Amyloodiniosis in cultured Dicentrarchus labrax: parasitological and molecular diagnosis, and an improved treatment protocol

Inactivation in vitro of the marine parasite Amyloodinium ocellatum

The ectoparasite Amyloodinium ocellatum is a dinoflagellate that causes severe morbidity and mortality in both brackish and marine warmwater aquaculture fishes worldwide. A. ocellatum has a triphasic life cycle based on a free-living flagellate (the dinospore), a parasitic stage (the trophont) and a resting and reproductive cyst (the tomont). Current chemical treatments have shown some efficacy in eliminating dinospores but fail to inactivate the tomonts. Here we evaluated the efficacy of alternative treatments in vitro through sporulation tests and the quantification of dinospore production and motility. Hydrogen peroxide and peracetic acid efficiently decreased dinospore production at low concentrations, but total inactivation of tomonts was only achieved with higher dosages. Tomont inactivation was also observed with disinfectants such as sodium hypochlorite and Virkon S. This work provides insights into effective and environmentally friendly alternatives for the elimination of resistant forms of the marine parasite A. ocellatum.

Study on amyloodiniosis outbreak in captive-bred percula clownfish (Amphiprion percula) and improved control regimens

The amyloodiniosis outbreak was documented with high mortality of percula clownfish (Amphiprion percula) and subsequently, various therapeutics were evaluated to control the infection. The affected fish exhibited symptoms such as discoloration of the skin and jerky movement with severe respiratory stress. Microscopic examination of the gill and body surface of the infested moribund fish showed the presence of brown coloured spherical shaped trophonts with the size range of 150-300 µM. Copper sulphate (10 ppm) and hydroxychloroquine phosphate (10 ppm) treatment with 14 days of continuous bath showed 76 ± 5.3 and 66 ± 4.3% survival rates. However, formalin (10 ppm) and malachite green (10 ppm) treated groups showed a survival of 41 ± 1.7 and 32.7 ± 1.2% respectively. The present results suggest that, use of copper sulphate to treat amyloodiniosis in clownfish will relatively more effective than other treatment options. The findings will be helpful to mitigate amyloodiniosis in marine finfish aquaculture, particularly marine ornamentals.

Investigating the etiologies behind emergent mass mortalities of farmed Liza carinata juveniles from coastal farms at Damietta, Egypt

This study aimed to identify the mortality present in private fish farm Amyloodinium ocellatum and Cryptocaryon irritans were isolated from this outbreak affecting Liza carinata fingerlings at an earthen-based aquaculture facility in Damietta, Egypt. A total of 140 moribunds, L. carinata, were collected from the fish ponds during the mortality events. Physico-chemical analysis of water was analyzed. The skin, fins, gills, and eyes of each fish specimen were scraped gently onto slides in areas over 2 cm area. All smears were examined separately under the light microscope. Molecular identification of the parasites using analysis of ITS rDNA regions flanking both 18S and 28S rDNA genes of Amyloodinium protozoa and C. irritans. Identities of the detected parasites were confirmed by gene sequence and phylogenetic analysis. The majority of the examined fish (90%) were infected, 66.42% had a mixed infection, and 23.57% had a single infection either with A. ocellatum (10.71%) or C. irritans (12.85%).The mean intensity of A. ocellatum was 16.5 ± 2.03 in the skin and 13.18 ± 1.90 in the gills of infected fish, while that of C. irritans was 4.75 ± 1.05 in gills and 7.43 ± 1.45 in the skin, respectively. To control the emergent mortalities, affected ponds were treated using copper sulfate pentahydrate, hydrogen peroxides solutions, and amprolium hydrochloride powder in feed. Fish across the treated ponds were gradually improved with low morbidity and mortalityrates during the treatment period. The clinical disease was almost diminished at the end of the second week of treatment. Coinciding with the clinical improvement of the treated juveniles, microscopical examination of skin/gill scraps exhibited a marked decline in the number of protozoan parasites at the end of the second week of treatment.

Skin transcriptomic analysis and immune-related gene expression of golden pompano (Trachinotus ovatus) after Amyloodinium ocellatum infection

Amyloodiniosis is a severe disease of marine and brackish water fish caused by Amyloodinium ocellatum. Golden pompano (Trachinotus ovatus) is often repeatedly infected by A. ocellatum, leading to extensive mortality. However, little is known about the immune response mechanisms of the T. ovatus following reinfection with A. ocellatum. In this study, an extensive analysis at the transcriptome level of T. ovatus skin was carried out at 24 h post-infection by A. ocellatum. During the transcriptomic analysis, 1367 differentially expressed genes (DEGs) in the skin of T. ovatus under A. ocellatum infection and control conditions were obtained. In Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway annotated analyses, the DEGs were significantly enriched in the immune-related pathways. To better understand the immune-related gene expression dynamics, a quantitative reverse transcription-polymerase chain reaction (RT-qPCR) was used to assess the primary and secondary infection groups of T. ovatus at different stages (3 h, 12 h, 24 h, 48 h and, 72 h post-infection) of infection with A.ocellatum. The results showed that innate immunity-related genes [interleukin (IL-8), chemokine ligand 3 (CCL3), toll-like receptor 7 (TLR7), and G-type lysosome (LZM g)] and adaptive immunity-related gene [major histocompatibility complex (MHC) alpha antigen I and MHC alpha antigen II] expression levels in the primary and secondary infection groups were significantly increased compared to the control group. The expression of MHC I and MHC II was more rapidly upregulated in the secondary infection group compared with the primary infection group after A.ocellatum infection. However, no significant differences of A.ocellatum load were observed in primary and secondary infection groups. In addition, the serum of the primary infection group had significantly higher concentrations of triglyceride (TG), higher alanine transaminase (ALT), aspartate transaminase (AST), and lactate dehydrogenase (LDH) activities than the control group. This study contributes to understanding the defense mechanisms in fish skin against ectoparasite infection.

Cytotoxic and Hemolytic Activities of Extracts of the Fish Parasite Dinoflagellate Amyloodinium ocellatum

The dinoflagellate Amyloodinium ocellatum is the etiological agent of a parasitic disease named amyloodiniosis. Mortalities of diseased fish are usually attributed to anoxia, osmoregulatory impairment, or opportunistic bacterial infections. Nevertheless, the phylogenetic proximity of A. ocellatum to a group of toxin-producing dinoflagellates from Pfiesteria, Parvodinium and Paulsenella genera suggests that it may produce toxin-like compounds, adding a new dimension to the possible cause of mortalities in A. ocellatum outbreaks. To address this question, extracts prepared from different life stages of the parasite were tested in vitro for cytotoxic effects using two cell lines derived from branchial arches (ABSa15) and the caudal fin (CFSa1) of the gilthead seabream (Sparus aurata), and for hemolytic effects using erythrocytes purified from the blood of gilthead seabream juveniles. Cytotoxicity and a strong hemolytic effect, similar to those observed for Karlodinium toxins, were observed for the less polar extracts of the parasitic stage (trophont). A similar trend was observed for the less polar extracts of the infective stage (dinospores), although cell viability was only affected in the ABSa15 line. These results suggest that A. ocellatum produces tissue-specific toxic compounds that may have a role in the attachment of the dinospores’ and trophonts’ feeding process.

Mass kills in hatchery-reared European seabass (Dicentrarchus labrax) triggered by concomitant infections of Amyloodinium ocellatum and Vibrio alginolyticus

Amyloodiniosis and vibriosis are serious diseases in European seabass (Dicentrarchus labrax) hatcheries with noticeable high mortality. This study was conducted on tank-cultured D. labrax frys at a private marine hatchery near Mariout Lake (Alexandria, Egypt). Frys showed a high mortality rate (70%), lethargy, darkening, asphyxia, ascites, and velvety skin appearance. Both infectious agents were presumptively identified in all investigated frys. The identities of the two recovered agents were confirmed by molecular assay and phylogenetic analysis. On the tissue level, histopathological examination of skin, splenic, and renal tissue indicated severe alterations due to the direct impacts of both infections. On the cellular level, scanning electron micrographs showed both protozoal and bacterial pathogens on/in gill epithelial cells in solitary and colonial forms. Vibrio alginolyticus showed variable results for tested antibiotics, with a higher sensitivity to florfenicol. A successful control strategy was strictly adopted to overcome infections and stop mortalities. Copper sulphate and hydrogen peroxide were efficiently applied to tank water to overcome A. ocellatum infections. Further, florfenicol was effectively used to overcome systemic V. alginolyticus infections. The efficacy of treatments was confirmed by the absence of infectious agents in randomly collected fish samples. To the best of the authors’ knowledge, this study is one of the earliest Egyptian studies that dealt with the dilemma of mass kills associated with external parasitic/systemic bacterial infections among hatchery-reared European seabass.

News Insights into the Host-Parasite Interactions of Amyloodiniosis in European Sea Bass: A Multi-Modal Approach

Amyloodiniosis is a disease resulting from infestation by the ectoparasitic dinoflagellate Amyloodinium ocellatum (AO) and is a threat for fish species such as European sea bass (ESB, Dicentrarchus labrax), which are farmed in lagoon and land-based rearing sites. During the summer, when temperatures are highest, mortality rates can reach 100%, with serious impacts for the aquaculture industry. As no effective licensed therapies currently exist, this study was undertaken to improve knowledge of the biology of AO and of the host-parasite relationship between the protozoan and ESB, in order to formulate better prophylactic/therapeutic treatments targeting AO. To achieve this, a multi-modal study was performed involving a broad range of analytical modalities, including conventional histology (HIS), immunohistochemistry (IHC) and confocal laser scanning microscopy (CLSM). Gills and the oro-pharyngeal cavity were the primary sites of amyloodiniosis, with hyperplasia and cell degeneration more evident in severe infestations (HIS). Plasmacells and macrophages were localised by IHC and correlated with the parasite burden in a time-course experimental challenge. CLSM allowed reconstruction of the 3D morphology of infecting trophonts and suggested a protein composition for its anchoring and feeding structures. These findings provide a potential starting point for the development of new prophylactic/therapeutic controls.

Livoneca redmanii (Isopoda, Cymothoidae) in meagre Argyrosomus regius: parasitological and molecular diagnosis and proposed control measure

Isopodiosis in cultured meagre Argyrosomus regius was investigated at 3 farms in the northern lakes of Egypt throughout 2018, based upon prevalence rate, parasitological examination, and molecular identification by PCR targeting the large ribosomal subunit 16S of the rRNA gene. Further, the susceptibility of A. regius to isopod infection was experimentally evaluated under hyposalination of 25, 15, and 8 ppt for 1 wk. The isolated isopod stages were morphologically identified as Livoneca redmanii Leach, 1818 with prevalence rates of 77.05 and 77.9% in Al-Madiyyah and Sidi Krir, respectively. The highest prevalence and salinity of 78.85% and 30-34 ppt were reported in El Matareya. DNA sequencing and molecular analysis confirmed the identification of L. redmanii. A. regius experimentally infected with L. redmanii under a hyposalination protocol at 15 and 8 ppt showed marked reduction in mortality (20 and 50%, respectively), and infection prevalence (40 and 63.33%, respectively). The serum lysozyme concentration and nitric oxide of treated fish at 15 and 8 ppt were significantly increased compared to those held at 25 and 33 ppt, whereas serum osmolality levels were higher at 25 and 33 ppt. To our knowledge, this is the first molecular characterization of L. redmanii in cultured A. regius in Egypt.

Comparative Therapeutic Effects of Natural Compounds Against Saprolegnia spp. (Oomycota) and Amyloodinium ocellatum (Dinophyceae)

The fish parasites Saprolegnia spp. (Oomycota) and Amyloodinium ocellatum (Dinophyceae) cause important losses in freshwater and marine aquaculture industry, respectively. The possible adverse effects of compounds used to control these parasites in aquaculture resulted in increased interest on the search for natural products with antiparasitic activity. In this work, eighteen plant-derived compounds (2′,4′-Dihydroxychalcone; 7-Hydroxyflavone; Artemisinin; Camphor (1R); Diallyl sulfide; Esculetin; Eucalyptol; Garlicin 80%; Harmalol hydrochloride dihydrate; Palmatine chloride; Piperine; Plumbagin; Resveratrol; Rosmarinic acid; Sclareolide; Tomatine, Umbelliferone, and Usnic Acid) have been tested in vitro. Sixteen of these were used to determine their effects on the gill cell line G1B (ATCC®CRL-2536™) and on the motility of viable dinospores of Amyloodinium ocellatum, and thirteen were screened for inhibitory activity against Saprolegnia spp. The cytotoxicity results on G1B cells determined that only two compounds (2′,4′-Dihydroxychalcone and Tomatine) exhibited dose-dependent toxic effects. The highest surveyed concentrations (0.1 and 0.01 mM) reduced cell viability by 80%. Upon lowering the compound concentration the percentage of dead cells was lower than 20%. The same two compounds revealed to be potential antiparasitics by reducing in a dose-dependent manner the motility of A. ocellatum dinospores up to 100%. With respect to Saprolegnia, a Minimum Inhibitory Concentration was found for Tomatine (0.1 mM), Piperine and Plumbagin (0.25 mM), while 2′,4′-Dihydroxychalcone considerably slowed down mycelial growth for 24 h at a concentration of 0.1 mM. Therefore, this research allowed to identify two compounds, Tomatine and 2′,4′-Dihydroxychalcone, effective against both parasites. These compounds could represent promising candidates for the treatment of amyloodiniosis and saprolegniosis in aquaculture. Nevertheless, further in vitro and in vivo tests are required in order to determine concentrations that are effective against the considered pathogens but at the same time safe for hosts, environment and consumers.

Influence of Age on Stress Responses of White Seabream to Amyloodiniosis

Amyloodiniosis is a disease that represents a major bottleneck for semi-intensive aquaculture, especially in Southern Europe. The inefficacy of many of the treatments for this disease on marine fish produced in semi-intensive aquaculture has led to a new welfare approach to amyloodiniosis. There is already some knowledge of several welfare issues that lead to amyloodiniosis as well as the stress, physiological, and immunological responses to the parasite by the host, but no work is available about the influence of fish age on the progression of amyloodiniosis. The objective of this work was to determine if stress, hematological, and histopathological responses are age dependent. For that purpose, we determined the mortality rate, histopathological lesions, hematological indexes, and stress responses (cortisol, glucose, lactate, and total protein) in “Small” (total weight: 50 ± 5.1 g, age: 273 days after eclosion (DAE)) and “Big” (total weight: 101.3 ± 10.4 g, age: 571 DAE) white seabream (Diplodus sargus) subjected to an Amyloodinium ocellatum infestation (8000 dinospores mL−1) during a 24-h period. The results demonstrated a strong stress response to A. ocellatum, with marked differences in histopathological alterations, glucose levels, and some hematological indexes between the fish of the two treatments. This work elucidates the need to take in account the size and age of the fish in the development and establishment of adequate mitigating measures and treatment protocols for amyloodiniosis.