Development and validation of an RNA- and DNA-based quantitative PCR assay for determination of Kudoa thyrsites infection levels in Atlantic salmon Salmo salar

Post-mortem 'soft flesh' in three commercial fish species from off Atlantic Morocco associated with the myxosporean parasites Kudoa thyrsites and K. encrasicoli (Myxozoa)

Small pelagic fishes represent one of the most important food resources off the Northwest coast of Africa. Despite their economic significance, little is known about the infections with flesh invading myxosporean parasites of genus Kudoa (Cnidaria, Myxozoa). Heavy infections in the flesh may be associated with post-mortem myoliquefaction, commonly known as 'soft flesh'. This condition may reduce the quality and marketability of the fish fillet, resulting in both economic losses to the fishing industry and loss of consumer confidence. In this study, we investigated Kudoa-induced 'soft flesh' occurrence in European anchovy Engraulis encrasicolus, European pilchard Sardina pilchardus, and Atlantic chub mackerel Scomber colias caught in 2019 off the Moroccan Atlantic coast. Five hundred specimens of each fish species were examined for 'soft flesh' by texture testing and visual inspection 48 h post-catch. 'Soft flesh' occurred in 0.2 % of the European anchovies, 1.4 % of the European pilchard, and in 4.4 % of the Atlantic chub mackerel. Microscopic examination of muscle samples revealed that 'soft flesh'-affected fish were infected with myxospores of K. thyrsites-like morphotype. Analysis of the kudoid SSU rDNA sequence obtained from European pilchard and the Atlantic chub mackerel identified these as K. thyrsites (100 % identity), whereas analysis of the sequence from European anchovy identified the presence of K. encrasicoli (100 % identity). Even if there are no known human health consequences associated with the ingestion of these Kudoa species, the unsightly appearance of some infected fillets is a food quality issue, that can eventually lead to reduced marketability and value.

Increased prevalence and severity of Kudoa thyrsites (Cnidaria: Myxosporea) in Atlantic salmon Salmo salar exposed to deeper seawater

Kudoa thyrsites is a myxozoan parasite of marine fish with a global distribution. In British Columbia (BC), Canada, severe infections are associated with an economically significant degradation of fillet quality in farmed Atlantic salmon. Exposures to naturally occurring actinospores at a coastal research laboratory were used to test the hypothesis that the prevalence and severity of K. thyrsites infections acquired by exposure of Atlantic salmon to seawater (SW) of various depths are not different. In Expt 1, fish were exposed to SW from 1, 7 or 13 m below the surface. Following exposure to deeper-sourced SW, the prevalence of K. thyrsites, determined from microscopic examination of muscle histology sections, was greater in all 4 trials and the severity of infection was greater in 2 trials. In Expt 2, infections were compared over time among salmon held in tanks supplied with deep-sourced SW (raw or UV-irradiated) or in a surface net-pen. The infection was observed in 35 of 40 fish sampled between 3 and 6 mo after tank exposure to raw SW. Coincidentally, the parasite was observed in 4 of 40 fish maintained in the net-pen. No consistent association of the parasite infection was observed with temperature; however, reduced salinity and solar radiation were not ruled out as factors which may reduce the risk of infection from surface SW. These findings require verification at commercial aquaculture sites in BC, as they will inform considerations related to farm siting and net-pen configuration.

Distribution of Kudoa thyrsites (Cnidaria, Myxozoa) myoliquefactive stages in Northeast Atlantic mackerel (Scomber scombrus) inferred from qPCR and histology

Kudoa thyrsites is a myxosporean parasite (Cnidaria, Myxozoa) that infects the skeletal and cardiac muscle of Northeast Atlantic (NEA) mackerel (Scomber scombrus). Heavy infections are associated with post-mortem myoliquefaction of the host skeletal muscle which reduces the quality of the fish product. The biological infection characteristics of the parasite in NEA mackerel are poorly known. This study examined the distribution of K. thyrsites in various organs of NEA mackerel from the northern North Sea, and elucidates the relationship between density of infection, developmental stage and parasite distribution in the musculature, and the extent of visible flesh myoliquefaction. Quantitative polymerase chain reaction (qPCR) data showed that K. thyrsites is unevenly distributed in the somatic musculature of the fish host, with highest density in the anterior ventral muscle sections—the belly flaps. A weak positive correlation was observed between the level of myoliquefaction and the parasite density in the fish host muscle. This relationship was also reflected by the amount and distribution of parasite developmental stages seen during histological examinations. Histological findings indicate an association between the dispersion of free myxospores and the level of myoliquefaction of the fish host muscle. Visceral organs were also found infected using qPCR, although at lower densities compared to the musculature.

Occurrence of Kudoa prunusi and K. lateolabracis (Myxozoa: Myxosporea: Multivalvulida) in Philippine-Sea Japanese parrotfish (Calotomus japonicus)

Multiple Kudoa spp. (Myxozoa: Myxosporea: Multivalvulida) have been recorded in Japanese parrotfish (Calotomus japonicus) from the Philippine Sea (Northwest Pacific Ocean), off southwestern Japan; Kudoa yasunagai in the brain, and K. igami, K. lateolabracis, and K. thalassomi in the muscles. This study examined eight Philippine Sea Japanese parrotfish samples collected in January and February 2019 and found K. prunusi in the brain (3-57 plasmodia/fish; average 17.9) and K. lateolabracis plasmodia in the trunk muscle of all fish individuals examined. The K. prunusi in this study was characterized by myxospores predominatetly with six shell valves (SVs) and a corresponding number of polar capsules (PCs), contrasting with the original description of the species from farmed Pacific bluefin tuna (Thunnus orientalis) brain that characterized the species as having predominately five SVs/PCs. Molecular-genetic characterization of 18S and 28S ribosomal RNA genes and mitochondrial DNA genes (cytochrome c oxidase subunit 1 and small and large ribosomal RNA subunits) clearly differentiated the K. prunusi isolate from K. yasunagai, commonly characterized by six or seven, but rarely five, SVs/PCs myxospores. The Japanese parrotfish is a new host record for K. prunusi and speculated to be an important reservoir host in its natural waters. Kudoa lateolabracis myxospores isolated from pseudocysts in the myofiber were morphologically and phylogenetically close to a clade of the Kudoa spp. that exhibit cruciform myxospores similar to K. thyrsites. This study is the first to sequence a mitochondrial DNA of small and large subunit ribosomal RNA of K. lateolabracis.

Cumulative Effects of Thermal and Fisheries Stressors Reveal Sex-Specific Effects on Infection Development and Early Mortality of Adult Coho Salmon (Oncorhynchus kisutch)

Multiple stressors are commonly encountered by wild animals, but their cumulative effects are poorly understood, especially regarding infection development. We conducted a holding study with repeated gill and blood sampling to characterize the effects of cumulative stressors on infection development in adult coho salmon. Treatments included chronic thermal stress (15°C vs. 10°C) and acute gill net entanglement with an air exposure (simulating fisheries bycatch release). The potential loadings of 35 infectious agents and the expression of 17 host immune genes were quantified using high-throughput quantitative polymerase chain reaction, while host physiology was characterized with chemical analysis of blood. Temporal increases in infectious agent richness and loads were concurrent with decreased expression of immune genes in fish sampled in the river. In the laboratory, mortality was minimal in cool water regardless of fishery treatment (<15%). Elevated water temperature under laboratory conditions increased mortality of males and females (8% and 28% mortality, respectively, delayed by >1 wk) and enhanced mortality associated with handling and biopsy (∼40% both sexes). Experimental gillnetting at high temperature further enhanced female mortality (73%). Fish held at high temperature demonstrated heavier infectious agent loads, osmoregulatory impairment, suppressed female maturation, and upregulation of inflammatory and extracellular immune genes. At high temperature, heavy Parvicapsula minibicornis loads were associated with premature mortality. Females exhibited physiological impairment from both stressors after 1 wk, and infection burdens correlated poorly with immune gene regulation compared with males. Cumulative effects of multiple stressors on female mortality are likely a function of physiological impairment and enhanced infections at high temperature.

A Comparison of Nonlethal and Destructive Methods for Broad-Based Infectious Agent Screening of Chinook Salmon Using High-Throughput qPCR

Molecular tools, such as high-throughput quantitative polymerase chain reaction (HT-qPCR), are useful for monitoring multiple infectious agents in wild animal populations (i.e., broad-based screening). If destructive tissue samples cannot be obtained due to experimental design requirements (e.g., bio-telemetry; holding with repeated biopsy) or the conservation status of host species, then nonlethally sampled tissues can be substituted. However, infection profiles have been found to differ between nonlethally and destructively sampled tissues. We present a comparative analysis of nonlethal (gill and blood) and destructive (pool of internal and external tissue) approaches for broad-based infectious agent screening of adult Chinook Salmon Oncorhynchus tshawytscha. Of a possible 47 agents, 16 were detected overall by nonlethal and destructive methods. Our results indicated moderate differences in infection profiles among tissues, with limitations of each tissue type dependent on the ecology of each agent. The gill was the most comprehensive screening tissue, as more infectious agents were detected overall in gill (n = 16) than in blood (n = 12) or multi-tissue pools (n = 15). The agreement in the estimated agent prevalence between tissue types ranged from poor to excellent, while overall agent community structure (the combined prevalence of all agents) showed low agreement between tissue types. Two agents occurred at 100% prevalence in all tissue types. Nine agents, including types of bacteria and gill parasites, were more prevalent in gill than in blood, while five agents, including one virus and several microparasites, were more prevalent in blood. Future studies should pair microscopy and histopathology with HT-qPCR to better characterize host health and disease development relative to molecular detection of agents across tissue types.

Host size influences prevalence and severity of Kudoa thyrsites (Cnidaria: Myxosporea) infection in Atlantic salmon Salmo salar

Kudoa thyrsites is a cosmopolitan myxozoan parasite of marine fish. The infection causes an economically important myoliquefaction in farmed Atlantic salmon in British Columbia, Canada. Laboratory exposure of Atlantic salmon smolts to infectious seawater was used to test the hypothesis that infection with K. thyrsites is more severe in age-matched, smaller salmon. In each of 2 trials approximately 4 mo apart, smolts were graded into small (80 and 68 g), medium (117 and 100 g) and large (142 and 157 g) initial weight groups (IWGs) and concurrently exposed to infectious seawater. The effects of IWG and time on fish size and infection severity were assessed by linear mixed-effects models. The fish were screened for infection by histological examination at intervals following exposure. Increases in mean length and weight were statistically significant in all IWG during both trials. The infection was detected in fish in both trials, and in Trial 2, the prevalence was significantly greater in larger fish 1000 degree-days (DD) after exposure. The severity of infection (plasmodia mm-2 muscle) was significantly higher in larger smolts: between medium and large IWGs at 2500 DD in Trial 1 and between small and medium IWGs at 1500 and 2000 DD in Trial 2. The hypothesis is rejected and possible explanations for the greater occurrence of K. thyrsites in larger smolts are discussed.

Morphological and molecular identification of a new Kudoa thyrsites isolate in Mediterranean silver scabbardfish Lepidopus caudatus

Myxozoans of the genus Kudoa (Myxosporea, Multivalvulida) infect marine and estuarine fish species worldwide. Some Kudoa species are of concern to the seafood industry since they may generate macroscopic cysts in the fish host's musculature, or cause post mortem myoliquefaction, commonly known as 'soft flesh'. One of the economically most important species is K. thyrsites, a myoliquefactive myxosporean parasite that occurs in many wild and cultured marine fish species worldwide. Here we identified a K. thyrsites isolate as the causative agent of myoliquefaction in silver scabbardfish Lepidopus caudatus from the Alboran Sea (western Mediterranean Sea). For comparative and validation purposes, the morphological and molecular characteristics of the isolate were compared with fresh spores of a K. thyrsites isolate infecting Atlantic mackerel Scomber scombrus from the Norwegian Sea. Myxospores of both isolates shared a stellate appearance and contained 4 unequal pyriform polar capsules (1 large, 1 small and 2 intermediate). These morphological traits were consistent with all other previously described K. thyrsites isolates. Moreover, the small subunit rDNA sequences of the Mediterranean and Norwegian Sea isolates revealed 100% similarity, and matched 100% with K. thyrsites isolates previously recorded in myoliquefactive Atlantic mackerel from the North Sea and off southern England. The findings suggest that K. thyrsites is the primary cause of myoliquefaction in silver scabbardfish from the Alboran Sea. This report represents the first morphological and molecular characterization of K. thyrsites in the Mediterranean Sea. A set of new allometric characters is proposed as additional descriptors for more accurate and specific description of kudoid myxospores.

Infection dynamics of Kudoa inornata (Cnidaria: Myxosporea) in spotted seatrout Cynoscion nebulosus (Teleostei: Sciaenidae)

Kudoa inornata is a myxosporean parasite that develops in the somatic muscle of spotted seatrout Cynoscion nebulosus, an economically and ecologically important fish in estuaries and harbors in southeastern North America. In South Carolina (SC), USA, over 90% of wild adult spotted seatrout are infected. To inform potential mitigation strategies, we conducted 3 experiments using naïve sentinel seatrout and infectious stages of K. inornata naturally present in raw water from Charleston Harbor, SC, to determine (1) if K. inornata infection follows a seasonal pattern, and (2) how long it takes for myxospores to develop in fish muscle. Infection by K. inornata was determined by visual detection of myxospores in fish muscle squashes, and any visually negative samples were then assayed for K. inornata ribosomal DNA using novel parasite-specific PCR primers. We observed that K. inornata infection in seatrout followed a seasonal pattern, with high prevalence when water temperature was highest (27-31°C; July-September) and infections that were either covert (at ~13-15°C) or not detected (<13°C) at the lowest water temperatures in January-February. Myxospore development occurred within 476 degree-days, i.e. 2 wk in a typical SC summer. Infection was dependent on fish density, which limited presumptive actinospore dose. Our findings suggest that the life cycle of the parasite may be disrupted by preventing spore-rich seatrout carcasses (e.g. at angler cleaning stations) being thrown back into harbors and estuaries throughout the year.

Capture severity, infectious disease processes and sex influence post-release mortality of sockeye salmon bycatch

Bycatch is a common occurrence in heavily fished areas such as the Fraser River, British Columbia, where fisheries target returning adult Pacific salmon (Oncorhynchus spp.) en route to spawning grounds. The extent to which these encounters reduce fish survival through injury and physiological impairment depends on multiple factors including capture severity, river temperature and infectious agents. In an effort to characterize the mechanisms of post-release mortality and address fishery and managerial concerns regarding specific regulations, wild-caught Early Stuart sockeye salmon (Oncorhynchus nerka) were exposed to either mild (20 s) or severe (20 min) gillnet entanglement and then held at ecologically relevant temperatures throughout their period of river migration (mid-late July) and spawning (early August). Individuals were biopsy sampled immediately after entanglement and at death to measure indicators of stress and immunity, and the infection intensity of 44 potential pathogens. Biopsy alone increased mortality (males: 33%, females: 60%) when compared with non-biopsied controls (males: 7%, females: 15%), indicating high sensitivity to any handling during river migration, especially among females. Mortality did not occur until 5-10 days after entanglement, with severe entanglement resulting in the greatest mortality (males: 62%, females: 90%), followed by mild entanglement (males: 44%, females: 70%). Infection intensities of Flavobacterium psychrophilum and Ceratonova shasta measured at death were greater in fish that died sooner. Physiological indicators of host stress and immunity also differed depending on longevity, and indicated anaerobic metabolism, osmoregulatory failure and altered immune gene regulation in premature mortalities. Together, these results implicate latent effects of entanglement, especially among females, resulting in mortality days or weeks after release. Although any entanglement is potentially detrimental, reducing entanglement durations can improve post-release survival.

Long-term epidemiological survey of Kudoa thyrsites (Myxozoa) in Atlantic salmon (Salmo salar L.) from commercial aquaculture farms

Kudoa thyrsites (Myxozoa) encysts within myocytes of a variety of fishes. While infected fish appear unharmed, parasite-derived enzymes degrade the flesh post-mortem. In regions of British Columbia (BC), Canada, up to 4-7% of fillets can be affected, thus having economic consequences and impacting the competitiveness of BC's farms. K. thyrsites was monitored in two farms having high (HP) or low (LP) historical infection prevalence. At each farm, 30 fish were sampled monthly for blood and muscle during the first year followed by nine samplings during year two. Prevalence and intensity were measured by PCR and histology of muscle samples. In parallel, fillet tests were used to quantify myoliquefaction. Infections were detected by PCR after 355 and 509 degree days at LP and HP farms, respectively. Prevalence reached 100% at the HP farm by 2265 degree days and declined during the second year, whereas it plateaued near 50% at the LP farm. Infection intensities decreased after 1 year at both farms. Blood was PCR-positive at both farms between 778 and 1113 degree days and again after 2000 degree days. This is the first monitoring project in a production environment and compares data between farms with different prevalence.

KUDOA SP. (Myxozoa: Multivalvulida) in skeletal muscle of captive bullnose eagle rays, Myliobatis freminvillei (Rajiformes: Myliobatidae)

Kudoa spp. (Myxozoa: Multivalvulida) are of significant concern in marine and estuarine teleosts because of trophozoites and spores in the skeletal muscle reducing the marketability of wild and cultured fish. Kudoa spp. have rarely been reported from elasmobranchs. Spores of a Kudoa sp. were identified in two captive, mature bullnose eagle rays (Myliobatisfreminvillei) (Rajiformes: Myliobatidae). The male bullnose eagle ray (case 1) showed intractable weight loss, muscle wasting, and hyperkalemia, and was euthanized. The skeletal muscle showed myofiber degeneration, vacuolation, and necrosis, and spores of a Kudoa sp. were present multifocally through the muscle. The female bullnose eagle ray (case 2) had a severe, rapidly progressive, erosive lesion extending through the pectoral fin and was euthanized. Spores of a Kudoa sp. were not found in the lesion but were present in two disparate sections of skeletal muscle. There was no inflammation or degeneration associated with the infected myofibers in case 2. From case 1, morphometrics of 15 spores in histologic section were apical width 6.5-8.0 μm, apical thickness 5.0-7.0 μm, lateral width 5.5-6.5 μm, length 4.5-6.5 μm; the equal-sized polar capsules were spherical, 1.5-2.5 μm in diameter; there was a clear halo 2 μm thick around the spores, considered to be a mucous envelope. Spores from case 2 were indistinguishable from those in case 1. This appears to be the first report of Kudoa sp. in bullnose eagle rays.

Characterization of the ribosomal RNA gene of Kudoa neothunni (Myxosporea: Multivalvulida) in tunas (Thunnus spp.) and Kudoa scomberi n. sp. in a chub mackerel (Scomber japonicus)

Kudoa neothunni is the first described Kudoa species having six shell valves and polar capsules, previously assigned to the genus Hexacapsula Arai and Matsumoto, 1953. Since its genetic analyses remain to be conducted, the present study characterizes the ribosomal RNA gene (rDNA) using two isolates from a yellowfin tuna (Thunnus albacares) with post-harvest myoliquefaction and a northern bluefin tuna (Thunnus thynnus) without tissue degradation. Spores of the two isolates localized in the myofiber of trunk muscles, forming pseudocysts, and showed typical morphology of K. neothunni with six equal-sized shell valves radially arranged in apical view: spores (n = 15) measuring 9.5-11.4 μm in width, 7.3-8.6 μm in suture width, 8.9-10.9 μm in thickness, and 7.3-7.7 μm in length; and polar capsules measuring 3.6-4.1 μm by 1.8-2.3 μm. In lateral view, the spores were pyramidal in shape without apical protrusions. Their 18S and 5.8S rDNA sequences were essentially identical, but variations in the ITS1 (62.4 % similarity across 757-bp length), ITS2 (66.9 % similarity across 599-bp length), and 28S (99.0 % similarity across 2,245-bp length) rDNA regions existed between the two isolates. On phylogenetic trees based on the 18S or 28S rDNA sequence, K. neothunni formed a clade with Kudoa spp. with more than four shell valves and polar capsules, particularly K. grammatorcyni and K. scomberomori. Semiquadrate spores of a kudoid species with four shell valves and polar capsules were detected from minute cysts (0.30-0.75 mm by 0.20-0.40 mm) embedded in the trunk muscle of a chub mackerel (Scomber japonicus) fished in the Sea of Japan. Morphologically, it resembled K. caudata described from a chub mackerel fished in the southeastern Pacific Ocean off Peru; however, it lacked filamentous projections on the shell valves of spores. Additionally, it morphologically resembled K. thunni described from a yellowfin tuna also fished in the Pacific Ocean; spores (n = 30) measuring 8.2-10.5 μm in width, 7.0-8.8 μm in thickness, and 6.1-6.8 μm in length; and polar capsule measuring 2.5-3.4 μm by 1.3-2.0 μm. The similarities of the 18S and 28S rDNA sequences between these two species were 98.5 % and 96.3 %, respectively. Simultaneously, the dimensions of cysts in the trunk muscle formed by K. thunni are clearly larger than those of the present species from a chub mackerel: 1.3-2.0 mm by 1.1-1.4 mm (n = 14) vs. 0.30-0.75 mm by 0.20-0.40 mm (n = 7), respectively. Thus, Kudoa scomberi n. sp. is proposed for this multivalvulid species found in the chub mackerel.

Development of a quantitative polymerase chain reaction assay for detection of Kudoa septempunctata in olive flounder (Paralichthys olivaceus)

Kudoa septempunctata is a newly identified myxosporean parasite that infects the trunk muscles of olive flounder (Paralichthys olivaceus) and a causative agent of the increasing number of foodborne gastroenteritis outbreaks with unknown etiology which have occurred in Japan over the last few years. Here, we developed a quantitative polymerase chain reaction (QPCR) assay for the detection of K. septempunctata 18S rDNA in olive flounder muscle tissue samples. Additionally, we compared the relative efficacy of four DNA extraction methods, including two commercial kits, and assessed intrafish variability in the distribution of K. septempunctata spores in flounder using this QPCR method in order to establish a more accurate quantitative measurement. Our QPCR assay displayed high sensitivity, specificity, and reproducibility, and had good correlation with a microscopic detection method. Our data also indicated that the DNeasy® Blood & Tissue Kit was more efficient method for the extraction of K. septempunctata DNA than the three other methods (heating, alkaline lysis, and FastDNA® SPIN Kit method). We believe that our method would be useful for investigating foodborne outbreaks caused by K. septempunctata and for the monitoring and quantification of this parasite in retail or aquacultured olive flounders to prevent such outbreaks.

Quantitative polymerase chain reaction (PCR) for detection of aquatic animal pathogens in a diagnostic laboratory setting

Real-time, or quantitative, polymerase chain reaction (qPCR) is quickly supplanting other molecular methods for detecting the nucleic acids of human and other animal pathogens owing to the speed and robustness of the technology. As the aquatic animal health community moves toward implementing national diagnostic testing schemes, it will need to evaluate how qPCR technology should be employed. This review outlines the basic principles of qPCR technology, considerations for assay development, standards and controls, assay performance, diagnostic validation, implementation in the diagnostic laboratory, and quality assurance and control measures. These factors are fundamental for ensuring the validity of qPCR assay results obtained in the diagnostic laboratory setting.

Influence of host reproductive state onSphaerothecum destruensprevalence and infection level

Sphaerothecum destruensis an obligate intracellular parasite with the potential to cause high mortalities and spawning inhibition in the endangered cyprinidLeucaspius delineatus. We investigated the influence ofL. delineatus’s reproductive state on the prevalence and infection level ofS. destruens. A novel real time quantitative polymerarse chain reaction (qPCR) was developed to determineS. destruens’ prevalence and infection level. These parameters were quantified and compared in reproductive and non-reproductiveL. delineatus. The detection limit of theS. destruensspecific qPCR was determined to be 1 pg of purifiedS. destruensgenomic DNA. Following cohabitation in the lab, reproductiveL. delineatushad a significantly higherS. destruensprevalence (P<0·05) and infection levels (P<0·01) compared to non-reproductiveL. delineatus. S. destruensprevalence was 19% (n=40) in non-reproductiveL. delineatusand 41% (n=32) in reproductiveL. delineatus. However, there was no difference inS. destruensprevalence in reproductive and non-reproductive fish under field conditions. Mean infection levels were 18 and 99 pgS. destruensDNA per 250 ngL. delineatusDNA for non-reproductive and reproductiveL. delineatusrespectively. The present work indicates thatS. destruensinfection inL. delineatuscan be influenced by the latter's reproductive state and provides further support for the potential adverse impact ofS. destruenson the conservation ofL. delineatuspopulations.

Development and validation of a quantitative PCR to detect Parvicapsula minibicornis and comparison to histologically ranked infection of juvenile Chinook salmon, Oncorhynchus tshawytscha (Walbaum), from the Klamath River, USA

Parvicapsula minibicornis is a myxosporean parasite that is associated with disease in Pacific salmon during their freshwater life history phase. This study reports the development of a quantitative (real-time) polymerase chain reaction (QPCR) to detect P. minibicornis DNA. The QPCR assay targets the 18S ribosomal subunit gene. A plasmid DNA control was developed to calibrate cycle threshold (C(T)) score to plasmid molecular equivalent (PME) units, a measure of gene copy number. Assay validation revealed that the QPCR was sensitive and able to detect 50 ag of plasmid DNA, which was equivalent to 12.5 PME. The QPCR assay could detect single P. minibicornis actinospores well above assay sensitivity, indicating a single spore contains at least 100 times the 18S DNA copies required for detection. The QPCR assay was repeatable and highly specific; no detectable amplification was observed using DNA from related myxozoan parasites. The method was validated using kidney tissues from 218 juvenile Chinook salmon sampled during the emigration period of March to July 2005 from the Klamath River. The QPCR assay was compared with histological examination. The QPCR assay detected P. minibicornis infection in 88.1% of the fish sampled, while histological examination detected infection in 71.1% of the fish sampled. Good concordance was found between the methods as 80% of the samples were in agreement. The majority of the disconcordant fish were positive by QPCR, with low levels of P. minibicornis DNA, but negative by histology. The majority of the fish rated histologically as having subclinical or clinical infections had high QPCR levels. The results of this study demonstrate that QPCR is a sensitive quantitative tool for evaluating P. minibicornis infection in fish health monitoring studies.