Incidences of shellfish poisoning and levels of toxicity are on the rise, which may be linked to global warming and the impact of humans on the environment. Shellfish such as cockles, mussels and oysters feed on microscopic algae (plankton). Some types of algae produce biotoxins that can accumulate in shellfish and in the tomalley (also referred to as hepatopancreas) of lobster and crab. The tomalley is the soft, green substance found in the body cavity of the lobster or crab that functions as the liver and pancreas. Research has shown that the tomalley can accumulate contaminants found in the environment, including the paralytic shellfish poisoning (PSP) toxin.
Types of Poisoning
There are four types of seafood poisonings caused by eating contaminated shellfish and crustaceans: paralytic shellfish poisoning (PSP), neurotoxic shellfish poisoning (NSP), amnesic shellfish poisoning (ASP) and diarrhetic shellfish poisoning (DSP).
Paralytic shellfish poisoning is caused by eating shellfish contaminated by saxitoxins or related compounds. The red-brown colour caused by these toxins can build up in the water and cause a “red-tide”; a natural warning sign. In Canada, red tides have occurred in the Gaspé region of Québec, the Bay of Fundy and along the British Columbia coastline. Generally, PSP is associated with eating contaminated mussels, clams, cockles, scallops, oysters, crabs and lobsters. The symptoms of PSP can appear 15 minutes to 10 hours after eating the contaminated shellfish and are usually mild, beginning with a tingling or numbness of the face, arms and legs, followed by headache, dizziness, nausea and poor muscle coordination. In cases of severe poisoning, muscle paralysis and respiratory failure occur and death may follow in two to twenty-five hours.
Neurotoxic shellfish poisoning is caused by eating shellfish, such as oysters, clams and mussels containing brevetoxins. Species harvested along the Atlantic coast of the southern United States and the Gulf of Mexico have led to cases of poisoning. Symptoms occur within a few minutes to a few hours after ingestion but illness does not last long. Symptoms include numbness, tingling in the mouth, arms and legs, poor coordination, stomach upset and severe muscle aches. NSP has never caused any fatalities and recovery normally occurs in two to three days.
Amnesic shellfish poisoningis caused by eating shellfish contaminated with the toxin, domoic acid. Domoic acid is known to concentrate in clams, mussels, oysters and scallops all over the world. There have been no known cases of illness in Canada since the first outbreak in 1987 on Prince Edward Island. Patients experience nausea, vomiting and diarrhea usually 30 minutes to six hours after eating the contaminated shellfish. Neurological symptoms such as dizziness, headache, disorientation and permanent short-term memory loss have also been recorded. In case of severe poisoning, seizures, paralysis and death may occur. The elderly and those with kidney problems are the most susceptible.
Diarrhetic shellfish poisoning is caused by eating shellfish contaminated with the toxin, okadaic acid. Shellfish associated with DSP outbreaks include mussels, oysters and scallops. DSP occurs seasonally in Europe and Japan, and periodically in other areas of the world. The onset of illness occurs 30 minutes to three hours after ingestion and includes symptoms such as vomiting and diarrhea that last for two to three days. This disease is not generally life threatening and patients recover completely.
Diagnosis and TreatmentGenerally, diagnosis of poisoning caused by shellfish is based on symptoms and a history of recently eaten food. Laboratory tests for specific toxins are difficult, as they require special techniques, equipment and specialized laboratories. Since there is no specific treatment for each shellfish toxin, there is generally no need to carry out these tests. Supportive care is the only treatment for poisoning caused by eating shellfish.
Only shellfish harvested from open harvest areas should be eaten. This is important because many outbreaks occur from shellfish recreationally harvested from closed areas. When an area is closed to shellfish harvesting, warnings are issued through signs posted around the area and media releases. Consumers who wish to harvest their own shellfish should find out from the Department of Fisheries and Oceans Canada (DFO) if the area is under a harvest prohibition.
To increase safety, purchase shellfish from a reputable store or restaurant, since shellfish sold in these establishments come from a federally inspected shellfish processing plant. Since these diseases can occur in other areas of the world, tourists should be aware of local conditions before consuming shellfish abroad.
Crustaceans such as crabs and lobsters that have fed on contaminated bivalve shellfish may contain PSP toxins in their tomalley. The Canadian Food Inspection Agency has reported that a small number of lobsters harvested during the late fall and early winter 2008 lobster fishing season contained levels of paralytic shellfish poison (PSP) in the tomalley that could represent a health risk. Health Canada has advised Canadians to limit their consumption of lobster tomalley, to the equivalent of one lobster daily for adults, due to the potential presence of PSP. Health Canada also recommends that children not consume lobster tomalley. The PSP toxin is not detected in lobster meat and there are no restrictions on the consumption of lobster meat.
Please note that marine shellfish toxins cannot be destroyed by cooking or freezing.
Monitoring by the Canadian GovernmentThe Canadian Food Inspection Agency (CFIA) operates a shellfish-monitoring program to provide early warning of the appearance of elevated marine shellfish toxins. As part of this program, the CFIA regularly analyzes shellfish samples taken from hundreds of harvesting sites in Atlantic Canada, Québec and British Columbia. When unacceptable levels are found in shellfish harvesting sites, the CFIA informs the DFO, which takes immediate measures to close the affected area to shellfish harvesting. Closed areas have signs posted and DFO fishery officers patrol the areas to prevent shellfish harvesting.
Canadian Food Inspection Agency. (2009). Specific Products and Risks. Retrieved fromhttp://www.inspection.gc.ca/english/fssa/concen/specife.shtml#fispoi
Centers for Disease Control and Prevention. (2004). Marine toxins. Retrieved fromhttp://www.cdc.gov/ncidod/dbmd/diseaseinfo/marinetoxins_g.htm
Fisheries and Oceans Canada. (2008). Shellfish biotoxins. Retrieved from http://www.pac.dfo-mpo.gc.ca/ops/fm/shellfish/Biotoxins/biotoxins_e.htm
Food and Drug Administration/Center for Food Safety & Applied Nutrition. (2004). Bad Bug Book: Various Shellfish-Associated Toxins. Retrieved fromhttp://www.cfsan.fda.gov/~mow/chap37.html
For updated link, click here.
Health Canada. (2009). Health Canada Updates Advice to Canadians on Consumption of Lobster Tomalley. Retrieved from http://www.hc-sc.gc.ca/ahc-asc/media/advisories-avis/_2009/2009_45-eng.php