Peter is a 55-year-old account executive who appeared to have become allergic to fish. He has no history of hay fever, asthma or eczema, his general health is fine and he takes no regular medication.
Three months ago he suffered suspected anaphylaxis in a fish restaurant.
He had chosen a chargrilled tuna steak on a bed of celery, mixed green salad, new potatoes and green beans, and the symptoms started immediately. Three other members of his party had the same meal but without adverse effect.
Your partner arranged RAST tests to various foods including tuna, shrimp, cod and salmon, but all the tests were negative. What should you do?
The patient's account
Just as Peter was finishing the fish, he noticed intense itching in his mouth and his head started throbbing.
His skin and conjunctivae became uniformly flushed, his pulse was racing and he felt disorientated and weak. He was taken to hospital and treated with an injection of chlorphenamine.
He then experienced an attack of abdominal pain and diarrhoea, but the other symptoms soon receded and all had gone by about six hours after the meal.
The hospital doctor advised him to request allergy tests, but as you have told him that the results were negative, he does not feel reassured and tells you he feels even more worried.
One possible option is to arrange for RASTs to additional foods. Green beans (and most other legumes) and celery can occasionally cause severe food allergy, and RASTs are available for both of these.
Various seeds might be found in salad dressings and there are RASTs for sesame seed, poppy seed and sunflower seed.
However, it was by no means certain that these tests would provide the answer, and so the patient was referred to see an allergy specialist.
Allergy specialist's view
As with many cases referred to the allergy clinic, it is the history rather than the results of allergy tests that provides the strongest clues to the likely diagnosis. Although Peter's symptoms were not inconsistent with a diagnosis of tuna allergy, the negative (<0.35kU/L) tuna-specific IgE test possesses greater than 95 per cent negative predictive value.
On the other hand, the negative RAST, tuna being the suspect food, and the prominence of flushing, tachycardia and diarrhoea all suggest an alternative diagnosis, the distressing type of false food allergy known as scombroid poisoning.
Scombroid (or histamine) poisoning is mainly caused by fish from the Scombridae family such as tuna, mackerel and sardines, but also mahi mahi, bluefish, amberjack or abalone.
These fish have the capacity to increase their natural histamine content due the autolytic conversion of histidine into histamine by bacterial action.
When this happens it obviously calls into doubt the restaurant's hygiene practices, particularly when an outbreak is traced to the same restaurant or same meal. Cooking provides no protection against the responsible histamine build-up and neither can its presence be detected by appearance or smell.
In most cases the condition is not dangerous, although deaths in frail or elderly individuals have been reported.
Severe outbreaks are rare, but isolated cases are certainly not uncommon and affected patients are seen regularly in the allergy clinic.
Tuna is by far the most common cause of scombroid poisoning in the UK and so almost all cases are referred to the allergy clinic on suspicion of fish allergy. It therefore seems highly likely that most cases of isolated and mild scombroid poisoning go unrecognised.
Although statutory reporting is not a requirement, it is advised that all suspected cases should be reported to the Environmental Health Department.
Allergy skin prick or blood tests should also be undertaken to rule out true food allergy. If the symptoms were severe and if doubt still exists, food provocation testing (undertaken by allergy clinics) may be considered.
Anyone is susceptible to scombroid (histamine) poisoning if the histamine dose is sufficiently high.
However, it appears that some people are more histamine-sensitive than others. Drinking alcohol at the same meal can increase the chance of a reaction and the symptoms are likely to be more severe among the frail and elderly.
Cases are also described if high, but not necessarily hazardous, levels of histamine have been consumed in conjunction with the medication isoniazid. In such cases such a reaction has been partly blamed on the capacity of isoniazid to inhibit diamine oxidase, an enzyme required in the degradation of histamine.
Unsurprisingly, MAOIs such as phenelzine can act in the same way, and there are reports of similar activity in the case of co-amoxiclav, doxycycline, metoclopropramide, verapamil and certain herbal treatments.
A quick-acting antihistamine such as acrivastine (oral) or chlorphenamine (oral or IM) is likely to be effective if given promptly in an acute attack. In severe cases supportive treatments (IV fluids) may be required.
Provided that true fish allergy has been excluded, patients need not avoid eating tuna and similar fish in future. But care should be taken to avoid implicated co-factors (for example alcohol or implicated drugs) and such patients should be advised to consume only fresh fish.
Dr Radcliffe is a consultant in allergy medicine at the Royal Free NHS Trust
- Morrow J, Margolies G, Rowland J, Roberts L. Evidence that histamine is the causative toxin in scombroid fish poisoning. N Engl J Med 1991; 324: 716-9.
- Hauser M, Baier H. Interactions of isoniazid with foods. Drug Intell Clin Pharm 1982; 16: 617-8.