Development of a rapid assay to detect the dinoflagellate Amyloodinium ocellatum using loop-mediated isothermal amplification (LAMP)

Fine structural features of the free-living stages of Amyloodinium ocellatum (Dinoflagellata, Thoracosphaeraceae): A marine fish ectoparasite

Amyloodinium ocellatum is a protozoan parasite that causes amyloodiniosis in marine and brackish water fish, threatening global aquaculture. The present study investigates the morphology and ultrastructure of the free-living stages of A. ocellatum (tomont and dinospore) using light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Dinospores measured 13.03-19.66 μm in length, 12.32-18.71 μm in width, and were laterally flattened. Dinospores had a transverse flagellum for propulsion and a longitudinal flagellum for direction control. The cyst wall had three distinct layers and included cellulose. The outer wall was coated with numerous bacteria. The orange-red speckled eyespot was observed all tomont developmental stages and in the dinospore of A. ocellatum. Tomonts proliferation required successive nuclear division, the formation of new cyst walls, and cytoplasmic segregation. The cytoplasm comprises mainly the matrix, organelles, and inclusions. The matrix was grainy and evenly distributed. In addition to organelles, including mitochondria with tubular cristae, Golgi apparatus, and endoplasmic reticulum, the cytoplasm had starch grains and lipid droplets as inclusions. The A. ocellatum cells lacked chloroplasts. This study provides the first ultrastructural view of the cytoplasmic structure of the free-living stages of A. ocellatum.

Development of LAMP assay for early detection of Yersinia ruckeri in aquaculture

Yersinia ruckeri is the causative agent of yersiniosis or enteric red mouth disease (ERM) that causes significant economic losses in the salmonid aquaculture industry. Due to an increasing number of outbreaks, lack of effective vaccines and the bacteria's ability to survive in the environment for long periods, there is a necessity for novel measures to control ERM. New techniques capable of rapidly detecting Y. ruckeri are critical to aid effective control programs. Molecular methods, like real-time polymerase chain reaction, can detect Y. ruckeri; however, that methodology is not field-deployable and cannot support local decision-making during an outbreak. We present a field-deployable molecular assay using loop mediated isothermal amplification (LAMP) and water filtering method for the detection of Y. ruckeri eDNA from water samples to improve current surveillance methods. The assay was optimised to amplify the glutamine synthetase gene (glnA) of Y. ruckeri in under 20 min. The assay demonstrated high specificity and sensitivity, as it did not amplify any non-target bacteria typically present in water sources. It achieved a limit of detection (LOD) of 0.5 × 10-7 ng/µl, significantly surpassing the LOD of 0.5 × 10-4 ng/µl obtained through conventional polymerase chain reaction (cPCR). When applied to environmental water samples spiked with transformed Escherichia coli containing the G-block of the Yersinia ruckeri (glnA) target gene, the Yr-LAMP method exhibited an analytical sensitivity of 0.08 cells/µl from the initial filtered water sample. Notably, the cumulative time for sample preparation and amplification was under 1 h. The simplicity of the developed field-deployable Yr-LAMP assay makes it suitable as a routine procedure to monitor fish for ERM infection. This will enable informed decision-making on mitigating pathogen prevalence in aquaculture farms.

Fish ectoparasite detection, collection and curation

Fish parasitology is a dynamic and internationally important discipline with numerous biological, ecological and practical applications. We reviewed optimal fish and parasite sampling methods for key ectoparasite phyla (i.e. Ciliophora, Platyhelminthes, Annelida and Arthropoda) as well as recent advances in molecular detection of ectoparasites in aquatic environments. Ideally, fish capture and anaesthesia as well as parasite recovery methods should be validated to eliminate potential sampling bias and inaccuracy in determining ectoparasite population parameters. There are considerable advantages to working with fresh samples and live parasites, when combined with appropriate fixation methods, as sampling using dead or decaying materials can lead to rapid decomposition of soft-bodied parasites and subsequent challenges for identification. Sampling methods differ between target phyla, and sometimes genera, with optimum techniques largely associated with identification of parasite microhabitat and the method of attachment. International advances in fish parasitology can be achieved through the accession of whole specimens and/or molecular voucher specimens (i.e. hologenophores) in curated collections for further study. This approach is now critical for data quality because of the increased application of environmental DNA (eDNA) for the detection and surveillance of parasites in aquatic environments where the whole organism may be unavailable. Optimal fish parasite sampling methods are emphasised to aid repeatability and reliability of parasitological studies that require accurate biodiversity and impact assessments, as well as precise surveillance and diagnostics.

Dinoflagellates Amyloodinium and Ichthyodinium (Dinophyceae), parasites of marine fishes in the South Atlantic Ocean

The morphology and molecular phylogeny of the parasitic dinoflagellates Ichthyodinium chabelardi and Amyloodinium ocellatum was investigated off Brazil (South Atlantic Ocean). This is the first record of Ichthyodinium and the first molecular data of both parasites from the southern hemisphere. I. chabelardi infected the yolk of eggs of wild populations of Argentine anchovy Engraulis anchoita (Engraulidae) and Brazilian sardinella Sardinella brasiliensis (Clupeidae) in different seasons. The small subunit (SSU) rRNA and internal transcribed spacer (ITS) gene sequences were identical and confirmed Ichthyodinium as a host generalist. The new sequences clustered with the type species I. chabelardi from the North Atlantic and environmental sequences from the Pacific Ocean. A second species from the western Pacific remains undescribed. A. ocellatum was isolated from the gills of a cultured cobia Rachycentron canadum after causing mortality. The SSU rRNA gene sequence of the Brazilian isolate was almost identical to those from the northern hemisphere. This suggests a single species with a widespread distribution, although it is uncertain whether the species has a natural pantropical distribution or is the result of artificial distribution due to human-induced fish transport.

Development of a rapid assay to detect the jellyfish Cyanea nozakii using a loop-mediated isothermal amplification method

Blooms of the harmful jellyfish Cyanea nozakii, which are a severe nuisance to fisheries and tourisms, frequently occur in the northern East China Sea, Yellow Sea, and Bohai Sea. To provide early warning of this species, a simple and effective molecular method for identifying C. nozakii was developed using the loop-mediated isothermal amplification method (LAMP). The LAMP assay is highly specific and uses a set of four primers that target six different regions on the mitochondrial cytochrome c oxidase subunit I (COI) gene of C. nozakii. The amplification conditions, including the dNTP and betaine concentrations, the inner primer to outer primer concentration ratio, reaction time and temperature, were optimized. The LAMP assay amplified DNA extracted from tissue samples of C. nozakii but did not amplify DNA from other common scyphozoans and hydrozoans collected in the same region. In addition, the LAMP assay was more sensitive than conventional PCR. Therefore, the established LAMP assay is a sensitive, specific, fast, and easily performed method for detection of C. nozakii at different stages in their life cycle.

Development and evaluation of a real-time fluorogenic loop-mediated isothermal amplification assay integrated on a microfluidic disc chip (on-chip LAMP) for rapid and simultaneous detection of ten pathogenic bacteria in aquatic animals

Rapid, low-cost, and user-friendly strategies are urgently needed for early disease diagnosis and timely treatment, particularly for on-site screening of pathogens in aquaculture. In this study, we successfully developed a real-time fluorogenic loop-mediated isothermal amplification assay integrated on a microfluidic disc chip (on-chip LAMP), which was capable of simultaneously detecting 10 pathogenic bacteria in aquatic animals, i.e., Nocardia seriolae, Pseudomonas putida, Streptococcus iniae, Vibrio alginolyticus, Vibrio anguillarum, Vibrio fluvialis, Vibrio harveyi, Vibrio parahaemolyticus, Vibrio rotiferianus, and Vibrio vulnificus. The assay provided a nearly-automated approach, with only a single pipetting step per chip for sample dispensing. This technique could achieve limits of detection (LOD) ranging from 0.40 to 6.42pg per 1.414μL reaction in less than 30 min. The robust reproducibility was demonstrated by a little variation among duplications for each bacterium with the coefficient of variation (CV) for time to positive (Tp) value less than 0.10. The clinical sensitivity and specificity of this on-chip LAMP assay in detecting field samples were 96.2% and 93.8% by comparison with conventional microbiological methods. Compared with other well-known techniques, on-chip LAMP assay provides low sample and reagent consumption, ease-of-use, accelerated analysis, multiple bacteria and on-site detection, and high reproducibility, indicating that such a technique would be applicable for on-site detection and routine monitoring of multiple pathogens in aquaculture.