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Epidemiologic Notes and Reports Paralytic Shellfish Poisoning -- Massachusetts and Alaska, 1990

Paralytic shellfish poisoning (PSP) is a foodborne illness caused by consumption of shellfish or broth from cooked shellfish that contain either concentrated saxitoxin, an alkaloid neurotoxin, or related compounds. This report summarizes outbreaks of PSP that occurred in Massachusetts and Alaska in June 1990. Massachusetts

On June 6, 1990, the Massachusetts Department of Public Health (MDPH) was notified that, on June 5, foodborne illness had occurred in six fishermen aboard a fishing boat in the Georges Bank area off the Nantucket coast. Onset of illness occurred after the men had eaten blue mussels (Mytilus edulis) harvested in deep water about 115 miles from the island of Nantucket.

The six men (age range: 24-47 years) developed symptoms 1-2 hours after consuming the shellfish (Table 1). Symptoms included numbness of mouth (five men), vomiting (four), paresthesia of extremities (four), numbness and tingling of tongue (two), numbness of face (two), numbness of throat (one), and periorbital edema (one). In all six men, lower back pain occurred approximately 24 hours after onset. The median duration of neurologic symptoms was 14 hours, and for lower back pain, 3.3 days. Approximately 10 hours after onset, when the fishermen presented to a local hospital emergency room, four were recovering; however, two, including one who had recovered from loss of consciousness, required hospitalization for 2-3 days.

The six fishermen had boiled the mussels for approximately 90 minutes before consuming them with baked fish, boiled rice, boiled potatoes, green salad, and other food items. They did not consume alcoholic beverages with the implicated meal.

Laboratory examination of the uneaten mussels detected saxitoxin concentrations of 24,400 ug/100 g in the raw mussels and 4280 ug/100 g in the cooked mussels (maximum safe level: 80 ug/100 mg). The difference in the levels of PSP toxin between raw and cooked mussels suggested that much of the saxitoxin had dissipated into the boiling water.

The implicated mussels had been harvested in an area of the Georges Bank where contamination of surf clams and sea scallops with saxitoxin had been detected through a deep-sea sampling survey conducted by the MDPH. The same area had been identified in a warning issued 2 weeks earlier by the MDPH and the National Marine Fisheries Service. The warning had been based on a report of a fisherman and his wife who had developed symptoms compatible with PSP after eating mussels obtained from that area. Because of the sampling survey and the first reported incident, the Georges Bank had been closed to harvesting of all shellfish except the adductor muscles of sea scallops shucked at sea. The closure notice had been sent to all appropriate Coast Guard stations and fishing vessels in the area; however, the six fishermen involved in the outbreak reported they had not received it. Alaska

On June 26, 1990, a physician reported to the Alaska Department of Health and Social Services (ADHSS) that a Native Alaskan man had died after consuming shellfish collected from a beach on the Alaska Peninsula. On the evening of June 25, while aboard a fishing boat, the decedent had consumed 25-30 steamed butter clams and 2 teaspoons of butter clam broth. Within an hour, he complained of numbness and tingling around his mouth, face, and fingers. Two hours later, he suffered a cardiopulmonary arrest; despite cardiopulmonary resuscitation efforts by emergency personnel, the patient died. Based on the symptoms reported, PSP was diagnosed. The patient's gastric contents contained 370 ug/100 g of PSP toxin, and a sample of the butter clam broth from the meal contained 2650 ug/100 g.

Two other crewmembers had also consumed butter clams. One developed numbness and tingling of the face and hands and dizziness approximately 1 hours later and recovered uneventfully; the other had no symptoms. Four crewmembers from two other fishing boats also had shared the butter clams presumed to be the vehicle for illness; all four had symptoms consistent with PSP.

As a consequence of this episode, ADHSS identified three additional episodes in the region of the Alaska Peninsula and Kodiak Island during June 1990. Each episode involved consumption of shellfish collected from a different area (Figure 1). When aggregated, the four episodes constituted a PSP outbreak with 13 cases among 21 persons (attack rate: 62%) who had consumed the implicated shellfish.

The four episodes occurred during June 17-25. Onset of symptoms ranged from 0 to 2 hours (median: 1 hour) after consumption of shellfish. Duration of illness ranged from 1 to 24 hours (median: 7.5 hours). Seven (54%) persons sought medical care. Only the index patient died (case-fatality rate: 8%); the others recovered uneventfully.

Seven (54%) cases resulted from consumption of butter clams, and six (46%), from mussels. Shellfish were consumed raw, boiled, or steamed. Affected persons consumed three to 30 shellfish each (median: four shellfish).

Because shellfish from the four episodes were not available for testing for PSP toxin, samples were collected from the four sites where the shellfish had been harvested. Butter clam samples from Volcano Bay and King Cove contained 7750 ug/ 100 g of PSP toxin, and mussel samples from Sand Point and Kodiak contained 1925-12,960 ug/100 g.

ADHSS, in conjunction with the Alaska Department of Environmental Conservation, issued a statewide press release warning of the risk for PSP for persons consuming shellfish collected from Alaskan beaches. Reported by: K Sharifzadeh, DVM, N Ridley, MS, R Waskiewicz, MS, P Luongo, Div of Food and Drugs, GF Grady, MD, A DeMaria, MD, RJ Timperi, MPH, J Nassif, MS, Massachusetts Dept of Public Health. M Sugita, Kodiak Health Clinic; V Gehrman, King Cove Clinic; P Peterson, Sand Point Clinic; A Alexander, Cold Bay Clinic; R Barrett, K Ballentine, Alaska Dept of Environmental Conservation; JP Middaugh, MD, State Epidemiologist, Alaska Dept of Health and Social Svcs. I Somerset, MS, Food and Drug Administration. Enteric Diseases Br, Div of Bacterial and Mycotic Diseases, Center for Infectious Diseases; Div of Field Epidemiology, Epidemiology Program Office, CDC.

Editorial Note

Editorial Note: The neurotoxins that cause PSP are among the most potent toxins known and can impair sensory, cerebellar, and motor functions. Saxitoxin is heat-stable and unaffected by standard cooking or steaming (1), is water-soluble, and can be concentrated in broth. Symptoms usually occur within 2 hours after ingestion of shellfish; high doses can lead to diaphragmatic paralysis, respiratory failure, and death (1,2).

The diagnosis of PSP is based on patient exposure history and clinical manifestations and on epidemiologic information. Predominant manifestations include paresthesia of the mouth and extremities, ataxia, dysphagia, and muscle paralysis (3-5); gastrointestinal symptoms are less common. Coma, total muscular paralysis, and respiratory arrest with death can occur. The prognosis is favorable for patients who survive beyond 12-18 hours (3). Because PSP has no specific treatment or antidote, treatment is supportive. Prompt evacuation of stomach contents may help by removing the remaining toxin-containing shellfish.

During 1973-1987, state health departments reported 19 PSP outbreaks (mean size: eight persons) to CDC's Foodborne Disease Outbreak Surveillance System. Outbreaks were caused by consumption of mussels, clams, oysters, scallops, and cockles. Outbreaks on the west coast have been reported from May through October, and on the east coast from August through October.

Most cases of PSP occur in individuals or small groups who gather shellfish for personal consumption. Although PSP has traditionally been considered a risk only in shellfish harvested from cold water, the incidence in tropical areas may be increasing: outbreaks have been reported recently from Central and South America, Asia, and the Pacific region (2,6).

The PSP-associated death in Alaska was the first in that state in greater than 14 years. From 1976 through 1989, 42 PSP outbreaks (accounting for 94 cases) were documented in Alaska. Butter clams were implicated in 23 (55%) of the outbreaks. Other shellfish implicated in Alaskan outbreaks included mussels, cockles, steamer clams, sea snails, and razor clams. Thirty-one (74%) of the 42 outbreaks occurred during May-July.

Shellfish can become toxic when toxin-producing dinoflagellates create massive algal blooms known as "red tides." However, shellfish can become toxic even in the absence of such blooms; detoxification may require a month or more in clean waters.

To prevent outbreaks of PSP and other shellfish intoxications, samples of susceptible mollusks are periodically collected in the coastal states and tested for toxin by mouse bioassay. When toxin levels exceed 80 ug/100 g, affected growing areas are quarantined, and sale of shellfish is prohibited. Warnings posted in shellfish-growing areas and on beaches and placed in the news media can alert the public to the hazard.


  1. Hughes JM, Merson MH. Fish and shellfish poisoning. N Engl J Med 1976;295:1117-20.

  2. Rodrigue DC, Etzel RA, Hall S, et al. Lethal paralytic shellfish poisoning in Guatemala. Am J Trop Med Hyg 1990;42:267-71.

  3. Eastaugh J, Shepherd S. Infectious and toxic syndromes from fish and shellfish consumption. Arch Intern Med 1989;149:1735-40.

  4. Sakamoto Y, Lockey RF, Krzanowski JJ. Shellfish and fish poisoning related to the toxic dinoflagellates. South Med J 1987;80:866-72.

  5. Wallace J. Disorders caused by venoms, bites, and stings. In: Isselbacher KJ, Adams RD, Braunwald E, Petersdorf RG, Wilson JD, eds. Harrison's principles of internal medicine. 9th ed. New York: McGraw-Hill Book Company, 1980:927.

  6. Maclean J, White A. Toxic dinoflagellate blooms in Asia: a growing concern. In: Anderson DM, White AW, Baden DG, eds. Toxic dinoflagellates. New York: Elsevier, 1985:517-30.

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