Ciguatera poisoning in French Polynesia: A review of the distribution and toxicity of Gambierdiscus spp., and related impacts on food web components and human health

Ciguatera Poisoning (CP) is a seafood poisoning highly prevalent in French Polynesia. This illness results from the consumption of seafood contaminated with ciguatoxins (CTXs) produced by Gambierdiscus, a benthic dinoflagellate. Ciguatera significantly degrades the health and economic well-being of local communities largely dependent on reef fisheries for their subsistence. French Polynesia has been the site of rich and active CP research since the 1960's. The environmental, toxicological, and epidemiological data obtained in the frame of large-scale field surveys and a country-wide CP case reporting program conducted over the past three decades in the five island groups of French Polynesia are reviewed. Results show toxin production in Gambierdiscus in the natural environment may vary considerably at a temporal and spatial scale, and that several locales clearly represent Gambierdiscus spp. "biodiversity hotspots". Current data also suggest the "hot" species G. polynesiensis could be the primary source of CTXs in local ciguateric biotopes, pending formal confirmation. The prevalence of ciguatoxic fish and the CTX levels observed in several locales were remarkably high, with herbivores and omnivores often as toxic as carnivores. Results also confirm the strong local influence of Gambierdiscus spp. on the CTX toxin profiles characterized across multiple food web components including in CP-prone marine invertebrates. The statistics, obtained in the frame of a long-term epidemiological surveillance program established in 2007, point towards an apparent decline in the number of CP cases in French Polynesia as a whole; however, incidence rates remain dangerously high in some islands. Several of the challenges and opportunities, most notably those linked to the strong cultural ramifications of CP among local communities, that need to be considered to define effective risk management strategies are addressed.

Concentration ciguatoxins in fillet of fish: A global systematic review and meta-analysis

In the current study, an attempt was made to meta-analyze and discuss the concentration of ciguatoxins (CTXs) in fillets of fish based on country and water resources subgroups. The search was conducted in Scopus and PubMed, Embase and Web of Science to retrieve papers about the concentration of CTXs in fillet fish until July 2022. Meta-analysis concentration of CTXs was conducted based on countries and water resources subgroups in the random effects model (REM). The sort of countries based on the pooled concentration of CTXs was Kiribati (3.904 μg/kg) > Vietnam (1.880 μg/kg) > Macaronesia (1.400 μg/kg) > French (1.261 μg/kg) > China (0.674 μg/kg) > Japan (0.572 μg/kg) > USA (0.463 μg/kg) > Spain (0.224 μg/kg) > UK (0.170 μg/kg) > Fiji (0.162 μg/kg) > Mexico (0.150 μg/kg) > Australia (0.138 μg/kg) > Portugal (0.011 μg/kg). CTXs concentrations in all countries are higher than the safe limits of CTX1C (0.1 μg/kg). However, based on the safe limits of CTX1P, the concentrations of CTXs in just Portugal meet the regulation level (0.01 μg/kg). The minimum and maximum concentrations of CTXs were as observed in Selvagens Islands (0.011 μg/kg) and St Barthelemy (7.875 μg/kg) respectively. CTXs concentrations in all water resources are higher than safe limits of CTX1C (0.1 μg/kg) and CTX1B (0.01 μg/kg). Therefore, it is recommended to carry out continuous control pans of CTXs concentration in fish in different countries and water sources.

Regional comparison on ciguatoxicity, hemolytic activity, and toxin profile of the dinoflagellate Gambierdiscus from Kiribati and Malaysia

The dinoflagellates Gambierdiscus and Fukuyoa can produce Ciguatoxins (CTXs) and Maitotoxins (MTXs) that lead to ciguatera poisoning (CP). The CP hotspots, however, do not directly relate to the occurrence of the ciguatoxic Gambierdiscus and Fukuyoa. Species-wide investigations often showed no association between CTX level and the molecular identity of the dinoflagellates. In the Pacific region, Kiribati is known as a CP hotspot, while Malaysia has only three CP outbreaks reported thus far. Although ciguatoxic strains of Gambierdiscus were isolated from both Kiribati and Malaysia, no solid evidence on the contribution of ciguatoxic strains to the incidence of CP outbreak was recorded. The present study aims to investigate the regional differences in CP risks through region-specific toxicological assessment of Gambierdiscus and Fukuyoa. A total of 19 strains of Gambierdiscus and a strain of Fukuyoa were analyzed by cytotoxicity assay of the neuro-2a cell line, hemolytic assay of fish erythrocytes, and high-resolution mass spectrometry. Gambierdiscus from both Kiribati and Malaysia showed detectable ciguatoxicity; however, the Kiribati strains were more hemolytic. Putative 44-methylgambierone was identified as part of the contributors to the hemolytic activity, and other unknown hydrophilic toxins produced can be potentially linked to higher CP incidence in Kiribati.

Synergistic effect of environmental food pollutants: Pesticides and marine biotoxins

Emerging marine biotoxins such as ciguatoxins and pyrethroid compounds, widely used in agriculture, are independently treated as environmental toxicants. Their maximum residue levels in food components are set without considering their possible synergistic effects as consequence of their interaction with the same cellular target. There is an absolute lack of data on the possible combined cellular effects that biological and chemical pollutants, may have. Nowadays, an increasing presence of ciguatoxins in European Coasts has been reported and these toxins can affect human health. Similarly, the increasing use of phytosanitary products for control of food plagues has raised exponentially during the last decades due to climate change. The lack of data and regulation evaluating the combined effect of environmental pollutants with the same molecular target led us to analyse their in vitro effects. In this work, the effects of ciguatoxins and pyrethroids in human sodium channels were investigated. The results presented in this study indicate that both types of compounds have a profound synergistic effect in voltage-dependent sodium channels. These food pollutants act by decreasing the maximum peak inward sodium currents and hyperpolarizing the sodium channels activation, effects that are boosted by the simultaneous presence of both compounds. A fact that highlights the need to re-evaluate their limits in feedstock as well as their potential in vivo toxicity considering that they act on the same cellular target. Moreover, this work sets the cellular basis to further apply this type of studies to other water and food pollutants that may act synergistically and thus implement the corresponding regulatory limits taking into account its presence in a healthy diet.

Deeper insight into Gambierdiscus polynesiensis toxin production relies on specific optimization of high-performance liquid chromatography-high resolution mass spectrometry

Ciguatera food poisoning affects consumer health and fisheries' economies worldwide in tropical zones, and specifically in the Pacific area. The wide variety of ciguatoxins bio-accumulated in fish or shellfish responsible for this neurological illness are produced by marine dinoflagellates of the genus Gambierdiscus and bio-transformed through the food web. The evaluation of the contents of ciguatoxins in strains of Gambierdiscus relies on the availability of standards and on the development of sensitive and specific tools to detect them. There is a need for sensitive methods for the analysis of pacific ciguatoxins with high resolution mass spectrometry to ensure unequivocal identification of all congeners. We have applied a fractional factorial design of experiment 2^^8-3 for the screening of the significance of eight parameters potentially influencing ionization and ion transmission and their interactions to evaluate the behavior of sodium adducts, protonated molecules and first water losses of CTX4A/B, CTX3B/C, 2-OH-CTX3C and 44-methylgambierone on a Q-TOF equipment. The four parameters that allowed to significantly increase the peak areas of ciguatoxins and gambierones (up to a factor ten) were the capillary voltage, the sheath gas temperature, the ion funnel low pressure voltage and the ion funnel exit voltage. The optimized method was applied to revisit the toxin profile of G. polynesiensis (strain TB92) with a confirmation of the presence of M-seco-CTX4A only putatively reported so far and the detection of an isomer of CTX4A. The improvement in toxin detection also allowed to obtain informative high resolution targeted MS/MS spectra revealing high similarity in fragmentation patterns between putative isomer (4) of CTX3C, 2-OH-CTX3C and CTX3B on one side and between CTX4A, M-seco-CTX4A and the putative isomer on the other side, suggesting a relation of constitutional isomerism between them for both isomers.

Gambierdiscus and Fukuyoa as potential indicators of ciguatera risk in the Balearic Islands

Gambierdiscus and Fukuyoa are genera of toxic dinoflagellates which were mainly considered as endemic to marine intertropical areas, and that are well known as producers of ciguatoxins (CTXs) and maitotoxins (MTXs). Ciguatera poisoning (CP) is a human poisoning occurring after the consumption of fish or more rarely, shellfish containing CTXs. The presence of these microalgae in a coastal area is an indication of potential risk of CP. This study assesses the risk of CP in the Balearic Islands (Western Mediterranean Sea) according to the distribution of both microalgae genera, and the presence of CTX-like and MTX-like toxicity in microalgal cultures as determined by neuro-2a cell based-assay (neuro-2a CBA). Genetic identification of forty-three cultured microalgal strains isolated from 2016 to 2019 revealed that all of them belong to the species G. australes and F. paulensis. Both species were widely distributed in Formentera, Majorca and Minorca. Additionally, all strains of G. australes and two of F. paulensis exhibited signals of CTX-like toxicity ranging respectively between 1 and 380 and 8-16 fg CTX1B equivalents (equiv.) • cell^-1. Four extracts of F. paulensis exhibited a novel toxicity response in neuro-2a cells consisting of the recovery of the cell viability in the presence of ouabain and veratridine. In addition, G. australes showed MTX-like toxicity while F. paulensis strains did not. Overall, the low CTX-like toxicities detected indicate that the potential risk of CP in the Balearic Islands is low, although, the presence of CTX-like and MTX-like toxicity in those strains reveal the necessity to monitor these genera in the Mediterranean Sea.

High sensitivity of rat cardiomyoblast H9c2(2-1) cells to Gambierdiscus toxic compounds

Ciguatera fish poisoning is a frequently reported non-bacterial food-borne illness related to the consumption of seafood contaminated with ciguatoxins, and possibly maitotoxins. These toxins are synthesized by marine dinoflagellate species of Gambierdiscus and Fukuyoa genera, and their abundance is a matter of great concern due to their adverse effects to aquatic life and human health. The present study aims to assess the sensitivity of rat cardiomyoblast H9c2(2-1) cells to Gambierdiscus toxic compounds using concentration- and time-dependent sulforhodamine B (SRB) colorimetric assays. Low concentrations of Gambierdiscus extracts (corresponding to 1.3-2.3 cells mL^-1) induced a concentration-dependent response. Specificity in time-dependent response of H9c2(2-1) cells was demonstrated for G. excentricus after a 180 min exposure compared to both G. cf. belizeanus and G. silvae species, with EC_50s obtained after 720 and 360 min, respectively. The sensitivity of H9c2(2-1) cells to dinoflagellate toxic compounds was also tested with other genera from benthic (Coolia malayensis, Ostreopsis cf. ovata, Prorocentrum hoffmannianum and P. lima) and planktonic (Amphidinium carterae and Lingulodinium polyedrum) habitats. Amphidinium, Coolia and Lingulodinium data did not present any concentration-response relationships, and EC₅₀ values could only be obtained after 720 and 1440 min of exposure to both Prorocentrum species and O. cf. ovata, respectively. This study demonstrated that the H9c2(2-1) SRB assay represents a promising and sensitive tool for the detection of Gambierdiscus toxic compounds present in water samples, particularly of G. excentricus at very low cell abundances.

Caribbean Ciguatoxin-1 stability under strongly acidic conditions: Characterisation of a new C-CTX1 methoxy congener

The recent emergence of ciguatera in the eastern Atlantic, particularly in the Canary Islands (Spain) and Madeira (Portugal) prompted the development and implementation of liquid chromatography tandem-mass spectrometry (LC/MS-MS) methods for the detection of ciguatoxins in fish. The complexity of fish tissue matrices, low concentrations of ciguatoxins in hazardous fish, and the scarcity of ciguatoxin standards present challenging issues for successful implementation of routine ciguatoxin analysis. A laboratory reference material of Caribbean Ciguatoxin-1 (C-CTX1), which was previously confirmed in fish responsible for ciguatera outbreaks in the Canary Islands, was used to assess the toxin's stability under strongly acidic conditions and solvent systems commonly used in LC-MS/MS. It was observed that strongly acidic conditions caused the transformation of C-CTX1 to a C56 methoxy congener, C-CTX1-Me. C-CTX1 was structurally characterised by LC-MS/MS and fragmentation pathways are presented showing the same fragmentation pattern as C-CTX1-Me. These results suggest that the use of strongly acidic conditions during sample pretreatment for C-CTX analysis, might produce significant artefacts, and risks failing to detect the presence of C-CTX1.

Ciguatera poisoning in French Polynesia: insights into the novel trends of an ancient disease

Ciguatera is a non-bacterial seafood poisoning highly prevalent in French Polynesia where it constitutes a major health issue and a major threat to food sustainability and food security for local populations. Ciguatera results from the bioaccumulation in marine food webs of toxins known as ciguatoxins, originating from benthic dinoflagellates in the genera Gambierdiscus and Fukuyoa. Ciguatera is characterized by a complex array of gastrointestinal, neurological and cardiovascular symptoms. The effective management of patients is significantly hampered by the occurrence of atypical forms and/or chronic sequelae in some patients, and the lack of both a confirmatory diagnosis test and a specific antidote. In addition, recent findings have outlined the implication of novel species of the causative organisms as well as new vectors, namely marine invertebrates, in ciguatera outbreaks. Another novel trend relates to the geographical expansion of this disease to previously unaffected areas, not only in certain island groups of French Polynesia but also in temperate regions worldwide, as a likely consequence of the effects of climate change.

A radioligand receptor binding assay for ciguatoxin monitoring in environmental samples: Method development and determination of quality control criteria

Ciguatoxins are algal toxins responsible for tens of thousands of human intoxications yearly, both in tropical and subtropical endemic regions as well as worldwide through fish exportation. Previously developed methods for biotoxin surveillance in the environment and seafood include analytical methods and in vivo and in vitro bioassays. The radioligand receptor binding assay (r-RBA) is among the in vitro methodologies currently used for the detection and quantification of marine biotoxins. For the ciguatoxin group, the r-RBA has been widely used as a means to characterize the mode of action and as detection method in various biological matrices. Yet, screening methods have not been standardized, and the details of the ciguatoxin-specific r-RBA are not well-documented, which limit interlaboratory comparison and progress toward method validation. This work presents the development of an optimized r-RBA for ciguatoxins and provides guidance on its use and quality control checks for analysis of environmental samples. We focus on the analysis of critical parameters involved in determining assay acceptability. Calculation of toxin concentrations in fish samples is illustrated with four examples. Thus, this paper provides the detailed information required for a full validation of the r-RBA, a necessary step toward the development and implementation of a regulatory monitoring programme for ciguatoxins in seafood products using the r-RBA.

Detection of pacific ciguatoxins using liquid chromatography coupled to either low or high resolution mass spectrometry (LC-MS/MS)

Ciguatera Fish Poisoning (CFP) is primarily caused by consumption of tropical and sub-tropical fish contaminated by Ciguatoxins (CTXs). These lipid-soluble, polyether neurotoxins are produced by dinoflagellates in the genera Gambierdiscus and Fukuyoa. While there is no regulatory level in Europe for CTXs, the European Food Safety Authority (EFSA) adopted the United States guidance level of 0.01 μg P-CTX1B eq.kg^-1 of fish. This limit is extremely low and requires significant improvement in the detection of CTXs. In this study, we compared analytical protocols based on liquid chromatography coupled to tandem low or high resolution mass spectrometry (LC-LRMS or HRMS) to find the best conditions for sensitivity and/or selectivity. Different approaches such as LC conditions, ion choice and acquisition modes, were evaluated to detect the Pacific-ciguatoxins (P-CTXs) on a triple quadrupole (API4000 Qtrap, Sciex) or a quadrupole time of flight (QTOF 6550, Agilent Technologies) spectrometer. Moreover, matrix effects were calculated using matrix-matched calibration solutions of P-CTX1B and P-CTX3C prepared in purified fish extract. Subsequently, the method performance was assessed on naturally contaminated samples of seafood and phytoplankton. With LRMS, the ammoniated adduct ion used as a precursor ion showed an advantage for selectivity through confirmatory transitions, without affecting signal-to-noise ratios, and hence limits of detection (LODs). As also reported by some studies in the literature, methanol-based mobile phase gave better selectivity and sensitivity for the detection of P-CTXs. While the LOD for P-CTX1B and P-CTX3C met the EFSA recommendation level when using LRMS, the findings suggested careful evaluation of instrumental parameters for determination of CTXs. LODs were significantly higher for HRMS, which currently results in the need for a significantly higher sample intake. Nevertheless, HRMS allowed for the identification of artefacts and may allow for improved confirmation of the identity of P-CTXs analogues. Consequently, LRMS and HRMS are considered complementary to ensure adequate quantitation and identification of P-CTXs.

Ciguatoxin prevalence in 4 commercial fish species along an oceanic exposure gradient in the US Virgin Islands

Ciguatera fish poisoning is a seafood-toxin illness resulting from consumption of fish contaminated with ciguatoxins. Managing ciguatera fish poisoning is complex. It is made easier, however, by local fishers from endemic areas reporting regional predictability for local fish species' ciguatera fish poisoning risk, which the present study then tested. We investigated the prevalence of ciguatoxins in 4 commonly marketed and consumed species (Balistes vetula, Haemulon plumierii, Ocyurus chrysurus, and Epinephelus guttatus) across an oceanic gradient (north, south, east, and west) from the US Virgin Islands. Fish muscle extracts were analyzed for Caribbean ciguatoxins using an in vitro mouse neuroblastoma (N2a) cytotoxicity assay and confirmed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Fish collected from the north location had 0 fish with detectable ciguatoxins; this site also had the greatest wave energy. Caribbean ciguatoxins in fish ranged from 0.01 to 0.11, 0.004 to 0.10, and 0.005 to 0.18 ng Caribbean ciguatoxin-1 eq/g, from the west, east, and south respectively. Ciguatoxin-like activity was detectable by the N2a assay in 40, 41, 50, and 70% of H. plumierii, O. chrysurus, B. vetula, and E. guttatus, respectively. Of the fish collected, 4% had Caribbean ciguatoxin levels exceeding the US Food and Drug Administration guidance of 0.1 ng Caribbean ciguatoxin-1 eq/g fish. These findings concurred with spatial ciguatera fish poisoning prevalence information provided by local fishers in the US Virgin Islands and demonstrate how partnerships between researchers and fishers can aid the improvement of science-based ciguatera fish poisoning management. Environ Toxicol Chem 2018;39:1852-1863. Published 2018 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Ciguatoxins activate the Calcineurin signalling pathway in Yeasts: Potential for development of an alternative detection tool?

Ciguatoxins (CTXs) are lipid-soluble polyether compounds produced by dinoflagellates from the genus Gambierdiscus spp. typically found in tropical and subtropical zones. This endemic area is however rapidly expanding due to environmental perturbations, and both toxic Gambierdiscus spp. and ciguatoxic fishes have been recently identified in the North Atlantic Ocean (Madeira and Canary islands) and Mediterranean Sea. Ciguatoxins bind to Voltage Gated Sodium Channels on the membranes of sensory neurons, causing Ciguatera Fish Poisoning (CFP) in humans, a disease characterized by a complex array of gastrointestinal, neurological, neuropsychological, and cardiovascular symptoms. Although CFP is the most frequently reported non bacterial food-borne poisoning worldwide, there is still no simple and quick way of detecting CTXs in contaminated samples. In the prospect to engineer rapid and easy-to-use CTXs live cells-based tests, we have studied the effects of CTXs on the yeast Saccharomyces cerevisiae, a unicellular model which displays a remarkable conservation of cellular signalling pathways with higher eukaryotes. Taking advantage of this high level of conservation, yeast strains have been genetically modified to encode specific transcriptional reporters responding to CTXs exposure. These yeast strains were further exposed to different concentrations of either purified CTX or micro-algal extracts containing CTXs. Our data establish that CTXs are not cytotoxic to yeast cells even at concentrations as high as 1μM, and cause an increase in the level of free intracellular calcium in yeast cells. Concomitantly, a dose-dependent activation of the calcineurin signalling pathway is observed, as assessed by measuring the activity of specific transcriptional reporters in the engineered yeast strains. These findings offer promising prospects regarding the potential development of a yeast cells-based test that could supplement or, in some instances, replace current methods for the routine detection of CTXs in seafood products.

Application of solid phase adsorption toxin tracking (SPATT) devices for the field detection of Gambierdiscus toxins

Ciguatera fish poisoning is a food-borne illness caused by the consumption of seafood contaminated with ciguatoxins (CTXs) produced by dinoflagellates in the Gambierdiscus genus. Since most surveillance programs currently rely on the survey of Gambierdiscus cell densities and species composition, supplementary toxin-based methods allowing the time- and spatially integrated sampling of toxins in ciguateric environments are needed for a more reliable assessment and management of the risks associated with Gambierdiscus proliferation. Solid Phase Adsorption Toxin Tracking (SPATT) filters use porous synthetic resins capable of adsorbing toxins directly from the water column. To assess the ability of these passive monitoring devices to retain Gambierdiscus toxins, SPATT bags filled with 10g of HP20 resin were deployed for 48h in two French Polynesian locations at high (Nuku Hiva Island) vs. low to moderate (Kaukura Atoll) risk of ciguatera. CTXs could be detected in SPATT bags extracts from Nuku Hiva Island, as assessed by the mouse neuroblastoma cell-based assay (CBA-N2a) and liquid chromatography - tandem mass spectrometry (LC-MS/MS) analyses. Results of in vitro experiments suggest that the saturation limit of CTXs on HP20 resin, for a deployment time of 48h, is ≃ 55ng P-CTX-3C equiv. g^-1 resin. Despite the non detection of maitotoxin (MTX), LC-MS/MS analyses showed that two other compounds also produced by Gambierdiscus species were retained on SPATT bags, i.e. iso-P-CTX-3B/C and a putative MTX analogue, known as MTX-3. This study, the first to demonstrate the suitability of SPATT technology for the in situ monitoring of Gambierdiscus toxins, highlights the potential application of this tool for routine ciguatera risk assessment and management programs.

Qualitative and quantitative assessment of the presence of ciguatoxin, P-CTX-1B, in Spanish Mackerel (Scomberomorus commerson) from waters in New South Wales (Australia)

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P-CTX-1B in 84 Spanish Mackerel from NSW and QLD were quantified using LC–MS/MS.

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Liver & flesh from 6 fish and liver from 4 other fish were positive for P-CTX-1B.

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Liver had significantly higher concentration (∼6 times) of P-CTX-1B than flesh.

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No relationship was observed between length/weight of fish & detection of P-CTX-1B.

Ciguatera Fish Poisoning (CFP) is a tropical disease caused by the consumption of fish contaminated with ciguatoxins (CTXs). Currently, the only feasible prevention methods for CFP are to avoid the consumption of fish of certain species from some regions, avoid larger fish of certain species, or avoid all fish caught from specific regions. Here, we quantified levels of P-CTX-1B in Spanish Mackerel (Scomberomorus commerson), which is the main fish species that causes CFP in New South Wales and Queensland, Australia, using LC–MS detection against a toxin standard. We found detectable P-CTX-1B in both flesh and liver tissues in fish from New South Wales (n = 71, 1.4% prevalence rate, with a confidence interval of 1%–4%, and 7% prevalence, 1%–12%, in flesh and liver, respectively). In the small sample of fish from Queensland, there was a 46% prevalence (19–73%, n = 13). Toxin levels found were 0.13 μg kg⁻¹ to <0.1 μg kg⁻¹ in flesh, and 1.39 μg kg⁻¹ to <0.4 μg kg⁻¹ in liver, indicating that liver tissue had a significantly higher concentration (∼5 fold) of P-CTX-1B. No apparent relationship was observed between the length or weight of S. commerson and the detection of P-CTX-1B in this study.

Footnote

Toxicity screening of 13 Gambierdiscus strains using neuro-2a and erythrocyte lysis bioassays

Species in the epi-benthic dinoflagellate genus Gambierdiscus produce ciguatoxins (CTXs) and maitotoxins (MTXs), which are among the most potent marine toxins known. Consumption of fish contaminated with sufficient quantities of CTXs causes Ciguatera Fish Poisoning (CFP), the largest cause of non-bacterial food poisoning worldwide. Maitotoxins, which can be found in the digestive system of fish, could also contribute to CFP if such tissues are consumed. Recently, an increasing number of Gambierdiscus species have been identified; yet, little is known about the variation in toxicity among Gambierdiscus strains or species. This study is the first assessment of relative CTX- and MTX-toxicity of Gambierdiscus species from areas as widespread as the North-Eastern Atlantic Ocean, Pacific Ocean and the Mediterranean Sea. A total of 13 strains were screened: (i) seven Pacific strains of G. australes, G. balechii, G. caribaeus, G. carpenteri, G. pacificus, G. scabrosus and one strain of an undetermined species (Gambierdiscus sp. Viet Nam), (ii) five strains from the North-Eastern Atlantic Ocean (two G. australes, a single G. excentricus and two G. silvae strains), and (iii) one G. carolinianus strain from the Mediterranean Sea. Cell pellets of Gambierdiscus were extracted with methanol and the crude extracts partitioned into a CTX-containing dichloromethane fraction and a MTX-containing aqueous methanol fraction. CTX-toxicity was estimated using the neuro-2a cytoxicity assay, and MTX-toxicity via a human erythrocyte lysis assay. Different species were grouped into different ratios of CTX- and MTX-toxicity, however, the ratio was not related to the geographical origin of species (Atlantic, Mediterranean, Pacific). All strains showed MTX-toxicity, ranging from 1.5 to 86pg MTX equivalents (eq) cell^-1. All but one of the strains showed relatively low CTX-toxicity ranging from 0.6 to 50 fg CTX3C eq cell^-1. The exception was the highly toxic G. excentricus strain from the Canary Islands, which produced 1426 fg CTX3C eq cell^-1. As was true for CTX, the highest MTX-toxicity was also found in G. excentricus. Thus, the present study confirmed that at least one species from the Atlantic Ocean demonstrates similar toxicity as the most toxic strains from the Pacific, even if the metabolites in fish have so far been shown to be more toxic in the Pacific Ocean.

Physiological and behavioural impacts of Pacific ciguatoxin-1 (P-CTX-1) on marine medaka (Oryzias melastigma)

Ciguatoxins (CTXs) are natural biotoxins produced by benthic dinoflagellates of the genus Gambierdiscus, which are bioaccumulated and biotransformed along food chains in coral ecosystems. They are neurotoxins that activate voltage-gated sodium channels and disrupt ion conductance in the excitable tissues. Pacific ciguatoxin-1 (P-CTX-1) is the most prevalent and potent CTX congener present in fishes from the Pacific Ocean. In this study, P-CTX-1 was administrated to larval marine medaka (2h post-hatch) via microinjection. Exposure to P-CTX-1 at sub-ppb levels led to adverse behavioural changes, altered physiological performances and reduced survivability of the larval marine medaka as early as 24h after exposure. P-CTX-1 decreased the rate of heartbeat and locomotion of the exposed larvae, probably owing to a series of physiological processes and morphological changes such as pericardial oedema, failure of swim bladder inflation and spinal curvature. The exposed larval marine medaka also demonstrated reduced, delayed and paralyzed responses to external stimulations. This may render them more susceptible to predation. P-CTX-1 could be effectively distributed from the yolk sac to all parts of the fish body, including head and trunk, 24h after exposure. Repeated low-dose P-CTX-1 exposure resulted in larval mortality comparable to that of a single high-dose exposure.

cells

Ciguatera Fish Poisoning (CFP) is a foodborne disease classically related to the consumption of tropical coral reef fishes contaminated with ciguatoxins (CTXs), neurotoxins produced by dinoflagellates of the Gambierdiscus genus. Severe atypical ciguatera-like incidents involving giant clams, a marine resource highly consumed in the South Pacific, are also frequently reported in many Pacific Islands Countries and Territories. The present study was designed to assess the ability of giant clams to accumulate CTXs in their tissues and highlight the potential health risks associated with their consumption. Since giant clams are likely to be exposed to both free-swimming Gambierdiscus cells and dissolved CTXs in natural environment, ex situ contamination experiments were conducted as follows: giant clams were exposed to live or lyzed cells of TB92, a highly toxic strain of G. polynesiensis containing 5.83±0.85pg P-CTX-3C equiv.cell^-1vs. HIT0, a weakly toxic strain of G. toxicus containing only (2.05±1.16)×10^-3pg P-CTX-3C equiv.cell^-1, administered over a 48h period at a concentration of 150cellsmL^-1. The presence of CTXs in giant clams tissues was further assessed using the mouse neuroblastoma cell-based assay (CBA-N2a). Results showed that giant clams exposed to either lyzed or live cells of TB92 were able to bioaccumulate CTXs at concentrations well above the safety limit recommended for human consumption, i.e. 3.28±1.37 and 2.92±1.03ng P-CTX-3C equiv.g^-1 flesh (wet weight), respectively, which represented approximately 3% of the total toxin load administered to the animals. In contrast, giant clams exposed to live or lyzed cells of HIT0 were found to be free of toxins, suggesting that in the nature, the risk of contamination of these bivalves is established only in the presence of highly toxic blooms of Gambierdiscus. Liquid chromatography-mass spectrometry (LC-MS/MS) analyses confirmed CBA-N2a results and also revealed that P-CTX-3B was the major CTX congener retained in the tissues of giant clams fed with TB92 cells. To the best of our knowledge, this study is the first to provide evidence of the bioaccumulation of Gambierdiscus CTXs in giant clams and confirms that these bivalve molluscs can actually constitute another pathway in ciguatera poisonings. While most monitoring programs currently focus on fish toxicity, these findings stress the importance of a concomitant surveillance of these marine invertebrates in applicable locations for an accurate assessment of ciguatera risk.

Voltage-sensor conformation shapes the intra-membrane drug binding site that determines gambierol affinity in Kv channels

Marine ladder-shaped polyether toxins are implicated in neurological symptoms of fish-borne food poisonings. The toxin gambierol, produced by the marine dinoflagellate Gambierdiscus toxicus, belongs to the group of ladder-shaped polyether toxins and inhibits Kv3.1 channels with nanomolar affinity through a mechanism of gating modification. Binding determinants for gambierol localize at the lipid-exposed interface of the pore forming S5 and S6 segments, suggesting that gambierol binds outside of the permeation pathway. To explore a possible involvement of the voltage-sensing domain (VSD), we made different chimeric channels between Kv3.1 and Kv2.1, exchanging distinct parts of the gating machinery. Our results showed that neither the electro-mechanical coupling nor the S1-S3a region of the VSD affect gambierol sensitivity. In contrast, the S3b-S4 part of the VSD (paddle motif) decreased gambierol sensitivity in Kv3.1 more than 100-fold. Structure determination by homology modeling indicated that the position of the S3b-S4 paddle and its primary structure defines the shape and∖or the accessibility of the binding site for gambierol, explaining the observed differences in gambierol affinity between the channel chimeras. Furthermore, these findings explain the observed difference in gambierol affinity for the closed and open channel configurations of Kv3.1, opening new possibilities for exploring the VSDs as selectivity determinants in drug design.

Contribution to the risk characterization of ciguatoxins: LOAEL estimated from eight ciguatera fish poisoning events in Guadeloupe (French West Indies)

From 2010 to 2012, 35 ciguatera fish poisoning (CFP) events involving 87 individuals who consumed locally-caught fish were reported in Guadeloupe (French West Indies). For 12 of these events, the presence of ciguatoxins (CTXs) was indicated in meal remnants and in uncooked fish by the mouse bioassay (MBA). Caribbean ciguatoxins (C-CTXs) were confirmed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. Using a cell-based assay (CBA), and the only available standard Pacific ciguatoxin-1 (P-CTX-1), the lowest toxins level detected in fish samples causing CFP was 0.022 µg P-CTX-1 equivalent (eq.)·kg(-1) fish. Epidemiological and consumption data were compiled for most of the individuals afflicted, and complete data for establishing the lowest observable adverse effects level (LOAEL) were obtained from 8 CFP events involving 21 individuals. Based on toxin intakes, the LOAEL was estimated at 4.2 ng P-CTX-1 eq./individual corresponding to 48. 4 pg P-CTX-1 eq.kg(-1) body weight (bw). Although based on limited data, these results are consistent with the conclusions of the European Food Safety Authority (EFSA) opinion which indicates that a level of 0.01 µg P-CTX-1 eq.kg(-1) fish, regardless of source, should not exert effects in sensitive individuals when consuming a single meal. The calculated LOAEL is also consistent with the U.S. Food and Drug Administration guidance levels for CTXs (0.1 µg C-CTX-1 eq.kg(-1) and 0.01 µg P-CTX-1 eq.kg(-1) fish).

Ciguatera caused by consumption of humphead wrasse

Humphead wrasse (Cheilinus undulatus) is an apex predator from coral reefs of the Indo-Pacific region. A food surveillance project using a validated mouse bioassay revealed the presence of ciguatoxins in significantly greater amounts in its flesh than in groupers and other coral reef fishes commonly available in Hong Kong wholesale market. Humphead wrasse has long been known to cause ciguatera, but there was a lack of clinical reports. A 45-year-old woman developed ciguatera after eating humphead wrasse. She required ICU care and infusions of intravenous fluids and dopamine for management of severe hypotension. All 5 published case series are also reviewed to characterise the types, severity and chronicity of ciguatera symptoms after its consumption. In addition to the gastrointestinal, neurological and other features that were typical of ciguatera, some subjects developed sinus bradycardia, hypotension, shock, neuropsychiatric features (e.g. mental exhaustion, depression, insomnia and memory loss), other central nervous system symptoms (e.g. coma, convulsions and ataxia) and myocardial ischaemia. Other subjects still experienced residual symptoms 6 months later; these were mainly neurological or neuropsychiatric complaints and skin pruritus. To prevent ciguatera, the public should avoid eating humphead wrasse and other large coral reef fishes. They should realise that consumption of the high-risk fish may result in more severe and chronic illness, including life-threatening complications and neuropsychiatric features.