Symptoms of seafood allergy are usually
obvious
Many allergic reactions to food are
mild and limited to localized hives or swelling. The most dangerous
symptoms are breathing difficulties or a a drop in blood pressure
(shock), either of which can be life threatening. Anaphylaxis
is the most severe form of allergic reaction, and death have been
described in association with seafood allergy. Other symptoms
include swelling of the face or throat, dizziness, difficulty
thinking, an intense sense of fear, tightness in the chest, vomiting
or diarrhoea. Occasionally, breathing difficulties may occur from
breathing in seafood allergen fumes, such as when food is being
cooked or in seafood processing factories.
Types of seafood
There are many varieties of seafood.
The major groups of marine animals associated with triggering
allergic reactions are:
VERTEBRATES
Scaly fish (eg. salmon, cod, mackerel, sardines, herring, anchovies,
tuna, trout, haddock, John Dory)
INVERTEBRATES
Crustaceans (eg. prawns/shrimps, lobster, crab, crayfish,
yabbies)
Molluscs
Seafood allergy usually only involves one
group of foods
As a rule, patients allergic to seafood
from one group can usually tolerate those from another. Seafood
allergy is most common in communities where seafood is an important
part of the diet, such as Asia and Scandinavia. Sensitivity is
more common in adults than children. While exceptions occur, once
established, seafood allergy usually remains a life-long problem.
Cross-reactivity between seafoods is important
There are a few studies examining the
actual allergens involved in triggering reactions. Most people
with seafood allergy are sensitive to only 1 or 2 proteins that
may be present in multiple species. Thus there is a risk of "cross-reactive"
allergic responses to other animals in that group as well.
In other words, allergy to one scaly fish often results in allergy to other scaly fish, and allergy to one crustacean usually means that all must be avoided. This risk is estimated at between 50 and 75 %. For this reason, On the other hand, patients allergic to seafood from one group can usually tolerate those from another. For this reason, complete avoidance of one or more groups of seafood is often advised.
Occasionally, intense cooking will partially or completely destroy the triggering allergen. This may explain why some patients allergic to fresh fish are able to tolerate tinned salmon or tuna.
Avoiding the allergen is an important part
of management
Complete avoidance of one or more groups
of seafood is often advised, yet may be difficult to achieve in
practice. Accidental exposure is more likely to occur when eating
away from home, particularly when eating at seafood restaurants.
Other potential sources of accidental exposure and cross-contamination
include:
Seafood allergy and iodine allergy are unrelated
Even though seafood is a rich source
of natural iodine, allergic reactions to seafood proteins have
a different mechanism to that described with iodine in topical
antiseptics (like Betadine or Povidine) or intravenous xray radio
contrast agents. Patients allergic to seafood are not at an increased
risk of allergic reactions to iodine. Those with iodine allergy
are not at increased risk of seafood allergy.
Anisakis simplex allergy and infestation
can mimic an allergic reaction
Anisakis simplex is a nematode (worm),
that acts as a fish parasite. It is distributed world-wide, and
infests a variety of hosts along the food chain while continuing
its maturation and development. Between 5 and 80 % of fish samples
have been found to be infested in various studies. The larvae
of this parasite are hard to kill. Cooking at temperatures above
60 degrees centigrade or storage in industrial freezers is required
to kill the parasite. Anisakis can cause two major problems in
humans: infection (anisakiasis) and allergic reactions.
Anisakis simplex infection (anisakiasis) results from eating raw or undercooked seafood such as sushi. Infection can cause nausea, vomiting, tummy pain, and sometimes appendicitis, bowel blockage or bleeding. Diagnosis is based on seeing the parasite using a fibreoptic telescope (endoscopy).
Anisakis simplex allergy resembles other allergic reactions to food. Allergic reactions to Anisakis simplex should be suspected when allergic-like reactions occur after eating seafood, yet the results of skin tests to seafood are negative. Reactions often occur intermittently after eating seafood rather than on every occasion. The allergens of Anisakis simplex appear to be not destroyed by heat or cooking and so allergic reactions may be triggered by dead parasites. While blood and skin allergy tests are available abraod, these are not easily accessible in Australia.
Scombroid fish poisoning can mimic an allergic
reaction
While not all reactions to seafood
are allergic in origin, some can resemble allergic reactions.
Scombroid fish poisoning describes an allergic-like reaction that
occurs after eating foods with a high histamine content. These
are generally fish that have been improperly refrigerated after
capture. This may occur at sea when dead fish are caught in long-line
nets and there is a delay in refrigeration, or after retail distribution.
Marine bacteria within the gut of unrefrigerated fish convert
the amino acid histadine into histamine. Fish involved include
mackerel, tuna, herring, sardines, marlin, anchovies and bluefish.
Affected fish often have a metallic or peppery taste. Symptoms usually commence within 30 minutes of eating, and include flushing, itch, urticaria/hives, nausea, vomiting, abdominal cramps, dizziness, palpitations and headache. Severe episodes may result in wheezing and a drop in blood pressure. Treatment involves administration of antihistamines and waiting until the swallowed histamine is broken down.
Toxic reactions to eating seafood
Not all reactions to seafood are allergic
in origin. Ciguetera poisoning and paralytic shellfish poisoning
are both caused by eating seafood that has been contaminated by
algae-derived toxins. These toxins interfere with the function
of nerve endings and are thus "neurotoxic". Symptoms
occur within 2-3 hours of eating contaminated food.
Ciguetera poisoning
Ciguetera poisoning is triggered by
eating tropical reef fish that have fed on toxic algae which produce
a "neurotoxin". As larger reef fish feed on smaller
ones, the toxin concentrates in the organs of larger fish such
as red snapper, barracuda, sea bass, eels and kingfish. Because
the toxin is heat-stable, it is unaffected by cooking. Symptom
consist of tingling of the lips, tongue and throat, often followed
by stomach upset, headache, fever and muscle aches and pains.
Numbness, collapse, coma and confusion have also been described.
Most people recover within a few days or weeks with supportive
treatment, but ongoing disability has occasionally been described.
Paralytic shellfish poisoning
Symptoms of paralytic shellfish poisoning
are similar to those of Ciguetera poisoning. Symptoms occur after
eating shellfish that have fed on toxic algal blooms. A heat-stable
"neurotoxin" known as saxitoxin is produced by toxic
algae, which concentrates in filter-feeding shellfish. Because
the toxin is heat-stable, it is unaffected by cooking. Toxin can
persist in contaminated shellfish for several weeks (and sometimes
months) after exposure to toxic algae.
Symptoms arise within a few hours of eating contaminated shellfish, and consist of tingling of the lips, tongue and throat, numbness, loss of balance, dizziness, rash, fever and sometimes muscle paralysis. Death may occur if the breathing muscles become paralysed. Recovery is usual if the patient survives the first 24 hours, although muscle weakness may persist for several weeks. There is no antidote to neutralise saxitoxins. Treatment is supportive, although activated charcoal has been used to bind and inhibit further absorption of toxin.
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