Paralytic Shellfish Poisoning

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Trigger for Investigation

On June 18, 2010 a commercial harvester advised an inspection specialist at CFIA of four cases of suspect PSP-related illness that occurred June 17, 2010. CFIA began an investigation and trace-back and advised relevant public health authorities of illnesses to ensure follow-up of cases. You are the epidemiologist working at the local health authority where the cases originated.

What is PSP?

Paralytic shellfish poisoning (PSP) occurs following the ingestion of bivalve shellfish (e.g., mussels, oysters, clams, and scallops), gastropods (abalone) and crustaceans (crab and lobster) containing biotoxins.1,2 The distribution of PSP biotoxins is worldwide, with biotoxins being produced by toxic marine micro-organisms that accumulate in exposed shellfish.1-3 There are more than 20 different PSP biotoxins, collectively called saxitoxins.3

What are the symptoms of PSP?

Paralytic shellfish poisoning can be life threatening; consequently, public health response to suspect PSP cases requires a timely and coordinated effort. Time between ingestion and onset of clinical effects ranges from 15 minutes to 10 hours, with median time being one hour.1,2 Initial symptoms include tingling or numbness of the tongue and lips that spreads to the face, neck, fingers, and toes. Headache, nausea, vomiting, diarrhea, hyper-salivation, fever, and diaphoresis may also occur. Exposed individuals may describe a feeling of dizziness or “floating,” owing to distortion of sensation and proprioception. Subsequent symptoms are generalized paraesthesia, arm and leg weakness, and ataxia. Rapid development of paralysis and respiratory failure may occur within 24 hours in severe cases. The rate of symptom progression is correlated with poisoning severity. In patients with mild to moderate poisoning, effects resolve over 2-3 days, but in severe cases, weakness may persist for up to a week. In most fatalities, death occurs rapidly, typically within 12 hours.

Illness information

Four persons ate raw oysters (quantity range 4-10 /person) and cooked Manila clams (quantity range 5-20/person) on June 17, 2010. Symptom onset occurred within 15 minutes with four out of four persons reporting tingling of the mouth and fingers. Symptoms resolved by early evening in all but one person who remained symptomatic the following morning with tingling in their toes. This individual was admitted to a hospital emergency department on June 18, 2010.

What can be done for individuals exhibiting the above symptoms and who are thought to be suspect-PSP illness cases?

There is no antidote for PSP, and all cases require immediate medical attention. Management of clinically ill individuals is primarily supportive. Close observation in the early stages of poisoning is critical so that any progression to paralysis and/or respiratory failure can be recognized and treated immediately and effectively.

What tests currently exist in Canada for PSP?

Laboratory testing for PSP in shellfish is via liquid chromatographic post-column oxidation (LC PCOX).4 This new test, which replaces the traditional mouse bioassay method used since the 1950s, separates fluid samples at the molecular level, thus allowing for individual toxic compounds to be identified and measured. Diagnosis of PSP in humans is primarily based on clinical presentation and recent exposure to shellfish.

Is PSP found in cooked shellfish?

Yes. Cooking will NOT destroy the toxins that cause PSP.

Would you expect these many cases of PSP?

No. In BC we would expect no cases of PSP.

What is an epidemic threshold? Has it been reached?

An epidemic threshold is the level of disease above which an urgent response is required. The epidemic threshold is specific to each disease and depends on the infectiousness or toxicity, other determinants of transmission or acquisition, and local endemnicity levels. As we would expect no cases of PSP in BC, and PSP can be life threatening, the epidemic threshold in this scenario has been reached.

If there was only one suspect case of PSP would you respond in the same or in a different manner?

The response would be the same, as PSP is life threatening and one case is enough to reach epidemic threshold and stimulate/initiate a response.

What would be your initial approach to these reports?

The answer should outline:

  1. Timely response (immediate)
  2. Active response
    1. Define cases and determine who may be at risk of becoming ill
    2. Initiate further case finding and interview cases already exposed. Use the shellfish related illness surveillance form (SRISF):
    3. Isolate suspect food product for testing if available
    4. Recommend closure of harvest site
    5. Communicate to community and to public health partners
  3. Collaborative response
    1. Inform EHO(s) or public health nurses in your health authority to assist with case ascertainment and food product collection
    2. Inform CFIA immediately. Follow up with CFIA as they have a duty to investigate further, especially in instances where the product has reached processors or distributors, or if there is risk of out-of-province export (remember that the area was not yet closed to harvesting). CFIA will notify and ask for involvement of other federal agencies if necessary. They would also be in a position to do further product testing, review previous biotoxin monitoring reports, recommend harvest area closure(s) if warranted, issue a health hazard advisory (recall) for the product, and review processor records.
    3. Follow up with BCCDC, especially in instances where multiple health authorities may be involved and/or if the product is to be submitted for testing.

Product information

Approximately 1000 dozen oysters and 400 lbs of Manila clams were commercially harvested on June 17, 2010. The above shellfish were processed by four processors, in addition to being sold to several local community members. The product sold to several of the local community members was consumed the same day and illnesses occurred as described above. Immediately following notification of symptoms, the commercial harvester called all affected processors and informed them to withdraw the product. CFIA was also informed immediately of the issue and in turn called all processors and advised withdrawal of the product. All implicated product was withdrawn voluntarily by all affected processors.

What information to date is worth communicating and to whom?

Person: who is at risk of becoming ill
Place: where was the shellfish harvested
Time: approximately when (time frame) were they harvested
Agent: PSP

To the public in particular, you will need to outline signs and symptoms (S/S) of PSP and inform them to seek medical attention immediately if they experience any S/S related to PSP post shellfish exposure (handling or ingestion).

To public health partners, you will need to inform of the number of illnesses, S/S experienced, outcomes, when exactly the event took place, and where the shellfish were harvested, and possibly in what quantity. If known, determine if any other shellfish were distributed or harvested in that time period and to whom.

What is the expected public health response in these cases?

Immediate action is required with regards to implementing control measures and public advisories. Control measures would be targeted towards the source and susceptible individuals. This would involve closure of the harvest site and further inspection (federal responsibility) and education to change behaviour and knowledge associated with self-harvesting of shellfish and S/S of PSP.


No further product was harvested from the implicated harvest site between June 10, 2010 and June 17, 2010. Details of the investigation were forwarded to the Office of Food Safety and Recall by CFIA. As no other product was left in distribution, a decision was made not to issue a health hazard alert to retailers or consumers.

A CNPHI alert was posted by BCCDC on June 19, 2010 to alert public health about the potential for illness in self-harvesters.

Can you name other examples in public health where a single case may be cause to respond?

  • Botulism
  • Anthrax
  • Diptheria
  • Human rabies
  • Salmonella typhi
  • Vibrio cholerae


We would like to thank Tom Kosatsky and Lorraine McIntyre for their invaluable input and review of this document.


  1. Isbister GK, Kiernan MC. Neurotoxic marine poisoning. Lancet Neurology. 2005;4(April):219-28.
  2. Brett MM. Food poisoning associated with biotoxins in fish and shellfish. Current Opinion in Infectious Diseases. 2003;16:461-5.
  3. La Barbera-Sánchez A, Franco Soler J, Rojas de Astudillo L, Chang-Yen I. Paralytic shellfish poisoning (PSP) in Margarita Island, Venezuela. Revista De Biología Tropical. 2004;52 Suppl 1:89-98.
  4. Canadian Food Inspection Agency. Government of Canada implements new, faster testing method for shellfish toxins. Government of Canada; 2011 [cited 2012 November 22].

Links to commonly used websites for shellfish related inquiries in BC


Environment Canada:

Department of Fisheries and Oceans:

June 2013