higher parasite burden at the implantation sites than did CCR5-/- mice at E6 (3 days postinfection [dpi]), T. gondii antigen was detected only in the uterine tissue and not in the fetoplacental tissues. At E8 (5 dpi), the embryos in infected wild-type mice showed poor development compared with those of infected CCR5-/- mice, and apoptosis was observed in poorly developed embryos. Compared to uninfected mice, infected wild-type mice showed increased CCR5 expression at the implantation site at E6 and E8. Furthermore, analyses of mRNA expression in the uterus of nonpregnant and pregnant mice suggested that a lack of the CCR5 gene and the downregulation of tumor necrosis factor alpha (TNF-α) and CCL3 expression at E6 (3 dpi) are important factors for the maintenance of pregnancy following T. gondii infection. These results suggested that CCR5 signaling is involved in embryo loss in T. gondii infection during early pregnancy and that apoptosis is associated with embryo loss rather than direct damage to the fetoplacental tissues.

Seroprevalence of Toxoplasma gondii in humans and pigs in North Sulawesi, Indonesia.

Toxoplasma gondii, an intracellular protozoan parasite, is a major public health concern throughout the world. Importantly, toxoplasmosis has several adverse effects, including neurological and ocular diseases. There are currently no data on the prevalence of T. gondii infection in humans or animals in North Sulawesi, although Indonesia is known to have a high seroprevalence of this parasite. In this study, the prevalence of T. gondii was determined in samples of humans and pigs from North Sulawesi using the latex agglutination test. In total, 856 human were sampled and 58.5% of whom were positive for T. gondii. Although the antibody prevalence in male and female children aged 0-9years was <10%, the prevalence in individuals over 10years old was >40% in both sexes, suggesting that the transmission rate of the parasite to humans is extremely high in this area. However, the overall prevalence of T. gondii in pigs was only 2.3%. Our study indicates a high incidence of T. gondii infection in humans. Therefore, a survey of the prevalence of T. gondii among different infection sources is required to determine the major risk factors for infection in North Sulawesi.

Development of real-time PCR assays for discrimination and quantification of two Perkinsus spp. in the Manila clam Ruditapes philippinarum.

The Manila clam Ruditapes philippinarum is infected with 2 Perkinsus species, Perkinsus olseni and P. honshuensis, in Japan. The latter was described as a new species in Mie Prefecture, Japan, in 2006. Ray's Fluid Thioglycollate Medium (RFTM) assay has been most commonly used to quantify Perkinsus infection. However, this assay cannot discriminate between species that resemble one another morphologically. We developed real-time PCR assays for the specific quantification of P. olseni and P. honshuensis. DNA was extracted using Chelex resin. Cultured P. olseni and P. honshuensis cells were counted and spiked into uninfected clam gill tissue prior to DNA extraction to generate standard curves, which allowed quantification based on the PCR cycle threshold values. We compared the RFTM assay with both real-time PCR assays by quantifying Perkinsus spp. in gill tissue samples from the same individual clams obtained from various localities in Japan. Infection intensities estimated by both assays were significantly correlated (r2 = 0.70). Our results suggest that the prevalence and infection intensity of P. honshuensis are much lower than for P. olseni in Manila clams.