Insect sting allergy
It is estimated that between two and three people die in New Zealand each year due to insect stings, however this figure may be an underestimate, as many fatal insect stings may not be properly documented.
Studies of the general population have suggested that approximately 1% of those surveyed report systemic allergic reactions (reactions involving the whole body) to stinging insects. The prevalence of positive skin test reactivity may be considerably higher than this figure. However, positive skin test reactions to insect venoms do not necessarily mean individuals are at risk of systemic reactions to stings.
Stinging insects in New Zealand
In New Zealand there are six dominant groups of stinging insects. The Italian honeybee (Apis mellifera) was introduced because of its high honey gathering potential. Most honeybees are domesticated in New Zealand and reside in apiaries. They make a major contribution to New Zealand's economy through their pollinating activities and their honey production. In general, bees do not sting unless provoked. Similarly, bumblebees (Bombus spp) are docile despite their burly appearance.
In contrast, the wasp species present in New Zealand are more aggressive. The common wasp (Vespula vulgaris) and the German wasp (Vespula germanica) may sting without provocation. Other wasp species include the Tasmanian wasp (Polistes humulus) and Asian paper wasp, (Polistes chinensis).
What causes an allergic reaction following a sting?
Insect venom consists of a complex mixture of proteins and peptides. The smaller peptides such as histamine and serotonin are responsible for the painful skin reactions that occur following a sting.
The larger proteins such as phospholipase are responsible for the local and systemic allergic reactions suffered by susceptible individuals. The immune system recognizes these proteins as foreign invaders and mounts an immune reaction against them.
In general, individuals allergic to wasp venoms are susceptible to the venom of other wasp species but react less frequently to bee venom. Those allergic to bee venom may also react to bumblebee venom.
Types of allergic reactions to venoms
In allergic individuals, the immune system reacts to the venom protein (or allergen) by producing IgE antibodies. These IgE antibodies bind to receptors on cells known as mast cells and basophils. When the individual is re-exposed to the same allergen, the IgE antibodies on mast cells trigger the release of a wide variety of chemical messengers, including histamine, which results in the allergic reaction.
Allergic reactions are classified as immediate or delayed. Those occurring within 4 hours are called immediate reactions while those occurring beyond 4 hours are classified as delayed. IgE antibodies against venom proteins are responsible for most immediate reactions. In contrast, the less common delayed reactions may not be caused by an immune reaction.
Immediate reactions are further subdivided into local and systemic. Local reactions, by definition, occur around the area of the sting. Systemic reactions include anaphylaxis, angioedema (swelling) or urticaria (hives) in other areas of the body.
For reasons that are not understood, many patients with systemic allergic reactions lose their sensitivity to insect venoms. This seems to be most common in younger children but no laboratory test is able to reliably identify these individuals.
Diagnosis of insect sting allergy
Skin prick testing and RAST blood testing are helpful in confirming the presence of IgE antibodies to venom components in individuals suffering a systemic reaction. It should be noted that the level of IgE antibodies to venom components does not appear to correlate with the severity of the reaction.
What should I do if I am allergic to insect stings?
Avoidance measures to reduce the probability of a sting are important for allergic individuals:
Patients with even mild systemic reactions may require adrenaline to prevent progress of the reaction — ask your doctor if this applies to you. Adrenaline is administered by subcutaneous or intramuscular injection; inhaled adrenaline may not reliably prevent or treat a systemic reaction.
All patients who have had systemic reactions to insect stings should carry adrenaline in an easy to administer form, such as an EpiPen, and should wear a Medic Alert bracelet. Get clear instructions from your doctor about the appropriate use of this medication; be sure to understand exactly when and how to use it.
Who can become allergic?
There are two main risk factors for becoming sensitised to insect stings, which can happen at any age. One is having multiple stings at one time, such as five, six or 10 stings at once. The other is being stung sequentially at close intervals, for example being stung over a repeatedly over a number of days or weeks. This is why forestry workers, beekeepers and their families or neighbours are at high risk of becoming allergic to stings.
Unlike food allergies, you don’t need to come from an allergic background — be atopic — in order to become allergic to insect stings. Asthma, however, may increase the severity of your reactions.
Like drug allergies, reactions to insect sting are more likely to involve the cardiovascular system and a drop in blood pressure, unlike food allergy, which is more like to involve the respiratory system.
If you have had an allergic reaction, you should be assessed by an allergy specialist to see if you still have the allergy, because there can be natural resolution. Children, particularly, lose their sensitisation over time.
The specialist will carry out a skin prick test, which is more sensitive than a Rast (blood) test. If you test positive and have a history of a clinical reaction, you will then be offered immunotherapy.