Authorities Issue Warnings about PSP Toxins in Shellfish, Clams, Mussels and Scallops


Dangerous levels of paralytic shellfish poisoning (PSP) toxins have been detected in mussels in different jurisdictions. The naturally occurring PSP toxins can cause illness or death in humans. Eating items such as mussels, cockles, oysters or razor fish from these areas may pose a health risk. Cooking or freezing does not remove the toxin.

The latest warning came from the Sabah Fisheries Department, Malaysia, which recently warned the people against selling or consuming shellfish as Paralytic Shellfish Poison (PSP) toxins reached dangerous levels in the Sungai Trayong waters. Californian and New Zealand authorities have issued similar warnings in the recent past. Tuaran District Fisheries enforcer Chin Tet Foh said:

“High levels of PSP toxins have been detected in the waters of sungai Trayong, Tuaran […] as a result of analysis of oyster (shellfish) samples conducted by the Fisheries Biosecurity Unit at Likas Fisheries Complex, the PSP level has reached a dangerous level in shellfish […] in this regard, the public is advised or prohibited from selling and consuming shellfish like oysters, Lokan, mussels and other types of shellfish.”

Frequent Mandatory Testing

The California Department of Public Health (CDPH) advised consumers not to eat sports-harvested mussels, clams, or whole scallops from Humboldt County. The warning did not apply to commercially sold clams, mussels, scallops or oysters from approved sources. State law permits only state-certified commercial shellfish harvesters or dealers to sell these products. Shellfish sold by certified harvesters and dealers are subject to frequent mandatory testing to monitor for toxins.

PSP toxins affect the central nervous system, producing a tingling around the mouth and fingertips within a few minutes to a few hours after eating toxic shellfish. These symptoms are typically followed by loss of balance, lack of muscular coordination, slurred speech and difficulty swallowing. In severe poisonings, complete muscular paralysis and death from asphyxiation can occur.

Biotoxin Affects the Nervous System

Paralytic Shellfish Poison (PSP) is a naturally occurring marine biotoxin produced by some microscopic algae species. Shellfish eat these algae and can retain the toxin. People can become ill from eating shellfish contaminated with Paralytic Shellfish Poison. This biotoxin affects the nervous system and paralyzes muscles, thus the term “paralytic” shellfish poison. High levels of Paralytic Shellfish Poison can cause severe illness and death.

How shellfish become contaminated with Paralytic Shellfish Poison

Shellfish are filter feeders. They pump water through their systems, filtering out and eating algae and other food particles. When shellfish eat biotoxin producing algae, the biotoxin can accumulate in their tissue.

Bivalve molluscan shellfish such as clams, mussels, oysters, geoduck, and scallops can accumulate Paralytic Shellfish Poison. Other marine species, such as sea cucumbers, might be affected. Crab, because they feed on shellfish, can also become toxic. Even if the crab meat is safe, toxins tend to accumulate in the crab gut and butter – the white-yellow fat inside the back of the shell. Clean crab thoroughly and avoid eating the crab butter and guts.  

Continuing Research

It’s normal for biotoxin producing algae to be present in marine water. They are usually in low numbers that cause no problems. But when the algae “blooms,” the amount of biotoxin-producing algae can increase. The increased algae become a greater food source for shellfish. The more algae the shellfish eat, the more biotoxin they accumulate. 

Biotoxins don’t harm shellfish, so the level in their tissue will rise until the bloom subsides. When the number of toxin-producing algal cells returns to normal low levels, the shellfish eventually flush the toxin from their bodies. It can be several days to several months or longer before they’re safe to eat again.

When water conditions become favorable, the algae “blooms” and reproduce, leading to increased PSP toxins. Continuing research has pointed to certain cause and effect situations, but the exact combination of conditions that cause blooms is unknown. 

Death from Paralytic Shellfish Poison

Early PSP poisoning symptoms include tingling of the lips and tongue, which may begin within minutes of eating toxic shellfish or may take an hour or two to develop. Symptoms may progress to tingling of fingers and toes and then losing control of arms and legs, followed by difficulty in breathing. Some people feel nauseous or experience a sense of floating. If a person consumes enough toxins, muscles of the chest and abdomen become paralyzed, including muscles used for breathing, and the victim can suffocate. Death from Paralytic Shellfish Poison has occurred in less than 30 minutes.


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