Summary

The honeybee, a cosmopolitan insect, is the only stinging member of the order Hymenoptera that leaves its barbed stinger and venom sac in the victim. Hymenoptera stings cause 48% of severe anaphylactic reactions occurring in European adults, and 20% of those occurring in children. In the United States, the prevalence of Hymenoptera-induced anaphylaxis is estimated at 3% in adults and 1% in children, with 40 to 100 Hymenoptera sting-induced fatalities being documented annually. Anaphylaxis is more common in adults than in children. Systemic reactions usually occur within minutes of being stung. The risk of repeated anaphylaxis is 30% to 70%. An estimated 9 to 42% of the general population is sensitized to Hymenoptera venom. Beekeepers, greenhouse workers, and rural populations are at higher risk of developing bee sting allergy. Mast cell disorders including hereditary α-tryptasemia, elevated baseline serum tryptase, or a family history of honeybee allergy are associated with an increased risk of occurrence and severity of Hymenoptera sting-induced reactions. A history of Hymenoptera-induced anaphylaxis is a red flag for an underlying clonal mast cell disorder.

Honeybee venom (HBV) contains 12 characterized allergens, of which five are available for in vitro diagnosis: Api m 1, Api m 2, Api m 3, Api m 5, and Api m 10. Sensitization to Api m 1 and to Api m 10 is most prevalent, at 57%–97% and 51.5%–61.8%, respectively. When testing for specific IgE to venom components, inclusion of all five major allergens allows identification of the patient’s sensitization profile, facilitates precise venom immunotherapy (VIT), and helps to improve specificity when HBV-sensitized patients have also tested positive for Vespid allergen extracts.

Epidemiology

Worldwide distribution

Hymenoptera stings cause 48% of severe anaphylactic reactions occurring in European adults, and 20% of those occurring in children [4].

It is estimated that the worldwide annual incidence of immunologic reactions to Hymenoptera stings ranges from 0.3–3.0% which equates to almost 100 million cases per year. Severity ranges from local wheal-and-flare reactions to death from anaphylactic shock [7].

In the US, Hymenoptera-induced systemic reactions are estimated to occur in 3% of adults and 1% of children, and approximately 40 to 100 fatalities are reported each year, although the figure is likely to be higher [8]. An Australian study reported honeybee to be the main cause of fatal insect venom anaphylaxis, however, there is a relatively high prevalence of honeybee allergy in Australia, and UK and European studies found that wasp was the most common cause of fatal and nonfatal venom anaphylaxis, respectively [9]. A consistent finding across studies in Australia, Canada, UK and US was that fatal insect venom anaphylaxis occurs at an approximate rate of 0.1 cases per million population [9]. In Europe, the prevalence of systemic reactions to Hymenoptera stings is 0.3% to 7.5% [10, 11].

Risk factors

Sensitization to Hymenoptera venom, estimated at 9.2% to 42% of the adult population, comprises a majority of asymptomatic individuals [4]. However, exposed populations, e.g. rural dwellers or beekeepers and their families, are at higher risk of developing HBV allergy [12]. In beekeepers, a 14% - 43% prevalence of systemic reactions to stings was reported [12, 13].

The prevalence and severity of Hymenoptera venom reactions are increased in patients with mast cell disorders including hereditary α-tryptasemia, with or without an elevated baseline serum tryptase concentration [14-17]. Hymenoptera venom allergy was observed in 50% of patients with systemic mastocytosis without hereditary α-tryptasemia and in 82% of those with concurrent hereditary α-tryptasemia [15].

Cardiovascular risk factors, male gender and older age have also been associated with an increased risk of severe reactions to Hymenoptera venom [18]. Atopy and a family history of allergy to bee stings have been inconsistently associated with severe reactions [6, 18].

Pediatric issues

The prevalence of systemic reactions to Hymenoptera venoms is estimated at 3.4% in children [19]. In children younger than 16 years experiencing a cutaneous reaction to Hymenoptera venom, the chance of anaphylaxis if re-stung is lower than 3% [8].

Route Of Exposure

Main

Exposure to HBV occurs through a honeybee sting, when the barbed stinger becomes embedded in the flesh and is pulled out of the abdomen of the insect along with the venom sac. Worker bees may survive for 18 to 114 hours after a stinging event but, unlike wasps, hornets, and yellow jackets, honeybees can only sting once [1, 6].

A single sting is capable of delivering an average of 140 µg to 150 µg of venom, although venom sacs may contain more than 300 µg of venom [1, 6] Within the first 20 seconds after stinging, 90% or more of the contents of the venom sac will already have been delivered [6].

Secondary

Ingestion of bee products such as honey, royal jelly, propolis, or “bee pollen” are usually considered as unrelated to Hymenoptera venom allergy, however, among the three isoforms of Api m 11, two are described as venom allergens and one as a food allergen [5, 22, 23].

Clinical Relevance

Five types of reactions to Hymenoptera stings are recognized: normal local reactions, large local reactions (LLR), systemic anaphylactic reactions, systemic toxic reactions, and unusual reactions. Of these, LLR and systemic anaphylactic reactions are the most common [1]. It can be difficult to differentiate systemic anaphylactic reactions from local reactions and toxic reactions based on signs and symptoms [24].

Anaphylaxis

Systemic reactions limited to cutaneous signs only carry a risk of anaphylaxis to a future sting below 3%. Conversely, a history of systemic sting-induced reaction and detectable venom IgE put the risk of a second systemic reaction at 60% [25]. The patient’s prior sting history – the severity and pattern of reactions, baseline tryptase level, age and concurrent medications all influence future risk [2]. Hymenoptera sting-induced anaphylaxis must prompt investigations for an underlying mast cell disorder including hereditary α-tryptasemia [14-17, 26].

Local reactions

In the general population, the reported prevalence of LLR ranges between 2.4% and 26.4%. In children, this figure is lower, while in beekeepers, it is higher, at 38% [27]. If local inflammation is contiguous with the sting site, it may be considered and managed as a local reaction [2]. LLR are not dangerous unless they cause compression, and compartment syndrome develops, or if a patient is stung in the oropharynx, when airway obstruction becomes a risk [28], or in the context of an underlying condition [19].

LLR patients exhibit a 10% risk of systemic reactions and a 3% risk of severe anaphylaxis if re-stung [2, 28].

Diagnostics

In vitro diagnostics

Detection of HBV sensitization and a convincing clinical history are required for the diagnosis of HBV [4]. Current guidelines recommend skin testing with allergen extracts as the first step for Hymenoptera venom allergy, however, the adjunction of in vitro testing with allergen extracts and allergen components (recombinant, CCD-free molecular allergens) helps refine the diagnosis and the therapeutic management decisions [4, 29]. However, recent data showed that in vitro and skin tests yielded complementary rather than overlapping results, and that in vitro diagnosis was best performed prior to skin tests [25].

As Hymenoptera venom IgE persist for extended periods, in vitro and skin testing can be done even a long time after the reported clinical reaction, however, it is recommended to observe a 2-week interval after the reaction before performing skin tests [4, 25]. In a prospective diagnostic study, the positive predictive value of ImmunoCAP in vitro testing of Hymenoptera extracts was 77% and the negative predictive value 59%, while skin tests yielded 87% and 55% respectively [25].

Besides allergen-extract specific IgE, in vitro investigation of Hymenoptera sting-induced reactions comprises allergen component IgE and CCD IgE determination to assess genuine versus cross-reactive sensitization, as double positivity to bee and wasp venom extracts during in vitro testing occurs in up to 50% of venom-allergic patients [4, 30, 31]. Total IgE measurements may be useful for calculating the specific-to-total IgE ratio, a proposed indicator for clinically relevant sensitization [4]. If, based on clinical history, the index of suspicion for anaphylactic reaction is high, but in vitro and skin tests are negative, testing should be repeated in three to six months [8].

Diagnostic investigation of Hymenoptera sting-induced systemic reactions also requires determination of baseline tryptase in search for a mast cell disorder, with levels at 8 µg/L or higher suggesting hereditary α-tryptasemia [4, 17]. Further investigations such as testing for the D816V c-kit mutation in peripheral blood may be considered [16].

In the approximately 5% of Hymenoptera venom-allergic patients with elevated baseline tryptase levels and/or mastocytosis, using molecular components and a decreased threshold sIgE level to 0.1 kUA/L may be needed, in order to enhance sensitivity [32].

The high prevalence of Hymenoptera venom sensitization in the general population (42%) explains why screening for Hymenoptera venom allergy is not recommended.

Skin prick tests

Diagnostic intradermal testing is usually carried out using venom extracts of concentrations of between 0.001 µg/mL and 1.0 µg/mL. The accuracy of the test is subject to proportionate representation of the relevant allergens in the extract, as false-negatives may be the result of underrepresented components and, conversely, irritant compounds may lead to false-positives [2].

Children have been shown to demonstrate lower intradermal testing sensitivity than adults. This could suggest that leaving out venom extract concentrations below 0.01 µg/L may facilitate less painful and less labor intensive testing without compromising accuracy [33].

Challenge tests

Live sting challenges are not a standard procedure in clinical practice [2, 4, 19].

Other topics

Prevention And Therapy

Allergen immunotherapy

Venom immunotherapy (VIT) is the only treatment that can prevent future sting-induced anaphylaxis in Hymenoptera venom-allergic patients and can induce tolerance in 75–98% of cases. The scale of success of VIT is attributable to its wide availability and to well-established protocols [2, 34].

VIT is most successful when treatment is selected based on specific IgE to venom allergens, i.e., bee and/or wasp molecular allergens [4, 19]. Candidates for VIT must have a documented history of a systemic reaction to a sting and evidence of IgE reactivity to a specific venom. Treatment with the incorrect venom, or with more than one venom without evidence of sensitization, can induce de novo sensitization, increase the likelihood of adverse effects, risk insufficient protection, and increase costs [34].

The usual duration of VIT is 3 to 5 years, although more prolonged or even lifelong VIT should be considered in patients with mast cell disorders [19]

Even in patients without diagnosed SM, an elevated baseline serum tryptase in a venom-allergic patient may be associated with very severe anaphylactic reactions [13] and may indicate the need for lifelong VIT [35].

Prevention strategies

Sting avoidance is difficult to achieve as it requires caution during outdoor activities [4].

Other topics

An emergency kit comprising autoinjectable epinephrin should be carried by HBV-allergic patients having experienced systemic reactions, including those having completed a successful VIT [4].

T cell epitope-bearing long and short peptides and mimotopes are being investigated with the aim of to overcoming VIT adverse effects [10]. Alternative routes of vaccine administration have also been investigated: intralymphatic delivery of low-dose vaccine shows potential for an enhanced immune response and a markedly reduced treatment duration, from five years to just 12 weeks [10].

Cross-Reactivity

Double positivity to bee and wasp venom during in vitro testing, seen in 40% to 50% of venom-allergic patients, is solved by the use of marker allergens (Api m 1, Api m 3, Api m 4 and Api m 10 for HBV, Ves v 1 and Ves v 5 for Vespids) and CCD, allowing distinction between genuine bee-wasp cosensitization and sensitization to only bee or wasp with cross-reactivity through shared allergens or CCD [4].

Honeybee and bumblebee venoms are highly cross-reactive and differential diagnosis is not currently available [4].

Natural Api m 1 carries a core glycan structure that reacts with IgE directed at CCDs. Recombinant venom components, including rApi m 1, lack all or part of that structure, which increases specificity since only IgE antibodies directed at the PLA2 protein epitopes will bind to rApi m 1 [31].