Cooliatoxin, the first toxin from Coolia monotis (Dinophyceae)

Coolia monotis is a benthic dinoflagellate previously thought to be non-toxic. We describe a new toxin, named cooliatoxin, purified from cultures of a strain of C. monotis isolated from Australia. Cooliatoxin is likely a mono-sulphated, polyether toxin (M = 1,062; i.p. LD50 = 1 mg/kg in mice) that induces hypothermia and respiratory failure in mice after a pronounced delay period during which there are no obvious signs of intoxication. These signs in mice are similar to those reported for the shellfish toxin named yessotoxin and the molecular weight of cooliatoxin corresponds to the mono-sulphated form of yessotoxin, suggesting that cooliatoxin may be an analogue of yessotoxin. Cooliatoxin has no effect on the mouse phrenic nerve or diaphragm musculature in vitro but causes initial stimulation and subsequent block of unmylenated nerves in vitro. In isolated guinea pig left atria, cooliatoxin (above 20 nm) induced a slow developing concentration dependent sustained inotropic response. Propranolol or tetrodotoxin reversed the positive inotropic effects, indicating that the majority of the cooliatoxin induced response was mediated by stimulation of nerves associated with the atrial musculature, resulting in the release of noradrenaline. Cooliatoxin induced transient contractions in isolated guinea pig vas deferens preparations. Atria and vas deferens preparations were tachyphylactic to a second equivalent dose of cooliatoxin applied after the effects of the first dose had diminished. The observed in vitro effects of cooliatoxin on peripheral nerves are unlikely to account for the lethal effects in mice and a central action of this toxin is suspected.

Temporal lobe epilepsy caused by domoic acid intoxication: evidence for glutamate receptor-mediated excitotoxicity in humans

We describe the development of temporal lobe epilepsy in an 84-year-old man who had suffered domoic acid intoxication. Following intoxication he had nausea, vomiting, confusion, and coma. Generalized convulsions and complex partial status epilepticus progressively developed. After 3 weeks he improved and was seizure free with severe residual memory deficit. Electroencephalograms initially showed periodic epileptiform discharges, later evolving to epileptic abnormalities over frontotemporal regions with diffuse slow waves. Eight months after the intoxication the electroencephalogram was normal. One year after the acute episode, complex partial seizures developed. Electroencephalograms showed epileptic discharges independently over both temporal lobes, with left-sided predominance. Magnetic resonance imaging revealed a hyperintense T2-weighted signal and atrophy of both hippocampi; a positron emission tomographic scan showed bitemporal decreased glucose metabolism. Pneumonia developed and the patient died 3 1/4 years after the intoxication. Autopsy disclosed severe bilateral hippocampal sclerosis. The seizures following acute domoic acid intoxication, the postmortem pathology, and the fact that temporal lobe epilepsy developed 1 year after intoxication indicate that the human hippocampus is also vulnerable to kainate receptor excitotoxicity, and provide strong evidence supporting the role of excitotoxic injury in epileptogenesis. This report provides a unique human parallel to, and validates the animal model of, kainate-induced epilepsy as an important tool for studying temporal lobe epilepsy.

Redtide in the Philippines

1. Redtide is a marine phenomenon that poses great risk to the health and economic livelihood of people in coastal areas. Paralytic shellfish poisoning develops when a person consumes molluscs containing toxic dinoflagellates and suffers neurological and/or gastrointestinal manifestations. 2. Four redtide incidents in the Philippines are presented. The manner in which the problems were managed are described. 3. The clinical features of redtide poisoning in the Philippines included gastro-intestinal and neurological features with deaths secondary to ventilatory failure. Mortality ranged from 0% to 12% in the different redtide episodes. 4. There are many lessons to be learned in handling this kind of natural disaster. For an effective toxicovigilance programme, there must be a central co-ordinating responsible organization, a clear definition of roles and functions and good inter-agency co-operation. Appropriate surveillance procedures, resources to intensify surveillance at times of risks, prompt warning system, and the ability to impose bans on consumption are also necessary. 5. Poisons centres can play an important role during times of redtide. This may include toxicovigilant activities, such as early warning and educational campaigns to consumers, and seminars in the recognition and management of paralytic shellfish poisoning. 6. The contribution of the epidemiologists in investigating and monitoring the extent of public health damage and patterns of poisoning in a coastal community is emphasized.

Diarrhetic shellfish poison in mussels: comparison of methods of detection and determination of the effectiveness of depuration

The lack of specificity of the Yasumoto mouse bioassay for Diarrhetic Shellfish Poisoning (DSP) toxin and the possibility of obtaining false positive results, prompted a search for a suitable method to confirm doubtful results. The mouse bioassay, an enzyme-linked immunoassay (ELISA) and a tissue culture bioassay were compared; in addition the efficiency of depuration of toxic mussels in ozonized water was evaluated. DSP assay using tissue cultures was found suitable for confirming results obtained using the mouse bioassay, particularly where interpretation was difficult. The ELISA kit gave satisfactory results, but it responded only to okadaic acid and dinophysistoxin 1. Depuration reduced toxicity in some cases after 3 days of treatment.

A competitive enzyme-linked immunoassay for domoic acid determination in human body fluids

A polyclonal antiserum was raised in mice against domoic acid. Two of three immunogens consisted of domoic acid coupled to ovalbumin (OVA) and keyhole limpet haemocyanin at molar ratios of 47:1 and 44:1, respectively using a carbodiimide reaction. Titres of both antisera exceeded 1/35,000 against domoic acid coupled to the non-relevant carrier. Domoic acid was also conjugated to bovine serum albumin at a molar ratio of 30:1 using N-hydroxysuccinimidyl-4-azidobenzoate, a photoreactive compound. This immunogen, however, produced no measurable serum titres against domoic acid. The antiserum produced against the OVA conjugate displayed the highest affinity for free domoic acid in competitive enzyme-linked immunosorbent assay (ELISA). Furthermore, this antiserum preparation did not significantly cross-react with glutamic acid, aspartic acid, the structural analogue kainic acid, or the paralytic shellfish toxin, saxitoxin. The competitive ELISA was used to quantify domoic acid concentrations in human body fluids spiked with pure domoate. The lower limits of accurate domoic acid determinations in competitive ELISA were 0.2 micrograms/ml in urine, 0.25 micrograms/ml in plasma and 10 micrograms/ml in milk. It was concluded that the competitive ELISA described herein could be used to quantitate directly the concentration of domoic acid in the body fluids of individuals with amnesic shellfish poisoning.

Poisoning by the red alga 'ogonori' (Gracilaria verrucosa) on the Nojima Coast, Yokohama, Kanagawa Prefecture, Japan

A food poisoning case due to the ingestion of 'ogonori', an edible red alga, occurred at Yokohama, Kanagawa Prefecture, Japan, in late October 1993, resulting in two victims, including one death (female). No causative agent present in the ogonori was found from a routine bioassay for marine toxins. From the production of increased amounts of prostaglandins (PGs), mainly PGE2, by the alga on stimulation by cutting or soaking in fresh water, and further increase of PGE2 by addition of arachidonic acid, it appeared that an enzyme, probably fatty acid cyclooxygenase, in the ogonori and the body of the victim, was acting on the highly unsaturated fatty acids in the oil of the ingested seafood and in the blood hemorrhaged from the stomach of the victim. This resulted in the production of over 30 mg of PGE2 and small amounts of other PGs in a comparatively short time. With this dosage the victim suffered from nausea, vomiting, and hypotension, and died of hypotensive shock. PGE2 seems to work more selectively on females. This type of poisoning is very unusual, and differs from the more familiar forms of poisoning occurring after ingestion of marine organisms.

A biochemical profile for predicting the chronic exposure of sheep to Microcystis aeruginosa, an hepatotoxic species of blue-green alga

Sheep which grazed on the shoreline of a fresh-water lake which had a toxic bloom of Microcystis aeruginosa were studied for evidence of chronic poisoning, and a serum biochemical profile was developed to indicate sub-lethal, chronic poisoning in the sheep which had been exposed to microcystins. The profile included measurements of glutamate dehydrogenase (GLDH), gamma-glutamyl transferase (gamma GT), bile acids, bilirubin and albumin. Of 18 sheep which were exposed to M aeruginosa for more than three months, 100 per cent had high serum concentrations of bile acids, 94 per cent had high activities of GLDH and gamma GT, 83 per cent had high bilirubin and 72 per cent had low albumin concentrations compared with the median values of unexposed animals. Other sheep which were exposed for shorter periods, showed evidence of hepatic injury after one week of exposure. The majority of the sheep showed no preference for an alternative, uncontaminated source of water.

Comparison of mouse bioassay, HPLC and enzyme immunoassay methods for determining diarrhetic shellfish poisoning toxins in mussels

Mussel specimens (Mytilus galloprovincialis) collected from two different areas of the Adriatic Sea were analysed for diarrhoetic shellfish poisoning (DSP) toxin by three methods: mouse bioassay, the DSP Check enzyme immunoassay kit, and high-performance liquid chromatography (HPLC). The results obtained confirm that Yasumoto's mouse bioassay, capable of detecting all the components of the DSP group, is still necessary to determine the wholesomeness of the product. The ELISA method has not always given quantitatively reliable results. The HPLC method is advantageous in terms of sensitivity, accuracy, specificity and rapidity. However, its application is limited so far to the determination of okadaic acid in mussels.

Genetic impact of an unusual group mortality among humpback whales

Mass mortalities, due to infectious disease or toxic algal blooms, are known to have severe demographic impacts on marine mammal populations. The genetic impacts of these events, however, have received little attention. To investigate the genetic consequences of an unusual group mortality among humpback whales, we compared the mitochondrial DNA haplotypes of 10 whales poisoned by mackerel contaminated with a dinoflagellate neurotoxin to those of 32 live whales from the same regional population. Two haplotypes that were rare in the reference sample of live whales accounted for eight of the 10 poisoned whales. A randomized test of independence, based on 500 permutations of the data matrix, showed significant differences in the frequencies of haplotypes in the two samples (P < .002). This is the first demonstration that group mortality events in marine mammals can have unpredictable genetic consequences and points to a need to evaluate ecological disasters within the context of the genetic mosaic of natural populations.

Origin and transfer of toxins involved in ciguatera

1. Ciguatera is a disease caused by sodium channel activator toxins and results from the consumption of warm water fish contaminated by the ciguatoxin class of polyether toxins. 2. Other toxins, including okadaic acid and maitotoxin, have no proven role in causing human illness associated with ciguatera. 3. Ciguatera often affects only a discrete region of a reef, with flare-ups of ciguatera being both temporally and spatially unpredictable. 4. The ciguatoxins likely arise through the biotransformation and acid-catalysed spiroisomerisation of gambiertoxin-4A produced by Gambierdiscus toxicus and it is unlikely that other toxic benthic dinoflagellates are involved. 5. Events leading to a ciguatera outbreak are initiated by environmental and genetic factors that favour the proliferation of gambiertoxins, with an apparent role for anthropomorphic effects; however, the precise factors involved are yet to be determined. 6. The gambiertoxins and/or ciguatoxins are transferred from the benthos to herbivorous species (fish, invertebrates etc) and then to carnivorous fish via marine food chains. 7. Factors influencing the concentration of ciguatoxins that accumulate in fish include the rate of dietary intake, the efficiency of assimilation, the degree and nature of any toxin biotransformation, the rate of depuration, and the rate of growth of fish.

Seastroke: a new entity?

Three previously healthy young male patients had brain stem infarction (stroke) after swimming or wading in shallow water along the southeastern North Carolina coastline. Each patient remembered an unusual encounter with something in the water. None sustained any obvious local envenomation. Although these cases may be a highly unusual coincidence, I believe they represent a previously unidentified entity--"seastroke." I speculate that the injuries could be caused by contact with a toxic marine animal.

Clinical and pathologic findings of blue-green algae (Microcystis aeruginosa) intoxication in a dog

A healthy dog developed signs of lethargy and vomiting after ingesting water from a tide pool containing blue-green algae. Fulminant hepatic failure occurred, and the dog was euthanized 52 hours later. At necropsy, the liver was large, friable, and discolored a dark red. Histopathology showed hepatocyte dissociation, degeneration, and necrosis. The alga was identified as Microcystis aeruginosa, a known hepatotoxin. The intraperitoneal administration of lyophilized cell material from the bloom caused hepatic necrosis in mice.

Immunological studies using monoclonal antibodies for detection of low dalton marine toxins

The general methodology employed in the development of an immunological assay system is initially described. These include: (a) the methods and reagents used for coupling low dalton epitopes; (b) selection of appropriate immunogenic carriers; (c) mode of administration and choice of animal; and (d) the method of assessing the antibody produced. The background and methodology, using ciguatoxin as a model, are presented. The stick enzyme immunoassay (S-EIA) has proven of value to evaluate implicated fish for verification of ciguatera poisoning and for screening of fish associated with ciguatera prior to consumption. No false negatives have occurred with the stick enzyme immunoassay system to date.

Phycotoxins in seafood--toxicological and chromatographic aspects

Two typical clinical types of algae-related seafood poisoning have attracted medical and scientific attention: paralytic shellfish poisoning (PSP) and diarrhetic shellfish poisoning (DSP). Therefore, it became necessary to establish methods for the evaluation of possible hazards caused by contamination of seafood with these phycotoxins. Bioassays with mice or rats are the common methods for the determination of the toxin content of seafood. However, biological tests are not completely satisfactory because of a lack of sensitivity and pronounced variations. Additionally, there is growing opposition against animal testing. Therefore, many efforts have been undertaken to determine phycotoxins by chromatographic methods. PSP determination is mainly based on high-performance liquid chromatographic (HPLC) separation by ion-pair chromatography followed by postcolumn oxidation of the underivatized toxins in alkaline solution and fluorescence detection. HPLC methods for the determination of the DSP toxins okadaic acid (OA) and dinophysistoxin-1 (DTX-1) are characterized by precolumn derivatization with 9-anthryldiazomethane (ADAM) and/or 4-bromomethyl-7-methoxycoumarin (Br-Mmc), followed by chromatographic separation of the DSP esters formed and fluorescence detection. The chromatographic methods discussed in this review allow the rapid, sensitive and non-ambiguous determination of individual species of the two most important phycotoxins in seafood, PSP and DSP.

Identification of anatoxin-A in benthic cyanobacteria (blue-green algae) and in associated dog poisonings at Loch Insh, Scotland

Dog deaths occurred in 1990 and 1991 after the animals drank water containing blooms of benthic cyanobacteria along the shoreline of Loch Insh, Scotland. Signs of poisoning in the affected animals and the high neurotoxicity of bloom extracts in laboratory bioassays indicated acute poisoning due to cyanobacterial neurotoxin(s). The neurotoxic blooms consisted largely of benthic Oscillatoria species which were also observed in the stomach contents of the poisoned dogs. Stomach contents were also neurotoxic in bioassays with the same signs of poisoning as the Oscillatoria blooms. The cyanobacterial alkaloid neurotoxin anatoxin-a was identified in bloom extracts and poisoned dog stomach contents by high-performance liquid chromatography and gas chromatography-mass spectrometry. A species of benthic Oscillatoria has been isolated from the neurotoxic bloom material and shown to produce anatoxin-a in laboratory culture. These findings are the first to associate anatoxin-a toxicoses with benthic, rather than planktonic, cyanobacteria. Procedures for anatoxin-a extraction and identification from the blooms and animal material are also detailed.

Marine trauma, envenomations, and intoxications

When humans encounter marine creatures a variety of maladies may occur, ranging from dermatitis to life-threatening trauma, allergy, envenomations, or intoxications. The emergency physician should be prepared to recognize quickly and address appropriately the potential life threats, which are primarily neurologic, respiratory, and cardiovascular. A high degree of suspicion for these illnesses is needed. Intoxications may be especially confusing. Although most of the syndromes are self-limited and treatment supportive, time is of the essence if neuromuscular paralysis, hypotension, or respiratory compromise is present. Much folklore exists regarding detection and prevention of these entities and should be regarded as such. The last several decades have seen a marked increase in our knowledge base regarding these fascinating envenomations and intoxications. Research in the next several decades probably will produce a variety of diagnostic and therapeutic tools, which will further our understanding of, and ability to specifically manage, these syndromes.

Scombroid poisoning. Case series of 10 incidents involving 22 patients

Scombroid poisoning is a form of ichthyosarcotoxism caused by the consumption of 'spoiled' fish of the dark meat varieties. It can be considered a mild-to-moderate form of 'food poisoning' and it occurs world-wide. Ten incidents, involving 22 patients, were reported to Tygerberg Hospital Pharmacology and Toxicology Consultation Centre in the first quarter of 1990. Cape yellowtail (Seriola lalandii) was involved in all the cases. The presenting symptoms and signs (in order of frequency) were: skin rash, diarrhoea, palpitations, headache, nausea and abdominal cramps, paraesthesia, an unusual taste sensation and breathing difficulties. The patients responded well to anti-histamines and, in most, the condition resolved within 12-24 hours. Although histamine plays an important role in the pathogenesis of scombroid poisoning, the exact mechanism is still unresolved. The condition should be recognised and not confused with a true seafood allergy. Health workers are urged to alert the authorities when outbreaks of suspected cases of scombroid poisoning are encountered in order to establish the possible cause and to prevent further cases.

Is there a role for amines other than histamines in the aetiology of scombrotoxicosis?

Mackerel fillets associated with an outbreak of scombrotoxicosis have been analysed for their contents of cadaverine, histamine, putrescine, spermidine, spermine and tyramine, and fed to informed, healthy volunteers of both sexes under medical supervision. Of the 86 fillets examined, 30 rapidly induced nausea/vomiting and/or diarrhoea when 50 g were consumed. The remaining fillets failed to provoke such symptoms, even though 17 of them were tested by volunteers proven to be susceptible to scombro-intoxication. Statistical analysis failed to detect any differences in amines content between fillets shown to be scombrotoxic and those failing to induce nausea/vomiting and/or diarrhoea, and failed also to establish any significant relationships between the amines doses and volunteer responses, even after manipulations to simulate additive or synergistic interactions. Accordingly it is concluded that the content of such amines in mackerel have little or no role in the aetiology of scombrotoxicosis.

The importance of endogenous histamine relative to dietary histamine in the aetiology of scombrotoxicosis

Deliberately spoiled mackerel samples and mackerel samples implicated in outbreaks of scombrotoxicosis were, under medical supervision, tested blind on normal, healthy volunteers of both sexes. These experiments identified batches of fish which could induce nausea/vomiting and/or diarrhoea when 50 g samples were consumed. It was also established that the fillets in a batch were neither of equal potency nor homogeneous with respect to histamine content. Strong evidence was obtained that dietary histamine is not a major determinant of scombrotoxicosis since potency was not positively correlated with the dose, and volunteers appeared to fall into susceptible and non-susceptible subgroups. However, there is no reason to suspect allergy as being solely responsible for these differences in sensitivity. It is also possible to discount body weight as a factor. While the data suggest that females may be more susceptible than males, this effect cannot be confirmed at the present time. Studies with susceptible volunteers predosed with either placebo or H1 antagonist (chlorpheniramine 4 mg) demonstrated convincingly that the antihistamine can abolish vomiting and diarrhoea associated with the ingestion of 50 g of scombrotoxic fish. It is therefore postulated that endogenous histamine released by mast cell degranulation has a significant role in the aetiology of scombrotoxicosis, whereas the role of dietary histamine is minor. The nature and origin of the agent responsible for mast cell degranulation is being investigated.

Clinical and epidemiological features of neurotoxic shellfish poisoning in North Carolina

BACKGROUND

In October 1987, a red tide due to P. brevis affected the North Carolina coast for the first time. The purpose of our study was to describe the clinical and epidemiological features of neurotoxic shellfish poisoning (NSP), an illness caused by eating shellfish contaminated with the neurotoxins of P. brevis.

METHODS

Active surveillance was established for cases of NSP. A descriptive study of the NSP cases was then completed.

RESULTS

Forty-eight persons, who had eaten oysters at 20 meals, met the case definition. A variety of gastrointestinal tract and neurological symptoms were reported. The illnesses were generally mild and of short duration, and there were no deaths. Forty-one (85 percent) affected persons lived in five communities located within a 70-kilometer area along the coast. Cases occurred from October 27 to December 9; 27 (56 percent) of the cases occurred before the first closure of affected shellfish waters on November 2. There was a significant increase in the illness attack rate with an increase in the number of oysters eaten.

CONCLUSIONS

Routine monitoring of coastal waters for P. brevis is needed to facilitate earlier recognition of red tides, closure of affected areas, and education of the public before substantial exposure to contaminated shellfish occurs.

Poisonous marine morsels

Consumption of seafood is increasing and physicians should know more about ichthyosarcotoxism, or fish-flesh poisoning. It appears in a variety of forms: poisoning, ciguatera, tetrodotoxin and scombroid poisoning. Ingestion of certain marine turtles and mammals also has been incriminated as the cause of illness. Paralytic shellfish poisoning can be deadly. In addition, red tide may cause respiratory problems.

Evidence that histamine is the causative toxin of scombroid-fish poisoning

BACKGROUND

The highest morbidity worldwide from fish poisoning results from the ingestion of spoiled scombroid fish, such as tuna and mackerel, and its cause is not clear. Histamine could be responsible, because spoiled scombroid fish contain large quantities of histamine. Whether histamine is the causative toxin, however, has remained in question. To address this issue, we investigated whether histamine homeostasis is altered in poisoned people.

METHODS

The urinary excretion of histamine and its metabolite, N-methylhistamine, was measured in three persons who had scombroid-fish poisoning (scombrotoxism) after the ingestion of marlin. We measured 9 alpha, 11 beta-dihydroxy-15-oxo-2,3,18,19-tetranorprost-5-ene-1,20-dioic acid (PGD-M), the principal metabolite of prostaglandin D2, a mast-cell secretory product, to assess whether mast cells had been activated to release histamine.

RESULTS

The fish contained high levels of histamine (842 to 2503 mumol per 100 g of tissue). Symptoms of scombrotoxism--flushing and headache--began 10 to 30 minutes after the ingestion of fish. In urine samples collected one to four hours after fish ingestion, the levels of histamine and N-methylhistamine were 9 to 20 times and 15 to 20 times the normal mean, respectively. During the subsequent 24 hours, the levels fell to 4 to 15 times and 4 to 11 times the normal values. Levels of both were normal 14 days later. PGD-M excretion was not increased at any time. Two persons treated with diphenhydramine had prompt amelioration of symptoms.

CONCLUSIONS

Scombroid-fish poisoning is associated with urinary excretion of histamine in quantities far exceeding those required to produce toxicity. The histamine is most likely derived from the spoiled fish. These results identify histamine as the toxin responsible for scombroid-fish poisoning.

Epiglottic enlargement: two unusual causes

We present two patients with airway obstruction due to supraglottic swelling; one was caused by a brown spider bite on an ear (loxoscelism) and the other from eating buffalo fish. In the latter patient it was likely due to scrombroid poisoning, a histamine-like reaction, but may have been an anaphylaxic reaction.

PET studies of domoic acid poisoning in humans: excitotoxic destruction of brain glutamatergic pathways, revealed in measurements of glucose metabolism by positron emission tomography

We used positron emission tomography to measure hippocampal and medial temporal lobe metabolism in brains of patients intoxicated by domoic acid from Prince Edward Island mussels. This analog of kainic acid specifically excites certain neurons in the hippocampus, and the study revealed a severe reduction of glucose metabolism in this part of the brain which paralleled the absence of long-, medium-, or short-term memory in these patients.

An outbreak of toxic encephalopathy caused by eating mussels contaminated with domoic acid

In Canada in late 1987 there was an outbreak of an acute illness characterized by gastrointestinal symptoms and unusual neurologic abnormalities among persons who had eaten cultivated mussels. Health departments in Canada solicited reports of this newly recognized illness. A case was defined as the occurrence of gastrointestinal symptoms within 24 hours or of neurologic symptoms within 48 hours of the ingestion of mussels. From the more than 250 reports received, 107 patients met the case definition. The most common symptoms were vomiting (in 76 percent of the patients), abdominal cramps (50 percent), diarrhea (42 percent), headache, often described as incapacitating (43 percent), and loss of short-term memory (25 percent). Nineteen patients were hospitalized, of whom 12 required intensive care because of seizures, coma, profuse respiratory secretions, or unstable blood pressure. Male sex and increasing age were associated independently with the risks of hospitalization and memory loss. Three patients died. Mussels associated with this illness were traced to cultivation beds in three river estuaries on the eastern coast of Prince Edward Island. Domoic acid, which can act as an excitatory neurotransmitter, was identified in mussels left uneaten by the patients and in mussels sampled from these estuaries. The source of the domoic acid appears to have been a form of marine vegetation, Nitzschia pungens, also identified in these waters in late 1987. The contaminated mussels from Prince Edward Island were removed from the market, and no new cases have occurred since December 1987. We conclude that the cause of this outbreak of a novel and severe intoxication was the ingestion of mussels contaminated by domoic acid, a potent excitatory neurotransmitter.