|
| Domoic Acid and
Amnesic Shellfish Poisoning (ASP)
What
is domoic acid?
Domoic
acid is a toxin, called a "biotoxin", that is produced
by the diatom Pseudo-nitzschia sp., which is a type of naturally-occurring
microscopic algae. Shellfish
eat this algae and can retain the toxin, sometimes at dangerous levels. Domoic acid causes
Amnesic Shellfish Poisoning (ASP) in humans
that eat contaminated shellfish.
Severe cases of ASP can result in permanent loss of short-term memory,
thus the name "Amnesic" Shellfish Poisoning.
What types of shellfish are affected?
Razor clams
are most often affected by domoic acid because the algae that
produces the toxin, Pseudo-nitzschia, is more commonly found in
coastal areas. However, in recent years domoic acid has been detected in mussels, clams, and
oysters in Puget Sound. Dungeness crab, because they feed on razor clams and other shellfish, can also
become toxic. Even if the crab meat is safe, domoic acid tends to accumulate in crab gut and butter
(the white-yellow fat inside the back of the shell). Always clean crab thoroughly. Remove
all butter and discard the gut. |
The diatom
Pseudo-nitzschia
sp.,
which produces domoic acid
that causes ASP.
Image courtesy of
Brian Bill,
Northwest Fisheries Science Center.
|
What causes unsafe levels of domoic acid?
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 domoic acid 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 Northwest Fisheries Science Center has a web site that further explains
harmful algal blooms (HABs) and Marine Biotoxins
and
algal bloom dynamics.
Does cooking the shellfish make it safe to eat?
No. The toxin is not destroyed by cooking or freezing.
What are the symptoms of Amnesic Shellfish Poisoning?
ASP symptoms include vomiting, nausea, diarrhea, and abdominal cramps
within 24 hours of ingestion. In more severe cases, neurological symptoms
develop within 48 hours and include headache, dizziness, confusion,
disorientation, loss of short-term memory, motor weakness, seizures,
profuse respiratory secretions, cardiac arrhythmias, coma, and possible
death. Short term memory loss can be permanent.
What should I do if I think I have ASP?
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. The only treatment for severe cases is the use
of life support systems until the toxin passes from the victim's system.
How can I protect myself from ASP?
Prevention is the best protection. We regularly test shellfish for biotoxins
and close areas when unsafe levels are detected. Always check for closures at
www.doh.wa.gov/shellfishsafety.htm or call the
Biotoxin Hotline at 1-800-562-5632 before harvesting. Also check with the
local health department in the area you plan
to harvest. 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.
Are there any other illnesses associated with
shellfish?
Yes. Other types of biotoxins found in the
northwest are Paralytic Shellfish Poison
and Diarrhetic Shellfish Poison. 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?
On current closures:
On biotoxins:
|
