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Paralytic Shellfish Poison (PSP)
What is paralytic shellfish
poison?
Paralytic shellfish poison (PSP) is naturally occurring toxin, called a "biotoxin",
that is produced by some species of microscopic algae. PSP can
concentrate in shellfish and, when eaten, can cause illness in humans.
This biotoxin affects the nervous system and paralyzes muscles, thus the term
"paralytic" shellfish poison. High levels of PSP can cause severe illness and
death. |
Microscopic view of
Alexandrium catenella,
which produces PSP toxin |
How do shellfish become contaminated with PSP?
Shellfish are filter feeders. They pump water
through their systems, filtering out and eating algae and other food particles.
When they eat biotoxin-producing algae, the toxin can accumulate in their
tissue.
What types of shellfish can transmit PSP to
humans?
All molluscan shellfish (those having a hinged shell) including clams, mussels, oysters, geoduck,
and scallops are capable of accumulating PSP. So can moon snails and other gastropods. Other marine species, such as sea cucumbers, might also
be affected. Crab feed on other shellfish, so crab gut can contain unsafe
levels of PSP, although the toxin is not known to accumulate in crab meat.
To be safe, clean crab thoroughly, removing all butter (the white-yellow fat
inside the back of the shell), and discard the gut.
What causes unsafe levels of PSP?
It's normal for biotoxin-producing algae to be present in marine water. They are usually
at very low concentrations and pose no problems. However, when the algae
"blooms" the concentration increases dramatically. The increased algae
simply becomes 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 continues to climb 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 systems. It can be several days to
several months or longer before they're safe to eat again.
What causes PSP blooms?
When water conditions are favorable, the algae "blooms" and reproduces.
Continuing research has pointed to certain cause and effect situations, but
the exact combination of conditions that cause the blooms is not yet known.
The National Office for Harmful Algal Blooms has an excellent site that further explains
harmful algal
blooms (HABs) and their
life cycle.
If the water looks dirty or red, does that mean the
shellfish are contaminated?
Not necessarily. PSP is rarely
associated with a red tinge to the water.
But isn't "Red Tide" the same as PSP?
The term "red tide" is commonly used to describe toxic blooms. This isn't
quite accurate. Some algal blooms turn water different colors, including
red, but many of these blooms are not harmful. Some of the most
spectacular red blooms recorded have been caused by non-toxic algal species.
If the water is not red, does that mean the shellfish
are safe to eat?
Not necessarily. PSP can be
present in large amounts even if the water looks clear. Also, the toxin can remain in shellfish long after the algae bloom is
over.
Can I tell if shellfish are toxic by
how they look?
No. Shellfish containing toxic levels
of PSP don't look or taste any different from shellfish that are safe to eat.
If I can't tell if shellfish are toxic by
how they look or taste, or by the color of the water, how can I tell?
Unfortunately you can't. Laboratory testing of shellfish meat is the only known method of
detecting PSP.
Does cooking the shellfish make it safe to eat?
No. PSP is not destroyed by cooking or freezing.
How can I protect myself from paralytic
shellfish poisoning?
We regularly test shellfish for PSP and
close areas that have unsafe levels. Before harvesting any kind of
shellfish, check the marine Biotoxin Bulletin, our
clickable maps, or call the Marine Biotoxin Hotline at 1-800-562-5632 to
find out what recreational areas are closed due to PSP. Beaches are
sometimes posted with warning signs, but don't
assume a beach is safe if there are no signs. Closure signs seem to be popular and can
"disappear" soon after they are placed.
What are the symptoms of PSP?
Early 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 loss of 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 toxin, muscles of the chest and abdomen become paralyzed,
including muscles used for breathing, and the victim can suffocate. Death
from PSP has occurred in less than 30 minutes.
Who is most at risk?
Anyone who eats PSP contaminated
shellfish is at risk for illness or death.
What should I do if I think I have paralytic shellfish poisoning?
If symptoms are mild, call your health care provider and
your local public health agency. If symptoms are severe, call 911 or have someone take you
to the emergency room immediately.
What is the treatment?
There is no antidote for PSP. The only treatment for
severe cases is the use of life support systems such as a mechanical respirator and oxygen until the toxin
passes from the victim's system. Survivors recover fully.
Are there any other illnesses associated with
shellfish?
Yes. There is another type of
biotoxin found in the northwest called Domoic Acid.
Harmful bacteria and viruses can cause intestinal upset (see
Vibriosis in Shellfish). Some people can
have an allergic reaction to shellfish.
What about shellfish offered by restaurants,
stores, and farmers' markets?
Are they safe to eat?
Shellfish harvested commercially and sold to the
public come from licensed,
certified growers. Commercial harvest operations must meet stringent state
and federal health standards, and the shellfish they harvest are regularly
tested for biotoxins.
Where can I get more information?
For current closures:
For more on biotoxins:
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