Symptoms of seafood allergy are usually obvious
Many food-allergic reactions are mild and limited to localized
hives or swelling. The most dangerous symptoms are breathing difficulties
or a hypotension, either of which can be life threatening. Anaphylaxis
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, respiratory symptoms
may occur from inhalation of allergen, 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 only a few studies examining the actual allergens
involved in triggering reactions. Muscle proteins such as parvalbumin
(in scaly fish) and tropomyosin (in crustaceans) appear to be
important allergens. As patients are usually allergic to 1 or
2 proteins that may be present in multiple species, there is a
risk of "cross-reactive" allergic responses to other
animals in that group.
In other words, allergy to one scaly fish often results in allergy to other scaly fish in the same way that allergy to one crustacean usually means that all must be avoided. This risk is estimated at between 50 and 75 %, and more likely to occur in patients allergic to crustaceans or shellfish than to scaly fish. On the other hand, patients allergic to seafood from one group can usually tolerate those from another.
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 radiocontrast 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, with wide geographical
variation. The larvae of this parasite are hard to kill, and are
unaffected by marinating or salting. Instead, cooking at temperatures
above 60 degrees centigrade or storage in industrial freezers
is required to kill the parasite. Two clinical syndromes are observed
in humans after eating contaminated fish; infestation by Anisakis
simplex (anisakiasis), and Anisakis simplex allergy.
Anisakis simplex infestation (anisakiasis) results from eating raw or undercooked seafood such as sushi. Infestation results in nausea, vomiting, abdominal pain, and sometimes appendicitis or bowel obstruction. Bleeding and bowel wall thickening are observed. Diagnosis is based on visualization of the parasite using endoscopic biopsy. Treatment involves administration of medications such as albendazole.
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. Unlike symptoms due to infestation (which require live parasites), allergic reactions may be triggered by dead parasites. Since the allergens of Anisakis simplex are not always destroyed by heat, allergic reactions may even occur after food is cooked. Blood and skin allergy tests for Anisakis sensitivity are available, but these are not easily accessible and have not been validated.
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 ingested histamine is metabolised.
Toxic reactions to ingestion of 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.
Neurological symptoms occur within 2-3 hours of eating contaminated
food.
Ciguetera poisoning
Ciguetera poisoning describes a neurological syndrome consisting
of tingling of the lips, tongue and throat, often followed by
gastrointestinal upset, headache, fever and myalgia. Peripheral
paresthesia and neuropathy, cardiovascular collapse, coma, confusion
have also been described.
Symptoms are triggered by eating tropical reef fish that have fed on toxic algae which produce a heat-stable lipid soluble neurotoxin. As larger reef fish feed on smaller ones, the toxin concentrates in the liver, viscera and muscle of larger fish such as red snapper, barracuda, sea bass, eels and kingfish. Because the toxin is heat-stable, it is unaffected by cooking. Most cases recover within a few days or weeks with supportive treatment and mannitol infusions, but chronic disability has been described.
Paralytic shellfish poisoning
Symptoms of paralytic shellfish poisoning are similar to those
of Ciguetera poisoning. Symptoms arise following ingestion of
contaminated shellfish that have fed on toxic algal blooms. A
heat-stable lipid soluble 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, peripheral
paresthesia, ataxia, dizziness, rash, fever and sometimes muscle
paralysis. When death occurs, it is usually by respiratory paralysis.
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 saxitoxin. Treatment is supportive,
although activated charcoal has been used to bind and inhibit
further absorption of toxin.
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