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Seafood Neurotoxins I: Shellfish Poisoning and the Nervous System
E. ANIMAL NEUROTOXINS
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Seafood Neurotoxins I: Shellfish Poisoning and the Nervous System Pratap Chand CHAPTER CONTENTS Introduction
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Shellfish Toxins
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INTRODUCTION Human shellfish poisoning can occur after eating clams, mussels, oysters, scallops, cockles, starfish, and crustaceans contaminated by toxins.1 Herbivorous dinoflagellates are the primary transvectors that accumulate the toxins via feeding in their digestive organs and soft tissues, apparently without harm to themselves. Shellfish consume dinoflagellate organisms while feeding, and the poison is stored in their bodies.2 This toxin has been found in these seafoods every month of the year, and butter clams have been known to store the toxin for up to 2 years. Shellfish poisoning is more common during red tides, when sea waters turn a reddish color because of the presence of large numbers of dinoflagellates (Figure 40-1; see color plate). These dinoflagellates produce at least 12 toxins, which are tetrahydropurines and are heat and acid stable. Saxitoxin was the first characterized and the best understood, and it produces paralysis.3 Humans, birds, and fish can all be affected by paralytic shellfish poisoning toxins.4 Shellfish contaminated by toxins do not have an abnormal taste, smell, or color; the toxins are water soluble and are not destroyed by acid, heating, or cooking.5 Shellfish poisoning can produce four clinical syndromes—paralytic shellfish poisoning, neurotoxic shellfish poisoning, diarrheal shellfish poisoning, and amnesic shellfish poisoning (Table 1).
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SHELLFISH TOXINS The main toxins responsible for each of the shellfish syndromes are as follows5: Paralytic shellfish poisoning—saxitoxin Neurotoxic shellfish poisoning—brevetoxin Diarrheal shellfish poisoning—okadaic acid Amnesic shellfish poisoning—domoic acid
Paralytic Shellfish Poisoning Gonyaulacoid dinoflagellates, the source of paralytic shellfish poisoning (PSP) marine toxins, develop algal blooms throughout the world (Figure 40-2; see color plate) for unknown reasons, although various factors have been studied, including change in weather, upwellings, temperature, turbulence, salinity, and transparency.6 PSP is a significant problem on both the East and the West coasts of the United States5 and is caused by several closely related species in the genus Alexandrium (Figure 40-3). On the East Coast, PSP is a serious and recurrent problem from Maine to Massachusetts and along the coasts of Connecticut, Long Island (New York), and New Jersey. On the West Coast, PSP is a recurrent annual problem along the coasts of northern California, Oregon,
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Seafood Neurotoxins I: Shellfish Poisoning and the Nervous System PSP
Figure 40-1. A red tide off the coast of La Jolla, California (see color plate). (Courtesy of Wikipedia.)
Washington, and Alaska. Florida red tides affect humans, wildlife, fishery resources, and the regional tourist-related economy. One of the highest concentrations of PSP in the world is reported to be in the shellfish in southeast Alaska.7
Toxins There are at least 21 molecular forms of PSP-associated toxins. Collectively, these PSP toxins are termed saxitoxins, deriving the name from the butter clam, Saxidomus giganteus, from which the toxin was originally extracted and identified. In mice, the saxitoxin LD50 parentally is 3 to 10 g/kg body weight and orally is 263 g/kg body weight.8 Humans are the most sensitive to saxitoxin; the
Table 1:
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Figure 40-2. Worldwide prevalence of paralytic shellfish poisoning in 2006 (see color plate). (Courtesy of U.S. National Office for Harmful Algal Blooms/Woods Hole Oceanographic Institution.)
oral dose in humans for death is 1 to 4 mg (5000 to 20,000 mouse units) depending upon the general and physical condition of the patient. It is rapidly absorbed through the gastrointestinal tract and excreted in the urine. The saxitoxins act by blocking sodium (Na⫹) ion movement through voltage-dependent Na⫹ channels in nerve and muscle cell membranes and have recently been found to also bind to calcium and potassium channels and neuronal nitric oxide synthase.9 Conduction block occurs principally in motor neurons and muscle. Saxitoxin inhibits the temporary permeability of Na⫹ ions by binding tightly to a receptor site on the outside surface of the membrane close to the external orifice of the Na⫹ channel. The resulting widespread blockade
Syndromes of Shellfish Poisoning
Disease
Toxin
Dinoflagellate
Symptoms or Signs
Treatment
Paralytic shellfish poisoning
Saxitoxin
Alexandrium
Headache, nausea, dizziness, pain, vomiting, anuria, paralysis, respiratory failure
Supportive Symptomatic Ventilation
Neurotoxic shellfish paralysis
Brevetoxin
Gymnodinium
Vomiting, rectal burning, myalgia, paresthesias, ataxia reversal of hot–cold sensation, vertigo, tremor, dysphagia, weakness, mydriasis, decreased reflexes
Supportive Symptomatic
Diarrheal shellfish poisoning
Okadaic acid
Dinophysis
Diarrhea, nausea, vomiting
Supportive Symptomatic
Amnesic shellfish poisoning
Domoic acid
Pseudonitzschia
Vomiting, abdominal cramps, diarrhea severe headache, loss of short-term memory, seizures, myoclonus, coma, hemiparesis, ophthalmoplegia, motor and sensory motor neuronopathy
Supportive Symptomatic
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Neurotoxic Substances: Animal Neurotoxins
decreased frequency, which returned to normal in a few weeks.12 The somatosensory evoked potentials confirmed normal peripheral and central sensory conduction. In another study, serial electrophysiological observations in a patient with acute bulbar and respiratory paralysis following ingestion of saxitoxin-contaminated clams showed prolonged distal motor and sensory latencies, slowed nerve conduction velocities, and moderately diminished amplitudes at the outset. All values returned to normal over 5 days.13 These findings, the result of incomplete Na⫹ channel blockade, distinguish PSP from most other acute paralytic illnesses. Figure 40-3. Alexandrium fundyense. (Courtesy of National Science Foundation.)
prevents impulse generation in peripheral nerves and skeletal muscles. Saxitoxin has a direct effect on skeletal muscle by blocking the muscle action potential without depolarizing cells; it abolishes peripheral nerve conduction but with no curare-like action at the neuromuscular junction.9
Clinical Features PSP develops usually within minutes after eating a contaminated shellfish, most commonly a mussel, clam, or oyster. From 5 to 30 minutes after consumption, there is slight perioral tingling progressing to numbness, which spreads to face and neck in moderate cases. Muscle weakness causing difficulty swallowing or speaking may occur. Other symptoms include sense of throat constriction, headache, dizziness, nausea, vomiting, rapid pain, and anuria.5 There is no loss of consciousness, and the reflexes are unaltered, except for perhaps pupillary size; sight may be temporarily lost. In severe cases within 2 to 12 hours there is complete paralysis, and death may occur from respiratory failure if the victims are not provided ventilatory support.7 The symptoms may last for 6 to 12 hours, after which most victims start to recover gradually but may continue to feel weak for a week or more.10 Some people have died after eating just one clam or mussel, others after eating many—each with a small amount of poison. The Guatemalan 1987 outbreak on the Pacific coast had a case fatality rate of 14%, which was even higher in young children (50%). It is possible that children may be more sensitive to PSP toxins than adults.11 In addition, the access to emergency medical services in acute cases is crucial to the prognosis. Nerve conduction studies in eight patients with PSP showed normal motor and sensory conduction velocities and amplitudes. The proximal conduction times, as assessed by F waves, showed delayed conduction and
Diagnosis The clinical scenario is the primary method of diagnosis initially. Recent shellfish ingestion, often but not always associated with a known red tide, and acute gastrointestinal illness with neurological symptoms forms the classic presentation. The differential diagnoses of an acute gastrointestinal illness with recent shellfish ingestion would be bacterial or viral gastroenteritis or recent organophosphate pesticide poisoning. It is important to obtain samples of contaminated tissues and their source. Each PSP epidemic is associated with different mixtures of the PSP toxins; this complicates the laboratory analysis of contaminated tissues. The mouse bioassay (time to death) of food extract was the conventional diagnostic method, but it cannot distinguish between tetrodotoxin and other PSP toxins. A mouse unit is defined as the minimum amount needed to cause the death of an 18- to 22-g white mouse in 15 minutes.14 Alternative methods include radioimmunoassay and indirect enzyme-linked immunoabsorbent assay (ELISA).15 High-performance liquid chromatography (HPLC) analysis methods for all PSP toxins have been developed with good correlation with mouse bioassay in terms of quantification.16 Human serum assays using HPLC have detected shellfish toxin but are not yet commercially available to clinicians.17 Research in this area is ongoing, and reliable serum assays for several of these specific toxins would be useful for diagnosis. Treatment In general, care is primarily symptomatic and supportive. Supportive measures are the basis of treatment for PSP, especially maintenance of the airway and ventilatory support with artificial ventilation in severe cases. Without supportive treatment, up to 75% of severely affected people die within 12 hours.18 It is important not to underestimate the seriousness of PSP. Once the symptoms begin to appear, the victim must be transported immediately to a medical care facility. Application of life support services at the medical care facility may be necessary to sustain the life of the victim. Nasogastric or orogastric lavage may be performed if the patient presents within 443
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1 hour of ingestion, but this is often unnecessary. If gastric lavage is performed, the use of isotonic Na⫹ bicarbonate solution for lavage has been suggested because many shellfish toxins have reduced potency in an alkaline environment. Care must be taken concerning aspiration with the neurologically compromised patient. Gastrointestinal decontamination with activated charcoal is recommended for patients who present within 4 hours of ingestion. The greatest danger is respiratory paralysis. Close monitoring for at least 24 hours and aggressive airway management at any sign of respiratory compromise should prevent severe morbidity and mortality. Reduction of symptoms normally occurs within 9 hours, and complete recovery usually occurs within 24 hours. Lactic acidosis can be seen in experimental animals and possibly humans and can be treated by assisted ventilation, fluid therapy, and periodic monitoring of the blood pH. It is possible that the fluid therapy also assists in the renal excretion of toxin.18 The prognosis for PSP is quite good, especially if the patient has passed the initial 12 hours of illness without needing breathing support. Most deaths occur during this period if breathing help is not available.
Neurotoxic Shellfish Poisoning With the ingestion of contaminated shellfish, neurotoxic shellfish poisoning (NSP) presents as a milder syndrome of gastroenteritis with neurological symptoms compared with PSP. The classic causative organism, Gymnodinium breve, is a dinoflagellate restricted to the Gulf of Mexico and the Caribbean, although similar species occur throughout the world.19 As G. breve cells die and break up, they release powerful neurotoxins, known collectively as brevetoxins.5 NSP is found especially during red tides in the late summer and autumn months almost every year off the west coast of Florida with massive fish and bird kills. Walker was the first to record NSP in 1880 on the west coast of Florida. The associated red tides are often characterized by patches of discolored water, dead or dying fish, and respiratory irritants in the air. Since then, NSP has been reported from the Gulf of Mexico, the east coast of Florida, and the North Carolina coast.20
Toxin Brevetoxins, the cause of NSP, are made by the dinoflagellate Ptychodiscus brevis and consist of two types of lipid-soluble toxins: hemolytic and neurotoxins.21 The major brevetoxin produced is PbTx-2, followed by lesser amounts of PbTx-1 and PbTx-3. Fish, birds, and mammals are all susceptible to the brevetoxins. The mouse LD50 is 0.20 mg/kg body weight (range 0.15 to 0.27) intraperitoneally. In human cases of NSP, the brevetoxin concentrations present in contaminated clams have been reported to be 30 to 18 g (range 78 to 120 g/mg). 444
Brevetoxins are polycyclic ethers that bind to and stimulate Na⫹ flux through voltage-gated Na⫹ channels in nerve and muscle. These toxins are depolarizing substances that open voltage-gated Na⫹ ion channels in cell walls, leading to uncontrolled Na⫹ influx into the cell.4 This alters the membrane properties of excitable cell types in ways that enhance the inward flow of Na⫹ ions into the cell; this current can be blocked by external application of tetrodotoxin.18 Like the other marine toxins, the brevetoxins are tasteless, odorless, and heat and acid stable. These toxins cannot be easily detected nor removed by food preparation procedures.5 Brevetoxin can be assayed by using a mouse bioassay, ELISA, and antibody radioimmunoassay.
Clinical Features The illness encountered with NSP is milder than that with PSP. Symptom onset ranges from 15 minutes to 18 hours after ingestion, and the duration of toxicity ranges from 1 to 72 hours (usually ⬍24 h) after ingestion.22 Presenting symptoms include gastroenteritis; rectal burning; paresthesias of the face, trunk, and limbs; myalgias; ataxia; and reversal of hot–cold sensations. Other less common features include vertigo, tremor, dysphagia, motor weakness, bradycardia, decreased reflexes, and mydriasis.23 Symptoms may last from several hours to a few days. Treatment Treatment of NSP is mostly symptomatic, with attention to fluid and electrolyte imbalance; the neurological symptoms are self-limiting, and patients recover spontaneously; respiratory compromise is not encountered as in PSP.5
Diarrheal Shellfish Poisoning Diarrheal shellfish poisoning (DSP) is a gastrointestinal illness without neurological manifestations reported worldwide caused by the consumption of contaminated shellfish.24 The causative organisms are the marine dinoflagellates Dinophysis and Procentrum, although there is an uneven distribution among species and location of toxin production. These dinoflagellates are widely distributed but do not always form red tides.
Toxin The associated toxins produced by the Dinophysis dinoflagellates are okadaic acid and its derivatives. At least nine toxins are produced by these dinoflagellates that bind to intestinal epithelial cells and increase their permeability; these have collectively been called pectenotoxins.25 D. fortii at levels of 200 cell/L in mussels and scallops becomes toxic for humans; the minimal amount of DSP toxins required to induce disease in humans was 12 mouse units. Pectenotoxin 1 causes liver damage in mice under similar circumstances. Okadaic acid is lipophilic. It is a potent inhibitor of protein phosphorylase
Section 4
phosphatase 1 and 2A in the cytosol of the mammalian cells that dephosphorylate serine and threonine.26 It probably causes diarrhea by stimulating the phosphorylation that controls Na⫹ secretion by intestinal cells. Okadaic acid also acts through the variations of cellular concentration of the calcium ion second messenger. It strongly increases the L-type inward calcium current in isolated guinea pig cardiac myocytes.
Clinical Features DSP is a self-limited diarrheal disease without known chronic sequelae. Gastroenteritis develops shortly after ingestion and generally lasts 1 to 2 days. There is no evidence of neurotoxicity, and no fatal cases have ever been reported. Diarrhea was the most commonly reported symptom, closely followed by nausea and vomiting, with onset 30 minutes to 12 hours from ingestion.27 Complete clinical recovery is seen even in severe cases within 3 days. Diagnosis A mouse bioassay using an intraperitoneal injection of toxin extracts with a 24-hour waiting period is used in Japan and shellfish with DSP toxin levels greater than 50 mouse units per kilogram are banned; similar surveillance systems have been established in the European countries.24 An HPLC method for detection of DSP toxins is available and is used in Sweden for monitoring purposes.28 Treatment Treatment is symptomatic and supportive with regards to short-term diarrhea and accompanying fluid and electrolyte losses. In general, hospitalization is not necessary; fluid and electrolytes can usually be replaced orally. Other diarrheal illnesses associated with shellfish consumption, such as bacterial or viral contamination, should be ruled out.24,27 Okadaic acid undergoes enterohepatic recycling that could be interrupted by charcoal administration.
Amnesic Shellfish Poisoning Amnesic shellfish poisoning (ASP) is caused by the consumption of shellfish contaminated with the neurotoxin domoic acid produced by diatoms in the genus Pseudonitzschia (see Figure 40-3). It may cause permanent short-term memory loss in victims, hence the name. It was first reported from Canada in 1987 and involved 150 reported cases, 19 hospitalizations, and four deaths after consumption of contaminated mussels. Later it was identified as a continuing problem in the Washington state and Oregon. After an initial gastroenteritis with neurological symptoms, some people with ASP develop apparent permanent neurological deficits.5 Domoic acid production has been confirmed for three species of Pseudonitzschia on the West Coast of the
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United States—P. australis, P. multiseries, and P. pungens. Domoic acid poisoning first became a noticeable problem in 1991, when pelicans and cormorants in Monterey Bay (California) died or suffered from unusual neurological symptoms similar to ASP. That same year, domoic acid was identified in razor clams and Dungeness crabs on the Oregon and Washington coasts. Since 1991, Pseudonitzschia species and domoic acid have recurred in Monterey Bay, but with relatively low cell numbers and concentrations. Toxic Pseudonitzschia species are present in the Northeast and Gulf of Mexico, and low levels of domoic acid have been detected in shellfish on the East Coast, but not at levels that necessitate quarantine.
Toxin Domoic acid is structurally similar to the excitatory neurotransmitter glutamate.29 Domoic acid binds to and stimulates the kainic acid glutamate receptor in the central nervous system, which allows Na⫹ influx and a small amount of potassium efflux with resulting neuronal depolarization. Lesions in the human brain, with necrosis of the glutamate-rich hippocampus and amygdala in autopsied cases, have been reported in the ASP cases and are similar to those seen in rats after kainic acid intravenous administration.30 When rats are exposed experimentally to domoic acid and its analogues, they experience limbic seizures, memory and gait abnormalities, and degeneration of the hippocampus. In animals, domoic acid is 3 times more potent than kainic acid and 30 to 100 times more potent than glutamic acid. Novelli et al. demonstrated that domoic acid from mussels is more neurotoxic for cultured human neurons than is purified domoic acid.31 This increase is believed to be due to domoic acid potentiation, even in subtoxic amounts, of the excitotoxic effect of glutamic and aspartic acids. Glutamic and aspartic acids are present in high concentrations in mussel tissue. This neurotoxic synergism may occur through a reduction in the voltage-dependent Mg2⫹ block at the N-methyl-d-aspartate (NMDA) receptor–associated channel, following activation of non-NMDA receptors by domoic acid. In humans, domoic acid appears to cause a nonprogressive acute neuronopathy involving anterior horn cells or a diffuse axonopathy predominantly affecting motor axons. The acute neuronal hyperexcitation syndrome presumably results from the stimulus of central and possibly peripheral neurons, followed by chronic loss of function in neural systems susceptible to excitotoxic degeneration (hippocampus and anterior horn cells of spinal cord). Clinical Features Gastroenteritis followed by headache and short-term memory loss is the typical scenario. In some cases, there followed confusion, hallucinations, loss of memory, and 445
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disorientation. Seizures, myoclonus, coma, hemiparesis, and ophthalmoplegia were noted in the most severe cases. The acute symptom frequencies were the following: vomiting (76%), abdominal cramps (50%), diarrhea (4%), severe headache (43%), and loss of short-term memory (25%). The mortality rate is 3%.32 Permanent neurological sequelae, especially cognitive dysfunction, were most likely in people who developed neurological illness with 48 hours, in males, in older patients (⬎60 years), and in younger people with preexisting illnesses such as diabetes, chronic renal disease, and hypertension with a history of transient ischemic attacks. Teitelbaum et al. studied 14 patients with severe neurological disease. In neuropsychological testing performed several months after the acute episode, 12 patients had severe antegrade memory deficits with relative preservation of other cognitive functions and 11 had clinical and electromyographic evidence of pure motor or sensory motor neuronopathy or axonopathy.30 Positron emission tomography results in 4 patients showed decreased glucose metabolism in the medial temporal lobes. The neuropathology for the 4 fatal cases revealed neuronal necrosis and loss, predominantly in the hippocampus and amygdala. All 14 patients with severe neurological disease reported confusion and disorientation within 1.5 to 48 hours after consumption. The maximal neurological deficits were seen 4 hours after ingestion in those least affected and 72 hours after ingestion in those most affected, with maximal improvement 24 hours to 12 weeks after ingestion. Acute coma was associated with the slowest recovery. Seizures ceased by 4 months but were frequent for up to 8 weeks.30
Diagnosis The mouse assay, HPLC, mass spectrometry, and ELISA techniques have been developed for detection of domoic acid from contaminated shellfish.33 Treatment Treatment of ASP is mainly supportive and symptomatic. Intensive care is required for those who have severe manifestations with unstable blood pressure, respiratory difficulties, and coma. Teitelbaum noted that seizures responded to intravenous diazepam and phenobarbital and were resistant to phenytoin in three patients.30
underreporting of PSP by people experiencing minor symptoms. In some instances, if victims had reported their PSP symptoms to a medical facility, more serious consequences could have been averted. Every effort should be made to obtain contaminated materials and their source. The most effective form of PSP prevention is to eliminate human contact with contaminated shellfish and other transvectors. Preventive measures include avoiding eating shellfish from an area with a high incidence of PSP. Purchasing shellfish from a seafood retailer or shellfish farm that sells only tested products is also a prudent measure. Cooking the shellfish does not prevent this disease. Routine surveillance of shellfish beds for known toxins and closures of the beds by monitoring the amount of toxins using the mouse assay are common practice throughout the world.34 In the United States, PSP levels in edible shellfish greater than 800 g of PSP per kilogram by mouse assay means that commercial beds are closed until they are monitored below this level; this action level is more than 10 times lower than the lowest level associated with human outbreaks. In Canada, it is recommended that commercial shellfish operations should be closed if concentrations of domoic acid exceed 20 g/g wet weight in shellfish. Furthermore, there is active monitoring of algal blooms with fish and bird kills. Ozonation can remove low levels of toxins from softshell clams but not if the clams have retained toxin for long periods; some industrial canning processes may lead to a decrease in PSP concentration.27,34 The feasibility and effectiveness of degrading saxitoxins through chlorination is being investigated. Biological controls to attack the red tide have been considered, such as using parasitic dinoflagellates like Amoebophrya ceratii that parasitizes various dinoflagellates responsible for PSP.35 Shellfish management regulations include a biotoxin control plan that is implemented during red tides to reduce the risk to humans from consumption of toxic mollusks. While some illness related to shellfish consumption occasionally has occurred, in general the highly cautionary regulations have been quite effective in preventing NSP.
REFERENCES PREVENTION OF SHELLFISH POISONING As with many marine toxin–induced diseases, the initial or index cases are often the tip of the iceberg. Any cases of PSP should be reported to the appropriate public health authorities for follow-up to ascertain other cases and to prevent further spread. A major problem is 446
1. Sobel J, Painter J. Illnesses caused by marine toxins. Clin Infect Dis. 2005;41:1290–1296. 2. Ciminiello P, Fattorusso E. Bivalve molluscs as vectors of marine biotoxins involved in seafood poisoning. Prog Mol Subcell Biol. 2006;43:53–82. 3. Van Dolah FM. Marine algal toxins: origins, health effects, and their increased occurrence. Environ Health Perspect. 2000;108 (Suppl 1):133–141.
Section 4 4. Baden DG. Marine food-borne dinoflagellate toxins. Int Rev Cytol. 1983;82: 99–150. 5. Baden DG, Fleming LE, Bean JA. Marine toxins. In: deWolf FA, ed. Handbook of Clinical Neurology: Intoxications of the Nervous System. Part II. Natural Toxins and Drugs. Amsterdam: Elsevier; 1995:141–175. 6. Anderson D. Toxic algal blooms and red tides: a global perspective. In: Okaichi T, Anderson D, Nemoto T, eds. Biology, Environmental Science and Toxicology. New York: Elsevier; 1989. 7. Gessner BD, Middaugh JP. Paralytic shellfish poisoning in Alaska: a 20-year retrospective analysis. Am J Epidemiol. 1995;141:766–770. 8. Erker EF, Slaughter LJ, Bass EL, et al. Acute toxic effects in mice of an extract from the marine algae Gonyaulax monilata. Toxicon. 1985;23:761–767. 9. Llewellyn LE. Saxitoxin: a toxic marine natural product that targets a multitude of receptors. Nat Prod Rep. 2006;23:200–222. 10. Isbister GK, Kiernan MC. Neurotoxic marine poisoning. Lancet Neurol. 2005;4:219–228. 11. Rodrigue DC, Etzel RA, Hall S, et al. Lethal paralytic shellfish poisoning in Guatemala. Am J Trop Med Hyg. 1990;42:267–271. 12. de Carvalho M, Jacinto J, Ramos N, et al. Paralytic shellfish poisoning: clinical and electrophysiological observations. J Neurol. 1998;245:551–554. 13. Long RR, Sargent JC, Hammer K. Paralytic shellfish poisoning: a case report and serial electrophysiologic observations. Neurology. 1990;40:1310–1312. 14. Park DL, Adams WN, Graham SL, et al. Variability of mouse bioassay for determination of paralytic shellfish poisoning toxins. J Assoc Off Anal Chem. 1986;69:547–550. 15. Usleber E, Dietrich R, Burk C, et al. Immunoassay methods for paralytic shellfish poisoning toxins. J AOAC Int. 2001;84:1649–1656. 16. Hungerford JM. Committee on Natural Toxins and Food Allergens: marine and freshwater toxins. J AOAC Int. 2006;89:248–269. 17. Gessner BD, Bell P, Doucette GJ, et al: Hypertension and identification of toxin in human urine and serum following a cluster of mussel-associated paralytic shellfish poisoning outbreaks. Toxicon. 1997;35:711–722. 18. Brown CK, Shepherd SM. Marine trauma, envenomations, and intoxications. Emerg Med Clin North Am. 1992;10:385–408. 19. Fleming LE, Stinn J. Shellfish poisonings. Travel Med. 1999;3: 1–6. 20. Stommel EW, Watters MR. Marine neurotoxins: Ingestible toxins. Curr Treat Options Neurol. 2004;6:105–114.
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21. Poli M, Mende TJ, Baden DG. Brevetoxins, unique activators of voltage-sensitive sodium channels bind to specific sites in rat brain synaptosomes. Mol Pharmacol. 1986;30:129–135. 22. Poli MA, Musser SM, Dickey RW, et al. Neurotoxic shellfish poisoning and brevetoxin metabolites: a case study from Florida. Toxicon. 2000;38:981–993. 23. Morris P, Campbell DS, Taylor TJ, et al. Clinical and epidemiological features of neurotoxic shellfish poisoning in North Carolina. Am J Public Health. 1991;81:471–473. 24. Aune T, Yndstad M. Diarrhetic shellfish poisoning. In: IR Falconer, ed. Algal Toxins in Seafood and Drinking Water. London: Academic Press; 1993:87–104. 25. Burgess V, Shaw G. Pectenotoxins: an issue for public health: a review of their comparative toxicology and metabolism. Environ Int. 2001;27:275–283. 26. Haystead TAJ, Sim ATR, Carling D, et al. Effects of the tumor promoter okadaic acid on intracellular protein phosphorylation and metabolism. Nature. 1989;337:78–81. 27. Halstead BW. Poisonous and Venomous Marine Animals of the World. 2nd ed, revised. Princeton, NJ: Darwin Press; 1988. 28. Lee JS, Murata M, Yasumoto T. Analytical methods for determination of diarrhetic shellfish toxins. In: Natori S, Hashimoto K, Ueno Y, eds. Mycotoxins and Phycotoxins. Amsterdam: Elsevier; 1989:327–334. 29. Chandrasekaran A, Ponnambalam G, Kaur C. Domoic acid–induced neurotoxicity in the hippocampus of adult rats. Neurotox Res. 2004;6:105–117. 30. Teitelbaum JS, Zatorre RJ, Carpenter S, et al. Neurologic sequelae of domoic acid intoxication due to the ingestion of contaminated mussels. N Engl J Med. 1990;322:1781–1787. 31. Novelli A, Kispert J, Fernandez-Sanchez T, et al. Domoic acid–containing toxic mussels produce neurotoxicity in neuronal cultures through a synergism between excitatory amino acids. Brain Res. 1992;577:41–48. 32. Jeffery B, Barlow T, Moizer K, et al. Amnesic shellfish poison. Food Chem Toxicol. 2004;42:545–557. 33. Lawrence JF, Charbonneau CF, Menard C, et al. Liquid chromatographic determination of domoic acid in shellfish products using the paralytic shellfish poison extraction procedure of the association of official analytical chemists. Journal of Chromatogr. 1989:462:349–356. 34. Silver MW. Protecting ourselves from shellfish poisoning. Am Sci. 2006;94:316–325. 35. Viviani R. Eutrophication, marine biotoxins, human health. Sci Total Environ. 1992;Suppl:631–662.
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Seafood Neurotoxins I: Shellfish Poisoning and the Nervous System Pratap Chand CHAPTER CONTENTS Introduction
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Shellfish Toxins
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INTRODUCTION Human shellfish poisoning can occur after eating clams, mussels, oysters, scallops, cockles, starfish, and crustaceans contaminated by toxins.1 Herbivorous dinoflagellates are the primary transvectors that accumulate the toxins via feeding in their digestive organs and soft tissues, apparently without harm to themselves. Shellfish consume dinoflagellate organisms while feeding, and the poison is stored in their bodies.2 This toxin has been found in these seafoods every month of the year, and butter clams have been known to store the toxin for up to 2 years. Shellfish poisoning is more common during red tides, when sea waters turn a reddish color because of the presence of large numbers of dinoflagellates (Figure 40-1; see color plate). These dinoflagellates produce at least 12 toxins, which are tetrahydropurines and are heat and acid stable. Saxitoxin was the first characterized and the best understood, and it produces paralysis.3 Humans, birds, and fish can all be affected by paralytic shellfish poisoning toxins.4 Shellfish contaminated by toxins do not have an abnormal taste, smell, or color; the toxins are water soluble and are not destroyed by acid, heating, or cooking.5 Shellfish poisoning can produce four clinical syndromes—paralytic shellfish poisoning, neurotoxic shellfish poisoning, diarrheal shellfish poisoning, and amnesic shellfish poisoning (Table 1).
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SHELLFISH TOXINS The main toxins responsible for each of the shellfish syndromes are as follows5: Paralytic shellfish poisoning—saxitoxin Neurotoxic shellfish poisoning—brevetoxin Diarrheal shellfish poisoning—okadaic acid Amnesic shellfish poisoning—domoic acid
Paralytic Shellfish Poisoning Gonyaulacoid dinoflagellates, the source of paralytic shellfish poisoning (PSP) marine toxins, develop algal blooms throughout the world (Figure 40-2; see color plate) for unknown reasons, although various factors have been studied, including change in weather, upwellings, temperature, turbulence, salinity, and transparency.6 PSP is a significant problem on both the East and the West coasts of the United States5 and is caused by several closely related species in the genus Alexandrium (Figure 40-3). On the East Coast, PSP is a serious and recurrent problem from Maine to Massachusetts and along the coasts of Connecticut, Long Island (New York), and New Jersey. On the West Coast, PSP is a recurrent annual problem along the coasts of northern California, Oregon,
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Seafood Neurotoxins I: Shellfish Poisoning and the Nervous System PSP
Figure 40-1. A red tide off the coast of La Jolla, California (see color plate). (Courtesy of Wikipedia.)
Washington, and Alaska. Florida red tides affect humans, wildlife, fishery resources, and the regional tourist-related economy. One of the highest concentrations of PSP in the world is reported to be in the shellfish in southeast Alaska.7
Toxins There are at least 21 molecular forms of PSP-associated toxins. Collectively, these PSP toxins are termed saxitoxins, deriving the name from the butter clam, Saxidomus giganteus, from which the toxin was originally extracted and identified. In mice, the saxitoxin LD50 parentally is 3 to 10 g/kg body weight and orally is 263 g/kg body weight.8 Humans are the most sensitive to saxitoxin; the
Table 1:
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Figure 40-2. Worldwide prevalence of paralytic shellfish poisoning in 2006 (see color plate). (Courtesy of U.S. National Office for Harmful Algal Blooms/Woods Hole Oceanographic Institution.)
oral dose in humans for death is 1 to 4 mg (5000 to 20,000 mouse units) depending upon the general and physical condition of the patient. It is rapidly absorbed through the gastrointestinal tract and excreted in the urine. The saxitoxins act by blocking sodium (Na⫹) ion movement through voltage-dependent Na⫹ channels in nerve and muscle cell membranes and have recently been found to also bind to calcium and potassium channels and neuronal nitric oxide synthase.9 Conduction block occurs principally in motor neurons and muscle. Saxitoxin inhibits the temporary permeability of Na⫹ ions by binding tightly to a receptor site on the outside surface of the membrane close to the external orifice of the Na⫹ channel. The resulting widespread blockade
Syndromes of Shellfish Poisoning
Disease
Toxin
Dinoflagellate
Symptoms or Signs
Treatment
Paralytic shellfish poisoning
Saxitoxin
Alexandrium
Headache, nausea, dizziness, pain, vomiting, anuria, paralysis, respiratory failure
Supportive Symptomatic Ventilation
Neurotoxic shellfish paralysis
Brevetoxin
Gymnodinium
Vomiting, rectal burning, myalgia, paresthesias, ataxia reversal of hot–cold sensation, vertigo, tremor, dysphagia, weakness, mydriasis, decreased reflexes
Supportive Symptomatic
Diarrheal shellfish poisoning
Okadaic acid
Dinophysis
Diarrhea, nausea, vomiting
Supportive Symptomatic
Amnesic shellfish poisoning
Domoic acid
Pseudonitzschia
Vomiting, abdominal cramps, diarrhea severe headache, loss of short-term memory, seizures, myoclonus, coma, hemiparesis, ophthalmoplegia, motor and sensory motor neuronopathy
Supportive Symptomatic
Section 4
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Neurotoxic Substances: Animal Neurotoxins
decreased frequency, which returned to normal in a few weeks.12 The somatosensory evoked potentials confirmed normal peripheral and central sensory conduction. In another study, serial electrophysiological observations in a patient with acute bulbar and respiratory paralysis following ingestion of saxitoxin-contaminated clams showed prolonged distal motor and sensory latencies, slowed nerve conduction velocities, and moderately diminished amplitudes at the outset. All values returned to normal over 5 days.13 These findings, the result of incomplete Na⫹ channel blockade, distinguish PSP from most other acute paralytic illnesses. Figure 40-3. Alexandrium fundyense. (Courtesy of National Science Foundation.)
prevents impulse generation in peripheral nerves and skeletal muscles. Saxitoxin has a direct effect on skeletal muscle by blocking the muscle action potential without depolarizing cells; it abolishes peripheral nerve conduction but with no curare-like action at the neuromuscular junction.9
Clinical Features PSP develops usually within minutes after eating a contaminated shellfish, most commonly a mussel, clam, or oyster. From 5 to 30 minutes after consumption, there is slight perioral tingling progressing to numbness, which spreads to face and neck in moderate cases. Muscle weakness causing difficulty swallowing or speaking may occur. Other symptoms include sense of throat constriction, headache, dizziness, nausea, vomiting, rapid pain, and anuria.5 There is no loss of consciousness, and the reflexes are unaltered, except for perhaps pupillary size; sight may be temporarily lost. In severe cases within 2 to 12 hours there is complete paralysis, and death may occur from respiratory failure if the victims are not provided ventilatory support.7 The symptoms may last for 6 to 12 hours, after which most victims start to recover gradually but may continue to feel weak for a week or more.10 Some people have died after eating just one clam or mussel, others after eating many—each with a small amount of poison. The Guatemalan 1987 outbreak on the Pacific coast had a case fatality rate of 14%, which was even higher in young children (50%). It is possible that children may be more sensitive to PSP toxins than adults.11 In addition, the access to emergency medical services in acute cases is crucial to the prognosis. Nerve conduction studies in eight patients with PSP showed normal motor and sensory conduction velocities and amplitudes. The proximal conduction times, as assessed by F waves, showed delayed conduction and
Diagnosis The clinical scenario is the primary method of diagnosis initially. Recent shellfish ingestion, often but not always associated with a known red tide, and acute gastrointestinal illness with neurological symptoms forms the classic presentation. The differential diagnoses of an acute gastrointestinal illness with recent shellfish ingestion would be bacterial or viral gastroenteritis or recent organophosphate pesticide poisoning. It is important to obtain samples of contaminated tissues and their source. Each PSP epidemic is associated with different mixtures of the PSP toxins; this complicates the laboratory analysis of contaminated tissues. The mouse bioassay (time to death) of food extract was the conventional diagnostic method, but it cannot distinguish between tetrodotoxin and other PSP toxins. A mouse unit is defined as the minimum amount needed to cause the death of an 18- to 22-g white mouse in 15 minutes.14 Alternative methods include radioimmunoassay and indirect enzyme-linked immunoabsorbent assay (ELISA).15 High-performance liquid chromatography (HPLC) analysis methods for all PSP toxins have been developed with good correlation with mouse bioassay in terms of quantification.16 Human serum assays using HPLC have detected shellfish toxin but are not yet commercially available to clinicians.17 Research in this area is ongoing, and reliable serum assays for several of these specific toxins would be useful for diagnosis. Treatment In general, care is primarily symptomatic and supportive. Supportive measures are the basis of treatment for PSP, especially maintenance of the airway and ventilatory support with artificial ventilation in severe cases. Without supportive treatment, up to 75% of severely affected people die within 12 hours.18 It is important not to underestimate the seriousness of PSP. Once the symptoms begin to appear, the victim must be transported immediately to a medical care facility. Application of life support services at the medical care facility may be necessary to sustain the life of the victim. Nasogastric or orogastric lavage may be performed if the patient presents within 443
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Seafood Neurotoxins I: Shellfish Poisoning and the Nervous System
1 hour of ingestion, but this is often unnecessary. If gastric lavage is performed, the use of isotonic Na⫹ bicarbonate solution for lavage has been suggested because many shellfish toxins have reduced potency in an alkaline environment. Care must be taken concerning aspiration with the neurologically compromised patient. Gastrointestinal decontamination with activated charcoal is recommended for patients who present within 4 hours of ingestion. The greatest danger is respiratory paralysis. Close monitoring for at least 24 hours and aggressive airway management at any sign of respiratory compromise should prevent severe morbidity and mortality. Reduction of symptoms normally occurs within 9 hours, and complete recovery usually occurs within 24 hours. Lactic acidosis can be seen in experimental animals and possibly humans and can be treated by assisted ventilation, fluid therapy, and periodic monitoring of the blood pH. It is possible that the fluid therapy also assists in the renal excretion of toxin.18 The prognosis for PSP is quite good, especially if the patient has passed the initial 12 hours of illness without needing breathing support. Most deaths occur during this period if breathing help is not available.
Neurotoxic Shellfish Poisoning With the ingestion of contaminated shellfish, neurotoxic shellfish poisoning (NSP) presents as a milder syndrome of gastroenteritis with neurological symptoms compared with PSP. The classic causative organism, Gymnodinium breve, is a dinoflagellate restricted to the Gulf of Mexico and the Caribbean, although similar species occur throughout the world.19 As G. breve cells die and break up, they release powerful neurotoxins, known collectively as brevetoxins.5 NSP is found especially during red tides in the late summer and autumn months almost every year off the west coast of Florida with massive fish and bird kills. Walker was the first to record NSP in 1880 on the west coast of Florida. The associated red tides are often characterized by patches of discolored water, dead or dying fish, and respiratory irritants in the air. Since then, NSP has been reported from the Gulf of Mexico, the east coast of Florida, and the North Carolina coast.20
Toxin Brevetoxins, the cause of NSP, are made by the dinoflagellate Ptychodiscus brevis and consist of two types of lipid-soluble toxins: hemolytic and neurotoxins.21 The major brevetoxin produced is PbTx-2, followed by lesser amounts of PbTx-1 and PbTx-3. Fish, birds, and mammals are all susceptible to the brevetoxins. The mouse LD50 is 0.20 mg/kg body weight (range 0.15 to 0.27) intraperitoneally. In human cases of NSP, the brevetoxin concentrations present in contaminated clams have been reported to be 30 to 18 g (range 78 to 120 g/mg). 444
Brevetoxins are polycyclic ethers that bind to and stimulate Na⫹ flux through voltage-gated Na⫹ channels in nerve and muscle. These toxins are depolarizing substances that open voltage-gated Na⫹ ion channels in cell walls, leading to uncontrolled Na⫹ influx into the cell.4 This alters the membrane properties of excitable cell types in ways that enhance the inward flow of Na⫹ ions into the cell; this current can be blocked by external application of tetrodotoxin.18 Like the other marine toxins, the brevetoxins are tasteless, odorless, and heat and acid stable. These toxins cannot be easily detected nor removed by food preparation procedures.5 Brevetoxin can be assayed by using a mouse bioassay, ELISA, and antibody radioimmunoassay.
Clinical Features The illness encountered with NSP is milder than that with PSP. Symptom onset ranges from 15 minutes to 18 hours after ingestion, and the duration of toxicity ranges from 1 to 72 hours (usually ⬍24 h) after ingestion.22 Presenting symptoms include gastroenteritis; rectal burning; paresthesias of the face, trunk, and limbs; myalgias; ataxia; and reversal of hot–cold sensations. Other less common features include vertigo, tremor, dysphagia, motor weakness, bradycardia, decreased reflexes, and mydriasis.23 Symptoms may last from several hours to a few days. Treatment Treatment of NSP is mostly symptomatic, with attention to fluid and electrolyte imbalance; the neurological symptoms are self-limiting, and patients recover spontaneously; respiratory compromise is not encountered as in PSP.5
Diarrheal Shellfish Poisoning Diarrheal shellfish poisoning (DSP) is a gastrointestinal illness without neurological manifestations reported worldwide caused by the consumption of contaminated shellfish.24 The causative organisms are the marine dinoflagellates Dinophysis and Procentrum, although there is an uneven distribution among species and location of toxin production. These dinoflagellates are widely distributed but do not always form red tides.
Toxin The associated toxins produced by the Dinophysis dinoflagellates are okadaic acid and its derivatives. At least nine toxins are produced by these dinoflagellates that bind to intestinal epithelial cells and increase their permeability; these have collectively been called pectenotoxins.25 D. fortii at levels of 200 cell/L in mussels and scallops becomes toxic for humans; the minimal amount of DSP toxins required to induce disease in humans was 12 mouse units. Pectenotoxin 1 causes liver damage in mice under similar circumstances. Okadaic acid is lipophilic. It is a potent inhibitor of protein phosphorylase
Section 4
phosphatase 1 and 2A in the cytosol of the mammalian cells that dephosphorylate serine and threonine.26 It probably causes diarrhea by stimulating the phosphorylation that controls Na⫹ secretion by intestinal cells. Okadaic acid also acts through the variations of cellular concentration of the calcium ion second messenger. It strongly increases the L-type inward calcium current in isolated guinea pig cardiac myocytes.
Clinical Features DSP is a self-limited diarrheal disease without known chronic sequelae. Gastroenteritis develops shortly after ingestion and generally lasts 1 to 2 days. There is no evidence of neurotoxicity, and no fatal cases have ever been reported. Diarrhea was the most commonly reported symptom, closely followed by nausea and vomiting, with onset 30 minutes to 12 hours from ingestion.27 Complete clinical recovery is seen even in severe cases within 3 days. Diagnosis A mouse bioassay using an intraperitoneal injection of toxin extracts with a 24-hour waiting period is used in Japan and shellfish with DSP toxin levels greater than 50 mouse units per kilogram are banned; similar surveillance systems have been established in the European countries.24 An HPLC method for detection of DSP toxins is available and is used in Sweden for monitoring purposes.28 Treatment Treatment is symptomatic and supportive with regards to short-term diarrhea and accompanying fluid and electrolyte losses. In general, hospitalization is not necessary; fluid and electrolytes can usually be replaced orally. Other diarrheal illnesses associated with shellfish consumption, such as bacterial or viral contamination, should be ruled out.24,27 Okadaic acid undergoes enterohepatic recycling that could be interrupted by charcoal administration.
Amnesic Shellfish Poisoning Amnesic shellfish poisoning (ASP) is caused by the consumption of shellfish contaminated with the neurotoxin domoic acid produced by diatoms in the genus Pseudonitzschia (see Figure 40-3). It may cause permanent short-term memory loss in victims, hence the name. It was first reported from Canada in 1987 and involved 150 reported cases, 19 hospitalizations, and four deaths after consumption of contaminated mussels. Later it was identified as a continuing problem in the Washington state and Oregon. After an initial gastroenteritis with neurological symptoms, some people with ASP develop apparent permanent neurological deficits.5 Domoic acid production has been confirmed for three species of Pseudonitzschia on the West Coast of the
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Neurotoxic Substances: Animal Neurotoxins
United States—P. australis, P. multiseries, and P. pungens. Domoic acid poisoning first became a noticeable problem in 1991, when pelicans and cormorants in Monterey Bay (California) died or suffered from unusual neurological symptoms similar to ASP. That same year, domoic acid was identified in razor clams and Dungeness crabs on the Oregon and Washington coasts. Since 1991, Pseudonitzschia species and domoic acid have recurred in Monterey Bay, but with relatively low cell numbers and concentrations. Toxic Pseudonitzschia species are present in the Northeast and Gulf of Mexico, and low levels of domoic acid have been detected in shellfish on the East Coast, but not at levels that necessitate quarantine.
Toxin Domoic acid is structurally similar to the excitatory neurotransmitter glutamate.29 Domoic acid binds to and stimulates the kainic acid glutamate receptor in the central nervous system, which allows Na⫹ influx and a small amount of potassium efflux with resulting neuronal depolarization. Lesions in the human brain, with necrosis of the glutamate-rich hippocampus and amygdala in autopsied cases, have been reported in the ASP cases and are similar to those seen in rats after kainic acid intravenous administration.30 When rats are exposed experimentally to domoic acid and its analogues, they experience limbic seizures, memory and gait abnormalities, and degeneration of the hippocampus. In animals, domoic acid is 3 times more potent than kainic acid and 30 to 100 times more potent than glutamic acid. Novelli et al. demonstrated that domoic acid from mussels is more neurotoxic for cultured human neurons than is purified domoic acid.31 This increase is believed to be due to domoic acid potentiation, even in subtoxic amounts, of the excitotoxic effect of glutamic and aspartic acids. Glutamic and aspartic acids are present in high concentrations in mussel tissue. This neurotoxic synergism may occur through a reduction in the voltage-dependent Mg2⫹ block at the N-methyl-d-aspartate (NMDA) receptor–associated channel, following activation of non-NMDA receptors by domoic acid. In humans, domoic acid appears to cause a nonprogressive acute neuronopathy involving anterior horn cells or a diffuse axonopathy predominantly affecting motor axons. The acute neuronal hyperexcitation syndrome presumably results from the stimulus of central and possibly peripheral neurons, followed by chronic loss of function in neural systems susceptible to excitotoxic degeneration (hippocampus and anterior horn cells of spinal cord). Clinical Features Gastroenteritis followed by headache and short-term memory loss is the typical scenario. In some cases, there followed confusion, hallucinations, loss of memory, and 445
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disorientation. Seizures, myoclonus, coma, hemiparesis, and ophthalmoplegia were noted in the most severe cases. The acute symptom frequencies were the following: vomiting (76%), abdominal cramps (50%), diarrhea (4%), severe headache (43%), and loss of short-term memory (25%). The mortality rate is 3%.32 Permanent neurological sequelae, especially cognitive dysfunction, were most likely in people who developed neurological illness with 48 hours, in males, in older patients (⬎60 years), and in younger people with preexisting illnesses such as diabetes, chronic renal disease, and hypertension with a history of transient ischemic attacks. Teitelbaum et al. studied 14 patients with severe neurological disease. In neuropsychological testing performed several months after the acute episode, 12 patients had severe antegrade memory deficits with relative preservation of other cognitive functions and 11 had clinical and electromyographic evidence of pure motor or sensory motor neuronopathy or axonopathy.30 Positron emission tomography results in 4 patients showed decreased glucose metabolism in the medial temporal lobes. The neuropathology for the 4 fatal cases revealed neuronal necrosis and loss, predominantly in the hippocampus and amygdala. All 14 patients with severe neurological disease reported confusion and disorientation within 1.5 to 48 hours after consumption. The maximal neurological deficits were seen 4 hours after ingestion in those least affected and 72 hours after ingestion in those most affected, with maximal improvement 24 hours to 12 weeks after ingestion. Acute coma was associated with the slowest recovery. Seizures ceased by 4 months but were frequent for up to 8 weeks.30
Diagnosis The mouse assay, HPLC, mass spectrometry, and ELISA techniques have been developed for detection of domoic acid from contaminated shellfish.33 Treatment Treatment of ASP is mainly supportive and symptomatic. Intensive care is required for those who have severe manifestations with unstable blood pressure, respiratory difficulties, and coma. Teitelbaum noted that seizures responded to intravenous diazepam and phenobarbital and were resistant to phenytoin in three patients.30
underreporting of PSP by people experiencing minor symptoms. In some instances, if victims had reported their PSP symptoms to a medical facility, more serious consequences could have been averted. Every effort should be made to obtain contaminated materials and their source. The most effective form of PSP prevention is to eliminate human contact with contaminated shellfish and other transvectors. Preventive measures include avoiding eating shellfish from an area with a high incidence of PSP. Purchasing shellfish from a seafood retailer or shellfish farm that sells only tested products is also a prudent measure. Cooking the shellfish does not prevent this disease. Routine surveillance of shellfish beds for known toxins and closures of the beds by monitoring the amount of toxins using the mouse assay are common practice throughout the world.34 In the United States, PSP levels in edible shellfish greater than 800 g of PSP per kilogram by mouse assay means that commercial beds are closed until they are monitored below this level; this action level is more than 10 times lower than the lowest level associated with human outbreaks. In Canada, it is recommended that commercial shellfish operations should be closed if concentrations of domoic acid exceed 20 g/g wet weight in shellfish. Furthermore, there is active monitoring of algal blooms with fish and bird kills. Ozonation can remove low levels of toxins from softshell clams but not if the clams have retained toxin for long periods; some industrial canning processes may lead to a decrease in PSP concentration.27,34 The feasibility and effectiveness of degrading saxitoxins through chlorination is being investigated. Biological controls to attack the red tide have been considered, such as using parasitic dinoflagellates like Amoebophrya ceratii that parasitizes various dinoflagellates responsible for PSP.35 Shellfish management regulations include a biotoxin control plan that is implemented during red tides to reduce the risk to humans from consumption of toxic mollusks. While some illness related to shellfish consumption occasionally has occurred, in general the highly cautionary regulations have been quite effective in preventing NSP.
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