Food anaphylaxis is now the leading single cause of anaphylactic reactions treated
in emergency departments in Westernised countries. In the US, it is estimated that
there are 29,000 anaphylactic reactions to foods treated in emergency departments
and 125-150 deaths each year.
Seafood (scaly fish, crustaceans and molluscs) is one of the most common causes
of food allergy. In adolescents and adults, peanuts, tree nuts, fish and shellfish
account for approximately 85% of true food allergies.
Higher prevalence of seafood allergy is found in the countries where seafood is
a staple part of the diet, like Scandinavian countries, Spain and Japan.
Seafood is potent allergens in sensitised individuals and cause life-threatening
adverse reactions that are usually life-long. Extreme sensitivity to minute quantities
of fish is occasionally noted, and even exposure to fumes of fish being cooked is
enough to precipitate reactions in certain individuals.
The Mollusc group includes three different classes of seafood with
species such as Abalone, oysters, mussels, and squid (Calamari).
The second group, the Crustacean, includes the rock lobsters ("crayfish"),
prawns, crabs and shrimps; and the third important group of seafood includes all
the common edible fish, such as Hake, Cod, Snoek, etc. Cod is the
most frequently reported cause of fish allergy, but reactions to other fish such
as haddock, herring, sprat, halibut, plaice, mackerel, trout and salmon are well
recognised. Very often patients are only allergic to certain species but are able
to eat other seafood species without problems.
CLASSIFICATION OF SEAFOOD CAUSING ALLERGIES
(Degree of relation can be judged from distance apart)
Abalone, Snails (Escargot)
Mussels, Oysters, Clams, Scallop, Cockle
Squids (Calamari), Octopus, Cuttlefish
Crabs, Lobsters, Shrimp, Prawn, Crayfish (freshwater), Rock Lobster
Salmon, Trout, Pike
Cod, Haddock, Hake
Snapper, Mackerel, Tuna, Bonito, Grouper
Sole, Flounder, Halibut, Plaice
Fish Allergens and cross-reactivity
Identified seafood allergens belong to a group of muscle proteins, namely the parvalbumins
in codfish and tropomyosin in crustaceans. In addition, there is strong immunological
evidence that tropomyosin is a cross-reactive allergen among crustaceans and molluscs.
In fish, the dominating allergen is the homologues of Gad c1 from cod, formerly
described as protein M. A close cross-reactivity exists within different species
of fish between this calcium-binding protein family, denominated the parvalbumins.
This cross-reactivity has been indicated to be of clinical relevance for several
species, since patients with a positive double blind, placebo-controlled food challenge
to cod will also react with other fish species, such as herring, plaice and mackerel.
In 61 children with a history of fish allergy exposed to 2 – 8 species, 34
(56%) reacted to all, and 27 (44%) tolerated some types.
In a study of 6 adults from Denmark with a positive DBPCFC result to at least 1
of 3 fish (catfish, codfish, and snapper) and challenged to at least 2 types, 4
reacted to more than 1 species.
In 16 atopic patients with shrimp allergy, greater than 80% had a positive SPT responses
to crab, crayfish, and lobster. In 11 patients with immediate reactions to shrimp
ingestion, the reaction rate to lobster, crab, and crayfish was 50% to 100% per
species. At the other end of the spectrum is a report of several individuals with
reactions to only particular species of shrimp.
Clinical characteristics of seafood allergy and classification of 10 seafood allergens
by cluster analysis
A study in Japan investigated the clinical characteristics of children who showed
Sensitisation to any type of seafood and to classify the 10 seafood allergens based
on IgE reactivities by a cluster analysis
Seafood allergens were classified into 4 groups by a cluster analysis:
1) Salmon, sardine, horse mackerel and mackerel
2) Cod and tuna
3) Octopus and squid, and
4) Crab and shrimp.
These findings corresponded to the biological classification and the classification
by the reported common allergens among various types of seafood. Based on our findings,
this classification is therefore considered to be useful when selecting allergens
to screen for sensitisation to seafood.
It has been estimated that if someone is allergic to a fish they have a 50% chance
of being allergic to at least one other fish, and if they are allergic to a shellfish
they have a 75% chance of being allergic to another shellfish. There appears to
be no cross-reaction between fish and shellfish, but concomitant allergy is possible,
so all fish allergic patients should be tested for shellfish and vice versa.
Symptoms of Seafood Allergy
- Urticaria (hives) and angioedema (swelling)
- Contact Urticaria of hands from handling seafood
- Vomiting, looses stools and abdominal pains
- Worsening Atopic Eczema
- Seafood-related, exercise-induced Anaphylaxis
Histamine Fish Poisoning (Scromboid poisoning)
The evaluation of a patient reacting to seafood can be difficult if the adverse
reaction to particular seafood was caused by a non-allergic reaction.
In the USA seafood is the leading single-food vehicle causing outbreaks of food
A major cause is the presence of a toxin in fish and shellfish, which produce symptoms
similar to allergic reactions. Histamine fish poisoning (HFP) is a chemical intoxication
that occurs after eating bacterially contaminated fish of the dark meat varieties.
Spoiled fish of the families, Scombridae and Scomberesocidae (eg. tuna, mackerel,
bonito), are commonly implicated in incidents of histamine poisoning, which leads
to the common usage of the term, "scombroid fish poisoning", to describe
this illness. However, certain non-scombroid fish, most notably mahi-mahi, bluefish,
and sardines, when spoiled are also commonly implicated in histamine poisoning.
It can be considered a mild-to-moderate form of 'food poisoning' and it occurs world-wide
Its incidence has been underestimated because of its frequently mild nature, lack
of mandatory reporting, and misdiagnosis (most often as seafood allergy). The fish
are non-toxic when caught, but increase in histamine content as bacterial numbers
increase. They may look and smell normal, and cooking does not destroy the histamine.
Although the condition is caused by histamine intoxication, its pathogenesis is
not fully understood, and other toxins or potentiators may be involved
Among fish species such as Yellowtail, Tuna and Mackerel, scombroid fish poisoning
is frequently encountered if the fish is improperly refrigerated or when refrigeration
The symptoms include nausea, vomiting, diarrhoea, an oral burning sensation or peppery
taste, hives, itching, red rash, and hypotension. The onset of the symptoms usually
occurs within a few minutes after ingestion of the implicated food and the duration
of symptom ranges from a few hours to 24 h. Antihistamines can be used effectively
to treat this intoxication. Histamine is formed in foods by certain bacteria that
are able to decarboxylate the amino acid, histidine
In filter-feeding shellfish such as black mussels and oysters, toxins produced by
"Red Tide" along the West Coast are found, mostly in later summer.
Nematode Anisakis simplex mimicking fish allergy
Recently allergic reactions to a parasitic worm, Anisakis, that is often found in
various fish species, has been described in people eating fish, and can mimic a
The nematode Anisakis simplex is a common intestinal parasite found in fish and
other seafood. It is considered to be a food allergen and to induce IgE-mediated
reactions. Hypersensitivity to Anisakis simplex is an unusual cause of anaphylaxis
but it should always be borne in mind in countries where a great deal of fish is
consumed, especially if it is eaten raw or undercooked. Several cases of anaphylaxis
due to hypersensitivity to A. simplex have been described.
A. Simplex has also been reported to cause rheumatology complaints after ingestion
of parasitised fishes. A. Simplex must be considered in the differential diagnosis
of arthralgia/ arthritis especially if associated with urticaria.
House dust mite-crustaceans-molluscs syndrome – A rare variant of food allergy
in primary sensitisation to inhaled allergens.
80% of all cases in food allergy in adults are preceded by a clinical or subclinical
sensitisation to inhalant allergens. Food allergy is caused by cross-reactions between
ingested food and inhaled particles. There are several reports of patients suffering
from the house-dust mite-crustaceans-molluscs-syndrome. Here, house-dust mite is
the sensitising agent, therefore even first ingestion of invertebrates such as snails,
shrimps, mussels or oysters can lead to severe anaphylaxis.
Allergenic cross-reactivity between the nematode Anisakis simplex and the dust mites.
Allergenic cross-reactivity between A. simplex and other nematodes has been reported,
as has cross-reactivity with arthropods: red mosquito larvae and German cockroach.
Recent studies show allergenic cross-reactivity between several allergens in A.
simplex and four dust-mite species. The clinical significance of this cross-reactivity
remains to be evaluated.
Occupational Seafood Allergy
Recent years have seen increased levels of production and consumption of seafood,
leading to more frequent reporting of allergic reactions in occupational and domestic
settings. Workers involved in either manual or automated processing of crabs, prawns,
mussels, fish, and fishmeal production are commonly exposed to various constituents
of seafood. Aerosolisation of seafood and cooking fluid during processing are potential
occupational situations that could result in sensitisation through inhalation. There
is great variability of aerosol exposure within and among various jobs with reported
allergen concentrations ranging from 0.001 to 5.061(microg/m(3)).
Occupational dermal exposure occurs as a result of unprotected handling of seafood
and its byproducts. Occupational allergies have been reported in workers exposed
to arthropods (crustaceans), molluscs, bony fish and other agents derived from seafood.
The prevalence of occupational asthma ranges from 7% to 36%, and for occupational
protein contact dermatitis, from 3% to 11%. These health outcomes are mainly due
to high molecular weight proteins in seafood causing an IgE mediated response. Cross
reactivity between various species within major seafood grouping also occurs.
Occupational asthma in snow crab-processing workers
The prevalence of occupational asthma was studied in two snow crab-processing industries
in operation since 1980. Before the 1982 season, all except 10 of the 313 employees
were investigated by a questionnaire, prick skin tests with common allergens, crab
n and crab-boiling water extracts, and spirometry. The diagnosis was confirmed in
46 (15.6%) workers (including 33 of 64 subjects with a history highly suggestive
of occupational asthma in the previous seasons) by (1) specific inhalation challenges
in 33 subjects (one immediate, nine dual, and 23 late asthmatic responses) and/or
(2) a combination of monitoring of peak expiratory flow rates (n = 12) and significant
changes in bronchial responsiveness to histamine (n = 16) as well as in spirometry
(n = 18) after reappearance of symptoms on return to work. Positive skin tests to
crab (p less than 0.001) and, to a lesser degree, smoking history (p = 0.03) but
not atopy (p greater than 0.05) were related to the presence of occupational asthma.
A high prevalence of rhinoconjunctivitis (35 of 46) and urticaria (16 of 46) was
also documented in the affected individuals.
Limited evidence from dose-response relations indicates that development of symptoms
is related to duration or intensity of exposure. Disruption of the intact skin barrier
seems to be an important added risk factor for occupational protein contact dermatitis.
The range of allergic disease associated with occupational exposure to crab is well
characterised, whereas for other seafood agents the evidence is somewhat limited.
The respective roles of skin contact and inhalation exposure in allergic sensitisation
and cross reactivity; and the contribution of host associated factors in the development
of occupational seafood allergies are important areas for future research.
Red Sea Coral Contact Dermatitis
In a study by JH Addy, from Korle Bu Teaching Hospital, University of Ghana Medical
School, Accra six of nine adults who developed Red Sea coral contact dermatitis
had seafood allergies. Contact with the "fire" coral was followed by a
series of skin eruptions starting with an immediate pruritic urticaria-like lesion,
which forced the victims out of the water. Within minutes the affected area became
erythematous and oedematous with eventual blister formation approximately 6 hours
after the initial contact. The blisters resolved, leaving violaceous papules and
plaques in a streaky fashion corresponding to where the coral brushed the skin.
The lesions became shiny and lichenoid in 3 weeks while pruritus persisted. Treatment
with topical corticosteroids and oral antihistamines reduced the severity of the
disease but did not stop its evolution to the lichenoid stage. Complete resolution
usually occurred after 15 weeks, leaving residual hyperpigmented macules.
"Aquarium allergy": fish food, another domestic allergen
A study by J. Knusel & B. Wuthrich in Germany reported 7 cases of allergic respiratory
diseases such as asthma, allergic rhinitis and bronchitis, induced by non-occupational
contact with pet-fish food. Food for pet fish contains proteins of a variety of
arthropod species (larvae and adult forms) --eg. Chironomus, Daphnia, brine shrimps
and others --as well as fresh-water worms such as Tubifex, Enchytraea etc. Thus
far only haemoglobins of Chironomus have been well studied with regard to their
antigenic role; sensitisation against Chironomus was noticed in most of our patients.
Daphnia is also known to be a specific allergen. Atopic patients often become sensitised
against further respiratory allergens (5 out of the 7 patients had a previous allergy
Accordingly, patients with respiratory diseases of already known allergic origin
should, in their opinion, "give up not only mammals or birds as household pets
but also an aquarium."
Diagnosis of Seafood Allergy
Diagnosis of seafood allergy is aided by clinical history,
skin prick testing
, and blood tests (RAST and Immuno-CAP);
however, double-blind placebo-controlled food challenges are the
most reliable method to confirm seafood allergy and to identify putative species.
A study done in Spain in 1994 on Shellfish allergy showed that the most frequent
causes of symptoms were shrimp (33 cases) and squid (24 cases).
These results suggest that prick test yields better results than CAP-RAST (blood
test) does in shellfish hypersensitivity, and that clinical association among shellfish
hypersensitivity can occurs within the same and different Phylum (groups) reflecting
common epitopes (functional part of allergen / antigen that binds the antibody).
It also showed that squid, octopus and limpet extracts contain a large amount of
Studies suggest that in the diagnosis of oyster sensitivity the RAST may not be
useful and that oyster and crustacean contain common antigenic structures.
In snow crab hypersensitivity cooking water and snow crabmeat extracts were more
sensitive than commercial preparations. Water extract was more potent and more sensitive
than meat extract.
Type-I reactions to commercial fish in non-exposed individuals.
Beck HI, Knudsen Nissen B.
The frequency of type-I reactions (contact urticaria) to plaice, whiting, common
dab, witch, codfish, Norwegian haddock, herring and halibut has been examined in
26 individuals; 23 of these carried through all the investigations. 65% of the volunteers
had a positive scratch patch test to one or more species of fish. All 20 min closed
patch tests were negative. 5 of the volunteers were atopics. They showed about twice
as many positive scratch patch tests when compared to the non-atopics. The investigation
shows that cooking but not by freezing destroys the "fish allergen". The
possibility of common and specific fish allergens is discussed.
Allergens in raw vs cooked seafood
Raw fish tends to be more allergenic that cooked fish, as cooking denatures the
The limpet, belonging to Phylum mollusca, is one of the most frequent molluscs in
the Canary Islands, as in all warm maritime regions. Two cases of atopic patients
were reported who developed anaphylactic reactions after ingestion of this mollusc.
Type I hypersensitivity to limpet antigens was demonstrated by means of immediate
skin test reactivity, specific IgE determination by RAST, and histamine release
test to cooked limpet extract. The controls did not react to any of these tests.
Allergic activity was only found with a cooked limpet extract; this suggests that
the offending antigen/s may have been released by cooking this food.
Management of Seafood Allergy
Atopic children should avoid seafood for the first three years of life, and start
with canned tuna, which is the least allergenic form of fish.
It is advisable that fish-allergic individuals avoid all fish (except canned tuna).
All patients who have had anaphylaxis to a seafood should be provided with (and
taught how to use) adrenaline for self-injection (eg Epi-pen).
Medic-Alert Identification should also be discussed.
Beware of "hidden" seafood allergens, like:
Surimi – is a processed food sold as imitation crabmeat,
but contains fish and egg white. Surimi and native codfish contain a common allergen
identified as a 63-kDa protein.
- Worcestershire sauce and Caesar salad dressing contains anchovies
Natural History of Seafood Allergy – usually life-long.