Development of specific PCR assays for the detection of Cryptocaryon irritans

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.

Transcriptomic Variation of Amphiprion Percula (Lacepède, 1802) in Response to Infection with Cryptocaryon Irritans Brown, 1951

Cryptocaryon irritans (Brown 1951) frequently infect the Pomacentridae fishes causing severe economic losses. However, the anti-C. irritans' molecular mechanism in these fishes remains largely unknown. To address this issue, we conducted RNA-Seq for C. irrtians-infected gills of the clownfish Amphiprion percula (Lacepède 1802) at the early (day 1) and late (day 3) stages of infection. A total of 1655 differentially expressed genes (DEGs) were identified. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses of DEGs showed a vast genetic variation related to the following aspects: ECM-receptor interaction, P13K-Akt signalling, cytokine-cytokine receptor interaction, and endocytosis. During the early phase of infection, key genes involved in ATP production, energy homeostasis, and stress control were abruptly increased. In the late phase, however, acute response molecules of the peripheral nervous system (synaptic transmission and local immunity), metabolic system triggering glycogen synthesis, energy maintenance, and osmoregulation were found to be critical. The highest number of upregulated genes (URGs) recovered during the early phase was included under the 'biological process' category, which primarily functions as response to stimuli, signalling, and biological regulation. In the late phase, most of the URGs were related to gene regulation and immune system processes under 'molecular function' category. The immune-related URGs of early infection include major histocompatibility complex (MHC) class-II molecules apparently triggering CD4+ T-cell-activated Th responses, and that of late infection include MHC class-1 molecules for the possible culmination of CD8+ T-cell triggered cytotoxicity. The high level of genic single nucleotide polymorphisms (SNPs) identified during the late phase of infection is likely to influence their susceptibility to secondary infection. In summary, the identified DEGs and their related metabolic and immune-related pathways and the SNPs may provide new insights into coordinating the immunological events and improving resistance in Pomacentridae fishes against C. irritans.

Host responses against the fish parasitizing ciliate Cryptocaryon irritans

The parasitic ciliate Cryptocaryon irritans, which infects almost all marine fish species occurring in both tropical and subtropical regions throughout the world. The disease, cryptocaryonosis, accounts for significant economic losses to the aquaculture industry. This review attempts to provide a comprehensive overview of the biology of the parasite, host-parasite interactions and both specific and non-specific host defense mechanisms are responsible for the protection of fish against challenge infections with this ciliate. Also, this article reflects the current interest in this subject area and the quest to develop an available vaccine against the disease. Due to the high frequency of clinical fish cryptocaryonosis, the study of fish immune responses to C. irritans provides an optimal experimental model for understanding immunity against extracellular protozoa.

Outbreak of Cryptocaryon irritans infection in silver pomfret Pampus argenteus cultured in China

Silver pomfret Pampus argenteus is a major cultivated marine fish species with a high market value. In summer 2021, Cryptocaryon irritans, a ciliate parasite, infected the cultured silver pomfret in aquaculture ponds in Ningbo, Zhejiang Province, China. The symptoms of infected fish include white spots on the skin and fins, increased body surface mucus, loss of appetite, irritability, and shedding of scales. After collecting white spots from moribund fish, the 18S ribosomal RNA sequence of the pathogen on the fish skin was amplified by PCR; phylogenetic analysis showed that it was closely related to C. irritans strains from Ningde, Fujian, China. Four groups of silver pomfret were tested in an artificial infection experiment over the course of 72 h, consisting of 3 infected groups (1600, 4000, and 8000 theronts fish-1) and 1 healthy group. White spots were observed on the skin and fins of the infected fish, but not on their gills. Samples were taken from the gills, liver, kidney, and spleen of both infected and healthy fish and were compared to evaluate any significant histopathological differences. As the dose of infection increased, symptoms became more pronounced. At 72 h, mortality rates were 8.3, 50, and 66.7% for the 3 different concentrations, respectively. The median lethal concentration was calculated to be 366 theronts g-1 at 72 h, 298 theronts g-1 at 84 h, and 219 theronts g-1 at 96 h. This study emphasizes the importance of developing early diagnosis methods and appropriate prevention strategies to decrease the impact of C. irritans infection in the silver pomfret aquaculture industry.

Quantitative detection of parasitic ciliate Cryptocaryon irritans in seawater with an optimized sample processing method

The protozoan Cryptocaryon irritans is one of the most important ectoparasites of marine fish, causing 'white spot disease' and mass mortality in aquaculture. To accurately predict disease outbreaks and develop prevention strategies, improved detection methods are required that are sensitive, convenient and rapid. In this study, a pair of specific primers based on the C. irritans 18S rRNA gene was developed and used in a real-time PCR (qPCR) assay. This assay was able to detect five theronts in 1 L of natural seawater. Furthermore, a linear model was established to analyse the log of Ct value and parasite abundance in seawater (y = -2.9623x + 24.2930), and the coefficient of determination (R² ) value was 0.979. A lysis buffer was optimized for theront DNA extraction and used for storage sample. This method was superior to the commercial water DNA kit, and there was no significant degradation of DNA at room temperature for 24-96 hr. A dilution method was developed to manage qPCR inhibitors and used to investigate natural seawater samples in a net cage farm with diseased fish, and the findings were consistent with the actual situation. This study provides a valuable tool for assisting in the early monitoring and control of cryptocaryoniasis in aquaculture.

Updating specific PCR primer for detection of Cryptocaryon irritans from reared Larimichthys polyactis

Artificial breeding of small yellow croaker (Larimichthys polyactis) was recently achieved, providing a bright future for its commercial farming. In May 2019, a disease outbreak occurred among small yellow croakers in an aquaculture farm near Xiangshan Bay, charactering by white spots spotted on the surface of fish skin, gills and fins. The parasite was preliminarily identified as Cryptocaryon irritans based on morphological feature of the parasite and the symptoms on fish. However, the previously published specific primer pairs failed to confirm the existence of C. iriitans. Six nucleotides mismatches were discovered after mapping specific forward primer back to targeted gene. Therefore, an updated PCR specific primer was developed within the 9th highly variable region of 18S rRNA gene and conserved in all C. irritans sequences available in GenBank database. The specificity was verified in silico by Primer-BLAST against GenBank nucleotide. Laboratory cultured ciliates (Mesanophrys, Pseudokeronopsis and Uronema) as well as natural microbial community samples collected from sea water and river water was used as negative control to verify the specificity of the primer in situ. Besides, tank transfer method was used to evaluate the treatment of the parasite infection. By tank transfer method, 2.00 ± 0.61 out of 10 fish that already sever infected were successfully survived after 8 days treatment, meanwhile the control group died out at d 6. More loss to the treatment group during first five days was observed and may attribute to the combined effect from infection and stress the recent domesticated fish suffered during rotation. Therefore, tank transfer method was also effective to prevent small yellow croaker from further infection, however the loss of the small yellow croaker suffered from stress during rotation also needs to be carefully concerned. In conclusion, this study reported the first diagnose of C. irritans infection on small yellow croaker, provided updated specific primer to detect C. irritans infection on fish body and reported the effect of tank transfer on small yellow croaker treatment.

Health assessment of the cleaner fish ballan wrasse Labrus bergylta from the British south-west coast

Wild-caught ballan wrasse Labrus bergylta are translocated en masse from the British south-west coast to Scotland for use as cleaner fish to tackle Atlantic salmon Salmo salar sea lice infestations; however, very little is known about the background health status of this species. This is the first health assessment of wild ballan wrasse from the British south-west. Wild-caught ballan wrasse (n = 75) from coastal populations off Dorset and Cornwall were subjected to a full health screen for viral, bacterial and parasitic infections and associated pathology. A range of metazoan and protozoan parasites were observed in histological sections, including copepods (sea lice Caligus centrodonti), nematodes, cestodes, digenean metacercariae, Cryptocaryon-like ciliates and an intestinal coccidian (Eimeria sp.) observed in 26.6% of the samples. The mycoplasma Acholeplasma laidlawii was associated with cytopathic effect in cell culture inoculated with tissue homogenates. The opportunistic pathogen Photobacterium damselae damselae was isolated from a single fish with a systemic infection. The isolate was confirmed to possess the virulence factors hlyAch and plpV, previously associated with cell toxicity and pathogenicity to fish. There are no immediate concerns for the continued mass translation of ballan wrasse, however careful monitoring of the population is recommended.

Cryptocaryon irritans recombinant proteins as potential antigens for sero-surveillance of cryptocaryonosis

Cryptocaryonosis is a major problem for mariculture, and the absence of suitable sero-surveillance tools for the detection of cryptocaryonosis makes it difficult to screen Cryptocaryon irritans-infected fish, particularly asymptomatic fish. In this study, we proposed a serum-based assay using selected C. irritans proteins to screen infected and asymptomatic fish. Eight highly expressed genes were chosen from an earlier study on C. irritans expressed sequence tags and ciliate glutamine codons were converted to universal glutamine codons. The chemically synthesized C. irritans genes were then expressed in an Escherichia coli expression host under optimized conditions. Five C. irritans proteins were successfully expressed in E. coli and purified by affinity chromatography. These proteins were used as antigens in an enzyme-linked immunosorbent assay (ELISA) to screen sera from experimentally immunized fish and naturally infected fish. Sera from both categories of fish reacted equally well with the expressed C. irritans recombinant proteins as well as with sonicated theronts. This study demonstrated the utility of producing ciliate recombinant proteins in a heterologous expression host. An ELISA was successfully developed to diagnose infected and asymptomatic fish using the recombinant proteins as antigens.

Elicited cross-protection and specific antibodies in Mozambique tilapia (Oreochromis mossambicus) against two different immobilization serotypes of Cryptocaryon irritans isolated in Hawaii

The objective of this study was to determine whether immunization of Mozambique tilapia with different Cryptocaryon irritans i-antigen serotypes elicited cross-protection against challenge infection by both serotypes. Fish were directly exposed to live theronts of isolate W1 or isolate K1, that express different surface i-antigens. There was no significant difference in the number of trophonts infecting the fish between the two isolates, W1 and K1, following primary exposure. Serum from immunized fish exposed to live theronts showed higher immobilization titres and ELISA values against homologous isolates than to heterologous isolates after the primary exposure. However, mucus antibody did not immobilize theronts although the ELISA results clearly indicated that mucus antibodies recognizing C. irritans were generated. In a study with Western blot analyses, serum antibodies recognized only an antigen of the corresponding serotype and no proteins common to both serotypes were identified. Sequence analyses of 754 bases of rDNA nucleotide sequence including complete nuclear ribosomal ITS-1-5.8S rDNA-ITS-2 region were conducted and found to be identical for W1- and K1-isolates. These findings confirmed that both isolates were members of the species, C. irritans, and that rDNA analysis would not distinguish the two isolates. In conclusion, despite the fact that the immobilization assays and ELISA detected two serotypes in vitro, challenge assays provided evidence for only one type of C. irritans.

Sequence homogeneity of internal transcribed spacer rDNA in Mikrocytos mackini and detection of Mikrocytos sp. in a new location

Mikrocytos mackini is a microcell parasite of Pacific oysters only known to occur on the Pacific coast of North America. It is the only described species in the genus, although a genetically divergent Mikrocytos sp. organism has been reported once in both the Atlantic Ocean and China. We developed methods for sequencing the internal transcribed spacer (ITS) of rDNA for the purpose of characterizing extant diversity within M. mackini throughout its known geographic range, and surveying for other evidence of Mikrocytos sp. organisms. Our specific aims were to examine relatedness of M. mackini among sites to make inferences about its recent evolutionary history, and to provide baseline data for future development of a species-specific molecular detection method. We found a total lack of genetic variation within M. mackini across the complete ITS1-5.8S-ITS2 array in over 70 samples collected throughout its range. We hypothesize that this could be a result of a founder effect if the parasite had been introduced into its known range alongside its host, which was imported from Asia beginning around 1914 to about 1961. We detected a single divergent sequence at a short stretch of 18S that was identical to the Mikrocytos sp. detected elsewhere, which adds to the recent and growing body of evidence that Mikrocytos is much more broadly distributed than the limited range of M. mackini suggests. A 1903 bp section of rDNA from Mikrocytos sp. was generated that contained regions of high divergence from M. mackini (in ITS1 and ITS2) that could be exploited for molecular diagnostics.