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Stinging Insect Allergy

Final Version 25 November 2000

Magnitude of the Problem:

Up to 3% of an exposed population may give a history of immediate systemic allergic reaction to an insect sting. While deaths from insect stings may be under-reported, deaths due to stinging insect allergy are still very rare in Australia ( < 1 per million/year). Stings are more common in younger age-groups, but deaths appear to concentrate in older individuals, perhaps relating to underlying cardiac disease.

Problem Insects

In regions with high mixing of horticulture and population, approximately 20% of the population have IgE antibodies to honey bee (Apis mellifera) venom and around 2% are likely to have experienced systemic allergic reactions following stings.

In rural and semi-rural Victoria, 2-3% of people give a history of systemic allergic reaction to ant stings, most commonly to the "Jack Jumper" Ant (Myrmecia pilosula). The distribution of the ant also includes Tasmania and the sandy high country of South Australia, ACT and New South Wales, suggesting that more than 10,000 subjects may be at risk.

There are other regional problems for example stings from Polistes and Ropalidia species of wasp and March Fly and tick bites in the central and northern east coast. The "European wasp" (Vespula gemanica) has the potential to become a major problem as its distribution is becoming more widespread.

Natural History of Stinging Insect Allergy:

The natural history of stinging insect allergy is best defined for bee and wasp allergy. About 40% of subjects with history of hypotensive reaction to a previous sting will have an immediate systemic allergic reaction following subsequent stings. A significant proportion of these will experience hypotensive anaphylaxis.

On the other hand, children are more likely to suffer generalised skin reactions to a sting. They have a less than 10% chance of suffering an immediate systemic reaction to with further stings and very few of these reactions are severe. Any individual with a history of a generalised reaction to an insect sting should be referred for a specialist opinion and consideration of immunotherapy.

By contrast, less than 10% of children or adults with a history of large local honey bee or wasp sting reactions will subsequently experience a generalised reaction, and immunotherapy is not indicated.

Diagnosis:

It is important to document the circumstances, insect prevalence, nature of the bite or sting, presence of a stinger or recognisable insect, previous sting history and the nature of the reaction. Particular care is required to document respiratory or hypotensive features. The insect causing the reaction should be identified and the presence of hypersensitivity confirmed with skin prick testing followed by intra dermal testing. Skin testing carries a small but defined risk of systemic reaction. Intra dermal testing requires considerable experience in performance and interpretation and should be performed by a consultant in clinical immunology/allergy. Blood testing for venom - specific IgE can also be performed.

Management Strategies

A) Avoidance Measures.

These are most effective when insect has a limited distribution e.g. jumper ant. For other insects avoidance of high risk activities such as bee-keeping in those known to be allergic, wearing shoes when outdoors, wearing long-sleeved, light coloured clothing, avoiding perfumes, the use of ventilation systems in vehicles rather than open windows and the avoidance of drinking "blindly" from containers may all be useful.

B) First Aid Measures

All individuals with known anaphylaxis to insect stings should carry adrenaline and have a written action plan. Such a plan will likely involve the following steps:

Administration of adrenaline. This should be self administered deeply into the front outer thigh; the advent of automated adrenaline syringes such as EpiPen, (0.15mg 3-5 years, 0.3mg from 6 years on) has greatly facilitated this step. Inhaled adrenaline devices are no longer routinely available.
Seek medical help
Oral administration of H1 antihistamines and in selected cases (e.g. those with history of severe delayed reactions) oral prednisolone may be also be appropriate.

Other recommended measures may include:

Removal of the bee sting (which should be done as soon as possible)
Applying an ice pack
Avoiding vigorous exercise or heat.

For subjects who have had mild reactions immediate removal of the stinger and administration of an antihistamine may be sufficient.

C) Immunotherapy.

Honey bee and wasp venom immunotherapy is of proven value. Unfortunately, such therapy is not yet available for stinging ants and many other insects. About 95% of individuals on maintenance immunotherapy with wasp venom and about 90% of individuals on such therapy with honey bee venom will not react to a sting from the respective insect.

Selection for, initiation and supervision of immunotherapy must be by a recognised consultant in clinical immunology and allergy as immunotherapy is complicated by a considerable risk of allergic reaction. In one very large representative study for example, 12% of subjects experienced one or more systemic reactions during the course of immunotherapy.

There are few contraindications to venom immunotherapy in individuals who have had serious allergic reactions to stinging insects (such as respiratory difficulty or hypotension) and where specific IgE reactivity to the venom is proven by in vitro test and/or skin testing. Venom immunotherapy is not appropriate to individuals with positive skin test or RAST alone in the absence of anaphylactic symptoms. Unless special risk factors (e.g. rural worker or adverse risk factors for cardiovascular disease) are present, the prognosis is sufficiently favourable in individuals who have had reactions confined to the skin that many of these can be managed conservatively.

When to discontinue immunotherapy is a complex issue. There is evidence from Europe and North America that therapy in most subjects after 3-5 years of well tolerated maintenance immunotherapy irrespective of skin or blood test results at the end of that therapy. Adverse prognostic factors, however, include being allergic to honey bee venom, hypotensive anaphylaxis and systemic reactions to venom immunotherapy injections. Consultant advice must therefore be sought before discontinuing immunotherapy.

Practical Points - Stinging Insect Allergy

Immediate systemic allergic reactions to insect stings are common but deaths following insect stings are relatively rare
The roles of honey bees and wasps are recognised but within its restricted distribution jack jumper ant is a major contributor
The natural history of bee and especially wasp sting allergy is more favourable than usually perceived. Those with a history of hypotensive anaphylaxis, however, have a substantial risk for a serious outcome
The advent of automated adrenaline syringes has facilitated emergency first aid care in the community
Those with a history of immediate allergic reactions with generalised, respiratory or hypotensive features should be referred to a Clinical Immunologist/Allergist for advice regarding venom immunotherapy.