The winged stinging insects are Honey Bee, Wasp, Yellow Jacket and Hornets while non-winged insects are fire ants and harvester ants. Identification of insect is difficult many a times but may be helpful in diagnosis and treatment of insect sting reactions (8). Yellow Jackets are ground dwelling insects and can be encountered during yard work, farming or gardening. They can also be found in wall tunnels or crevices and in hollow logs. These insects are aggressive and they sting with minimal provocation. These insects are attracted to meats or sweets easily and become aggressive during late summer. Hornets build large nests of several feet in diameter and are usually found in trees or shrubs. Hornets are extremely aggressive particularly near their nests. They get excited on being provoked especially on trimming hedges and shrubs. Wasps build honeycomb nests that are several inches or more in diameter and might be seen on the outside of the nest. The nests can be found in shrubs, under the caves of houses or barns and occasionally in pipes on playgrounds. Yellow jackets, hornets and wasps feed on human foods and are especially attracted to sweet food. Domestic honeybees are found in commercial hives. Wild honey bees are found in tree hollows, old logs or in buildings. Even a slight disturbance near their hives can cause hundreds of bees to become air borne and they may follow the fleeing victim for even one kilometer (9,10).
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The stinging apparatus of insects originates in the abdomen of the female insect. It consists of a sac containing venom attached to a barbed stinger. A sting occurs when the sac contracts and the venom is deposited in the tissue. The honeybee stinger has multiple barbs, which usually cause the stinging apparatus to detach from the insect, leading to its death. In contrast the stingers of vespids have few barbs, and these insects can inflict multiple sting.
The honey bee venom contains phospholipase A2, hyaluronidase, acid phosphatase and mellitin. Mellitin causes hemolysis of red blood cells while phospholipase A2 causes disruption of cell membrane resulting in release of products of arachidonic acid that increases the vascular permeability. These events lead to microcirculatory failure, hypotension and shock. Vespid venom contains phospholipase, hyaluronidase and a protein known as antigen 5. There is no difference between the venom of Europeanized honeybees or much publicized, Africanized honey bees (also known as killer honey bees). There is no difference in allergic potentials or toxicity of either. There is a cross reactivity between Honey bee and Vespid ââ‚¬" IgE antibody against hyaluronidase.
Bee sensitized patients are not at a higher risk for anaphylaxis when stung by yellow jackets. Allergic manifestations in an individual are dependent on the atopic background of the patient and his ability to form IgE against the venom. The mediators of allergic manifestations are histamine, prostaglandins, leukotrienes and tryptase.
The fire ant solenopis invicta attaches to the person by biting with its jaws. It rotates around its head and stings at multiple sites. Bite leads to formation of a sterile pustule which is diagnostic of fire ants sting (11). Anaphylaxis may occur after bite by deerfly, lone nose (kissing bug) and bed bug. Large local reactions, associated with IgE antibodies are however common (12).
The common sites of insect stings are head and neck as these are the usual exposed sites. The various reactions to insect stings can be local reaction, toxic reactions and anaphylaxis.
Insect stings usually cause local burning pain of varying degree. The other manifestations are swelling and erythema. These minor reactions usually improve within few hours. Stings deposit venom allergens while bites of mosquitoes deposit salivary glands secretions and are therefore usually painless. The local reactions following sting occasionally may be more severe with swelling extending from the sting site over a large area and are called large local reactions (13). These increase over next 48 hours and last for as long as seven days. This can be mistakenly diagnosed as infected bites and antibiotics may be administered inadvertently. These local reactions are IgE mediated.
Cellulitis rarely develops following insect stings and is suspected when the local reaction is large and is associated with nausea, fatigue or systemic features. These may be associated with an ascending lymphangitis and lymphadenitis. Patients with large local reactions are likely to manifest similarly following another bite but the risk of anaphylaxis is less than 5% per episode (14).
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These reactions are very difficult to differentiate from systemic allergic manifestations. Presence of nausea, vomiting, diarrhea, headache, syncope and fever favours a toxic reaction.
Systemic allergic manifestations
0.3 to 0.5% of individuals following insect stings can have manifestations of anaphylaxis. One of the most important causes of anaphylaxis in adults is insect sting allergy. It is to be remembered that the sudden death near pool side or near the buildings in young individuals could be because of this phenomenon. No age is immune from these manifestations. Anaphylaxis is more common in males (twice as common as females), younger individuals and patients with a history of atopy. Clinical features are the same as of anaphylaxis due to any other cause. The most common site of manifestation is skin that includes urticaria, flushing and angioedema. Respiratory manifestations are those of upper airway obstruction and shortness of breath. Cardiovascular manifestations are hypotension and shock and can occur in absence of urticaria or angioedema (15). Symptoms usually start within 10-15 minutes following stings but rarely have occurred as late as 72 hours after a sting.
Unusual reactions following insect sting include acute renal failure, vasculitis, encephalitis and serum sickness (16). Acute renal failure has been described following honey bee stings and hornet stings (17). This has been attributed to hypotension, intravascular hemolysis, myoglobinuria due to rhabdomyolysis and probably due to direct toxic effects of the massive quantity of the injected venom. Similarly acute diffuse encephalomyelitis has been described following insect stings. Serum sickness may occur 7-10 days after an insect sting (18).
Diagnosis of local reactions following insect sting is not difficult as the history of insect sting is straight forward. Similarly, diagnosis of anaphylaxis is established based on the clinical manifestations and a history of precedent insect sting. However, methods used in the diagnosis of Hymenoptera venom induced immediate type hypersensitivity are controversial. This issue is important especially in patients who have experienced a systemic reaction following an insect sting. Controversy exists on how individuals who had systemic reaction following an insect sting should be investigated and diagnosed to have insect venom allergy. This becomes particularly problematic in individuals where IgE antibody test are discordant with a clinical history of sting reactions.
Plasma histamine levels may be measured in select group of patients where the diagnosis is not straight forward and the facilities are available. Similarly metabolites of histamine in the urine can be estimated. Serum tryptase level rises 1-2 hours following an insect sting and persists up to 6 hours.
Testing for venom allergy
Venom extracts typically contained phospholipase and hyaluronidase and can be used for skin testing. Skin test typically detects IgE antibody against the venom. 30% of individuals following an insect sting would have an elevated level of IgE antibody. These levels would decrease over a period of time. Therefore, it is recommended that the skin testing should be performed six weeks following an insect sting. This is because patients who had a systemic reaction following an insect sting will continue to have an elevated persistent level of IgE antibody. The skin test indicates a prior antigen exposure but is poor in predicting a reactor from a non reactor. Skin test is not indicated in an asymptomatic individual or a patient who had only local reaction following the insect sting. Purified and freeze dried venom from single type of insect (Yellow jackets, honeybees, while faced hornets, yellow hornets and wasps) as well as mixed vespid venom (venom in equal parts from yellow jackets, white faced hornets and yellow hornets) can be used. The same venoms that are used for the therapy can be used for the diagnosis. An immediate reaction to a skin test with venom dose less than 0.1 to 1.0 ug/ml is immunologically specific and indicates that person has venom specific IgE (19,20).
Radioallergosorbent assay (RAST)
Serological assays of IgE antibodies against insect venoms can be measured by radioallergosorbent test (RAST) assay technology that has been improved further as a result of second generation assay methods. There is a new capture allergic protein radioallergosorbet tests (CAP-RAST). These tests have an increased precision due to automation and highly purified and reproducible venoms.
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Type 1 hypersensitivity is confirmed by detecting venom specific IgE in serum using RAST or by prick skin tests.
Alternative diagnosis testing methods
There are a number of alternative testing methods that have been used to supplement skin test and serology. These include in vivo provocation and invitro provocation tests. In vivo provocation tests have been rarely performed because of an inherent risk of complications in the individual. In vitro provocation test include activation of basophils and measurement of mediator release or detection of cell surface markers of basophil activation. CD63 flow based basophil studies and CD203 flow based basophil studies are under the realm of research at present (21).
Local treatment forms the mainstay in the treatment of insect stings in majority of the patients. These include cleaning of the wound, application of ice to the site of the sting to reduce the pain and removal of stinger with its sac. Systemic antihistamines ââ‚¬" Diphenhydramine can be given to alleviate the itching. Topical steroids (Dexamethasone 0.05%) can be applied. Antibiotics are not given routinely. However, if the erythema, warmth and fluctuation persist 48 hours later, antibiotics may be considered. Insect stinging apparatus should be flicked of with care taken not to squeeze the sac as it would cause more deposition of venom.
Treatment of anaphylaxis include subcutaneous epinephrine (0.2-0.5ml) (1:1000 dilution) which can be repeated every 30 minutes. Intravenous epinephrine (1:10,000 dilutions) may be rarely required in case of profound shock.
Basic management of airway, breathing and circulation may be required as in any other emergency. Aerosolized epinephrine may help treat upper airway edema. Acute symptoms of anaphylaxis usually subside within 15-30 min. If symptoms persist, steroids such as intravenous methyl prednisolone (60-80mg) followed by prednisolone 60mg may be given for 1-2 days.
People who have had anaphylactic reactions to insect stings and have positive venom skin tests are at risk for reactions to subsequent stings. They should be told about their venom sensitivity. Prophylactic recommendations include minimizing exposure to stinging insects, having medication for treatment of anaphylaxis at hand and considering venom immunotherapy. Subsequent stings can be avoided by wearing shoes, slacks and shirts with long sleeves. Cosmetic hairsprays which attract insects should be avoided. Insect repellants are usually not effective against stinging Hymenoptera.
Venom immunotherapy (VIT) is recommended for patients who have experienced a systemic reaction to an insect sting and who have specific IgE to venom allergies as demonstrated by skin test. Following considerations are to be taken into account. VIT is generally not necessary in children below 16 years of age or younger who have experienced cutaneous systemic reactions without other systemic manifestations of a reaction after an insect sting from a wasp, hornet, yellow jacket or a bee (22). Adults who have experienced only cutaneous manifestations to an insect sting are generally not considered candidates for VIT, although the need for immunotherapy in this group is controversial. Because the natural history of fire ant hypersensitivity in children who have only cutaneous manifestations have not been well elucidated and there is increased risk of fire ant stings in children, who live in areas where fire ants are prevalent, immunotherapy may be considered for such children.
VIT injections are administered generally at weekly intervals, usually beginning with levels not greater than 0.1 to 0.5 ug and increasing to maintenance dose of up to 100ug per insect venom (23,24). More accelerated schedules for VIT have been published and can be used successfully and safely (25,26). The interval between the maintenance doses is usually increased to 4 weeks during the first year and eventually to every 6-8 or even 12 weeks during subsequent years. Safety considerations related to VIT are same as with other forms of allergen immunotherapy. The major risk is anaphylaxis. Patients on beta blockers might not respond readily to treatment if they experience an allergic reaction. Therefore they should not take beta blockers, unless absolutely necessary. Once begun the VIT should usually be continued for at least 3-5 years. Although most patients can then safely discontinue immunotherapy, some patients may need to continue VIT definitely. Guidelines for discontinuing VIT are evolving (27). Criteria suggested for stopping are a decrease in serum venom specific IgE to insignificant levels, conversion to negative skin test or finite period of time (3-5 years).
Insect bites are commonly inflicted by the family Culicidae (order Dipetra) and are only families, local reactors are few and systemic features are related to the diseases the insect can transmit. The mosquitoes include the genera Anopheles, Culex and Aedes that are usually responsible for human bites. It is only the female insect which bites as she requires blood to produce eggs. Painful and erythematous reactions are common which last from 2-7 days. Triatomas include the reduviid group the so called kissing bugs or assassin bugs (28). These insects bite at night time. These are also responsible for Chagas disease.Flies rarely induce systemic reactions (29). Allergic reactions can be caused by Black flies, Horseflies, Deerflies and Sand flies. Fleas (order Sphonoptera) can induce papular urticaria but no anaphylaxis. Ticks are recognized cause of infectious disease causation including Lymes disease, Rocky Mountain Spotted Fever, Ehrlichiosis.
The bites are usually painless. Occasionally they may induce inflammatory reaction at the local site that subsides within a few hours but sometimes, it may be followed by delayed skin reactions. Systemic reactions due to insect bite are very rare.
Diagnosis is simple. Severe systemic reactions need to be evaluated, if present. In rare situations, skin testing with whole body extracts have been used as saliva was not commercially available till recently but recombinant salivary antigens have been produced and new diagnostic tests will be available soon. RAST and histamine release from peripheral basophils are helpful diagnostic tools.
Insect bite site should be washed with soap and water. For local reactions cooling (ice or cold pack) to reduce local edema, topical lotions like calamine to reduce itching and systemic antipruritic drugs as hydroxyzine and cetrizine are used. For systemic reactions, epinephrine should be used for anaphylaxis. In these cases, immunotherapy with whole body extracts from mosquitoes and triatomas is reportedly useful (30), but has systemic reactions (31). Insect repellents like N.N.-diethyl-3-methyl benzamide (DEET) are effective in protection against mosquitoes. It is extremely safe though adverse neurological side effects have been reported with massive exposure and chronic use.