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Toxicity, Shellfish

Author: Thomas Arnold, MD, Medical Director, Louisiana Poison Control Center, Associate Professor and Chairman, Department of Emergency Medicine, Section of Clinical Toxicology, Louisiana State University Health Sciences Center
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Updated: Jun 7, 2007

Introduction

Background

Infectious agents cause most shellfish-associated illness. Hepatitis A, Norwalk virus, Vibrio parahaemolyticus, and Vibrio vulnificus all have been transmitted through shellfish ingestion. Toxic illness caused by shellfish has been recognized for several hundred years.

Native Americans are known to have warned early settlers to avoid shellfish during the summer months. Since that time, at least 4 distinct shellfish-poisoning syndromes have been identified, as follows:

• Paralytic shellfish poisoning (PSP)
• Neurologic shellfish poisoning (NSP)
• Diarrheal shellfish poisoning (DSP)
• Amnestic shellfish poisoning (ASP)

All 4 syndromes share some common features and primarily are associated with bivalve mollusks (eg, mussels, clams, oysters, scallops). These shellfish are filter feeders and, therefore, accumulate toxins produced by microscopic algae in the form of dinoflagellates and diatoms.

Pathophysiology

The toxins responsible for most shellfish poisonings are water-soluble, are heat and acid-stable, and are not inactivated by ordinary cooking methods. The main toxins responsible for each of the shellfish syndromes are as follows:

• PSP - Saxitoxin
• NPS - Brevetoxin
• DSP - Okadaic acid
• ASP - Domoic acid

The saxitoxins act by blocking sodium ion movement through voltage-dependent sodium channels in nerve and muscle cell membranes. Conduction block occurs principally in motor neurons and muscle. The toxin is made by dinoflagellates of the Gonyaulax species (red tide). Brevetoxins are polycyclic ethers that, like ciguatoxin, bind to and stimulate sodium flux through voltage-gated sodium channels in nerve and muscle. Brevetoxins are made by the dinoflagellate Ptychodiscus brevis. Okadaic acid binds to intestinal epithelial cells and increases their permeability. This toxin is made by dinoflagellates of the species Dinophysis and Prorocentrum. A group of these toxins associated with diarrheal shellfish poisoning has collectively been called pectenotoxins.

Domoic acid is structurally similar to the excitatory neurotransmitter glutamate. Domoic acid binds to and stimulates the kainic acid glutamate receptor, which allows sodium influx and a small amount of potassium efflux—neuronal depolarization results. Domoic acid has been associated with necrosis of the glutamate-rich hippocampus and amygdala in autopsied cases. Domoic acid is produced by the diatom Nitzschia pungens.

Frequency

United States

Toxic outbreaks often are associated with algal blooms of single-celled dinoflagellates, which can cause a red-brown discoloration of the water. This proliferation of toxic dinoflagellates, known as red tide, is favored by warmer weather. This phenomenon has led to the general teaching in North America that shellfish are safe to eat only if harvested in a month containing the letter "r."

Education, surveillance, and strict regulation by public health officials appear to be decreasing the incidence of shellfish poisoning in the United States. Additionally, enzyme-linked immunosorbent assay (ELISA) screening techniques are making detection of these toxins simple and rapid. Most recent cases of PSP have occurred along the northeast Atlantic coast, northwest Pacific coast, or Alaska. Most cases have involved recreational shellfish collectors, not commercial vendors. Since 1927, a total of 500 cases of PSP and 30 deaths have been reported in California. Sporadic and continuous outbreaks of NSP occur along the Gulf coast from Florida to Texas. In May 2002, 13 cases of saxitoxin poisoning were reported in Florida residents who ate pufferfish caught in waters near Titusville, Florida.

International

Sporadic outbreaks have been reported in Europe, Asia, Africa, and the Pacific Islands. Red tide and its resultant massive kills of various birds and marine animals have become an enormous concern in Europe, prompting numerous international congresses to address the problem.

Mortality/Morbidity

Fatality rates from PSP, the most severe of the 4 syndromes, ranges from 1-12% in isolated outbreaks. Its high mortality rate in some areas is caused by poor access to advanced life support capabilities. The mortality rate in the only known outbreak of ASP was 3%. To date, no deaths have been reported for NSP or DSP.

Age

Based on mortality figures from recent outbreaks, children appear to be more sensitive to the saxitoxins of PSP than adults. To date, all the reported deaths from ASP have been in elderly persons who had more severe neurologic symptoms.

Clinical

History

All 4 shellfish syndromes can produce symptoms lasting from a few minutes to several hours after ingestion of contaminated shellfish.

• Paralytic shellfish poisoning
• • The onset generally is noted with paresthesias of the lips, tongue, and gums. Symptom onset usually occurs within 30 minutes of ingestion.
  • Symptoms at onset rapidly progress to involve the distal extremities.
  • Other symptoms include a sensation of floating, headache, ataxia, muscle weakness, paralysis, and cranial nerve dysfunction.
  • Gastrointestinal symptoms are less common and may include nausea, vomiting, diarrhea, and abdominal pain. Fatalities are usually within the first 12 hours of symptom onset and are caused by unsupported respiratory failure.
  • PSP usually lasts 3 days, but muscle weakness may persist for weeks.
• Neurologic shellfish poisoning
• • The illness encountered with NSP is milder than that with PSP. Symptom onset ranges from 15 minutes to 18 hours postingestion, and the duration of toxicity ranges from 1-72 hours (usually <24 h) postingestion.
  • Presenting symptoms include gastroenteritis; rectal burning; paresthesias of the face, trunk, and limbs; myalgias; ataxia; vertigo; and reversal of hot/cold sensation.
  • Other less common features include tremor, dysphagia, bradycardia, decreased reflexes, and mydriasis.
  • This syndrome presents much like ciguatera poisoning but without a paralytic component, and it may last from several hours to a few days.
  • The brevetoxins, unlike the other shellfish toxins, can become aerosolized by the surf and produce an allergic response characterized by rhinorrhea, conjunctivitis, bronchospasm, and cough in sensitive individuals along the shore.
• Diarrheal shellfish poisoning
• • DSP is most common in Japan and Europe.
  • Gastroenteritis develops shortly after ingestion and generally lasts 1-2 days.
• Amnestic shellfish poisoning
• • The only reported outbreak occurred in 1987 and affected more than 100 people after eating mussels harvested off Prince Edward Island, Canada.
  • Gastroenteritis followed by headache and short-term memory loss occurred.
  • In a few cases, severe cognitive dysfunction to the point of interfering with the patient's ability to perform normal daily activities was noted.
  • Seizures, coma, hemiparesis, and ophthalmoplegia were noted in the most severe cases. The mortality rate is 3%.

Physical

• Findings vary according to the syndrome involved.
• • Gastrointestinal symptoms occur less often in PSP than in the other syndromes.
  • Paresthesias of the face and extremities are noted only in PSP and NSP.
  • ASP is the only shellfish syndrome with cognitive dysfunction as an early finding.
• Volume depletion from gastrointestinal symptoms is common to all syndromes.

Causes

Ingestion of raw or cooked mollusks that contain the toxin in sufficient quantities ensures the development of symptoms.

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