Allergy Capital

Stinging insect allergy / anaphylaxis

Version 10 February 2002

Around 1 in 300 people are susceptible to severe allergic reactions to stinging insects, and some will die. Identification of patients at risk of anaphylaxis allows them to commence specific immunotherapy.

Introduction
Stings from ants, bees and wasps are common. Unfortunately, around 1 in 300 people are susceptible to severe allergic reactions to stinging insects, and some will die. Identification of patients at risk of anaphylaxis allows for the commencement of specific immunotherapy.

Triggers
Allergic reactions to stings by bees, wasps, "jumper ants" (and sometimes ticks) are most common in the warmer months of the year. Jumper ants are large primitive ants, aggressive, can sting through clothing and are prevalent in Black Mountain reserve, the Botanic Gardens other areas around Canberra and south-eastern Australia. The recent identification of the South American "Imported Fire Ant" (Solenopsis richteri) in South Eastern Queensland is of particular concern, as systemic reactions to stings occur in up to 1 in 6 cases. As their venoms are all different, an allergic reaction to one does not increase the risk of sensitivity to another.

Not all reactions are serious

Minor local swellings can be treated with ice, Calomine lotion and perhaps an antihistamine or paracetamol.
Large local reactions cause much fear, but are usually benign. Pain, heat and swelling typically increase over a 48 hour period and can last up to a week. In some cases, an entire limb may swell after being stung on an extremity. Resembling cellulitis, many patients are unnecessarily given antibiotics. These reactions are not associated with fever and regional lymphadenopathy is rare. First Aid measures plus application of potent topical corticosteroids under a Band Aid or Gladwrap is sometimes useful. More severe cases will require systemic corticosteroids for 2-3 days.
Systemic reactions are characterised by symptoms remote from the site of sting, such as generalised urticaria or throat swelling after being stung on the foot. The presence of bronchospasm or hypotension has more sinister implications for management.

Who is at risk of anaphylaxis?
Skin testing "out of interest" does not predict risk; up to 1 in 5 in the community will have positive skin test to bee venom yet most will not react clinically. Minor local reactions are common and do not indicate a significant risk of anaphylaxis. There is a common community perception that each reaction will be worse than the one that precedes it. Long term followup studies suggest the opposite. Less than 10 % of large local reactors ever progress to anaphylaxis, immunotherapy is ineffective at reducing the severity of local swellings and is not offered as a routine.

Patients with mild systemic reactions (such as urticaria alone) do not always need immunotherapy; less than 10 % will have worse reactions with subsequent stings. The prognosis is even better in young children who will usually "grow out of" their allergy after a few years. In patients with dangerous features, however, immunotherapy should be considered "compulsory" regardless of age. Anaphylactic shock after stings has been observed by the author in patients ranging from 3 years to 73 years.

Immunotherapy
is a worthwhile hassle for those at risk of anaphylaxis. Conventional therapy involves starting at the equivalent dose of 1/1000 sting, building to 2 stings per injection over a few months. Injections are then continued monthly for about 18 months, then 6-8 weekly thereafter for a total duration of 5 years, sometimes longer in selected patients. This form of therapy is not trivial, and so only those at risk are normally advised to commence treatment, although special circumstances (eg. living or working in remote areas, intercurrent cardiac disease) sometimes need to be taken into account. A novel immunotherapy trial is being undertaken by the author as part of a multinational study at John James Memorial Hospital, aiming to increase the potency of treatment with fewer visits.

Reducing the risk
Children allergic to venomous insects should be advised to wear shoes when outside, and bee and wasp nests should be removed from the vicinity of their home and school. While the effectiveness of this approach remains unproven, the author recommends that venom-sensitive subjects take prophylactic antihistamines if walking in remote areas during the "active" insect season. One of the author's patients has probably only survived an episode of anaphylaxis after activating an emergency satellite beacon when stung in the wilderness.

Management

Wearing a MedicAlert bracelet increases the likelihood that adrenaline will be administered in an emergency.
Avoid medication that may enhance the severity of anaphylaxis or complicate its treatment. Beta-blockers (and perhaps ACE inhibitors) inhibit counterregulatory mechanisms that may protect from uncontrolled hypotension.
Patients at risk of anaphylaxis should carry adrenaline and be trained in its use.
ALL patients, regardless of age, who have suffered a systemic allergic reaction following envenomation should be evaluated and offered specific immunotherapy if appropriate.

Specific Immunotherapy
Immunotherapy is effective for the treatment of inhalant allergies and honey-bee or wasp stings. The risk of a serious systemic allergic reaction is reduced from approximately 65 % down to 5 % per sting. Unfortunately, there is no vaccine available for the treatment of "jumper ant" at this time. A pilot study is currently being undertaken in Tasmania, and results will be of great interest to the hundreds of patients for whom there is currently no specific treatment.

Emergency Treatment of Anaphylaxis
Adrenaline is the only medication available for the emergency treatment of anaphylaxis. It rapidly reduces mediator release and their end organ impact. In an outpatient setting, it should be administered intramuscularly. Subcutaneous injection is associated with significantly delayed absorption and inhaled adrenaline is ineffective. Intravenous adrenaline should only be given by those experienced in its use, and even then only slowly using 1/10,000 dilution.

Special Circumstances: Anaphylaxis in Schools
A recent South Australian study of over 4000 school-aged children demonstrated that 1 in 170 children had suffered an episode of anaphylaxis, and 1/6 episodes had occurred at school. Children spend over a third of their waking hours at school and are totally dependent on carers to provide a safe environment and administer First Aid in an emergency. That is why the Australasian Society for Clinical Immunology and Allergy recommends that:

Children who have had anaphylaxis should be identified at school enrolment, and staff should be made aware of all relevant information.
Appropriate strategies should be in place to minimise the risk of accidental exposure at school
Staff should be trained how to recognise symptoms of anaphylaxis.
All children at risk of anaphylaxis should have an Emergency Action Plan
Similar to the South Australian programme, a school-based training programme is under development in the ACT (2001-2) and has already been piloted in over 16 schools.