How common is anaphylaxis

Much local data has been collated in order to try to gain estimates of how common anaphylaxis might be. Yocum and Khan22 estimated that food caused one-third of anaphylactic reactions that occurred outside hospitals in the US. Danish data suggest prevalence of one in 30 000 population per year.23

A prospective study by Bock in Denver, Colorado, found 25 cases of food-related anaphylaxis over a three-year period, a third of which were due to peanut.24 This number of reactions in a state whose population was 3.3 million gives a minimum incidence of severe reactions to foods of approximately one per 264 000 people per annum. Extrapolation of this minimum figure to the entire US population would suggest a national incidence of 950 individuals per annum.

A similar study in Munich, over the course of one calendar year (1992), showed that food caused 17 of 150 cases (11.3%) of anaphylaxis requiring emergency room treatment. The minimum overall incidence of food-related anaphylaxis in Munich was estimated to be 1.1 per 100 000 people. The incidence of actual reactions would be higher, due to the recalcitrant nature of severe reactions3,4 and, in the case of peanut, at least, to the difficulty in avoiding exposure26 and the underreporting of reactions, even to airline staff during a flight,27 let alone seeking medical help.

A French study28 supports Bock's findings from Colorado. This multi-centre study investigated the presentation rate of food-induced anaphylactic shock to 46 emergency departments, 29 dermatology units and 19 internal medicine departments. In 794 reported cases of anaphylaxis, food was implicated in 81 cases (10%). Unusually, only 19 patients (23.4%) had known food allergy. The presence of the causative allergen in 'hidden form' contributed to 25 cases (31%) of food-related anaphylaxis. An enhancing factor, such as alcohol consumption or exercise,29 was present in 221 cases (27.8%).

A retrospective British study suggests that the incidence of food-related anaphylaxis is broadly similar in Britain and continental Europe. Stewart and Ewan30 analysed clinical records of attendances at the casualty unit of Addenbrooke's Hospital, Cambridge (catchment area population 350 000), over the full calendar year of 1993 and over a three-month period in 1994. They found nine cases of collapse (severe anaphylaxis) and 15 cases of generalised allergic reaction (without hypotension) in 55 000 attendances in 1993. The rate of generalised allergic or severe anaphylaxis (combined total 24 cases) was, therefore, one case in every 2300 casualty attendances, or 6.8 per 100 000 of the local population per year. Three of the nine severe anaphylaxis reactions were due to foods (one per 18 000 casualty attendances, or 1.16 per 100 000 population per year). This figure is very similar to those of Bresser et al25

Stewart and Ewan noted an increased rate of diagnosis of anaphylaxis from any cause in the follow-up three-month study the following year, probably due to an increased rate of ascertainment and to the study falling in the peak season for wasp and bee stings. Stings accounted for eight of the nine anaphylactic reactions noted over the three-month period. With increased staff awareness of anaphylaxis the incidence increased to one per 1500 casualty attendances.

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