Biting Insects

The most common biting insects are mosquitoes, flies, midges, gnats, fleas, ticks and bedbugs.

Itching and a wheal may develop immediately and mostly disappear after about 2 hours, but they are often followed by a small itchy lump (papule) that develops up to 24 hours later and may last for several days before fading away. Large local reactions from mosquito bites are more common in young children. Over time, and with repeated exposure, the reactions become less intense and are less frequent problems in adolescents and adults. Anaphylaxis has been described after the bites of mosquitoes, deer flies, bed bugs and black flies.

Mosquito bites may be associated with IgE and perhaps IgG antibodies. Elevated titers correlate with the intensity of the local reactions and appear to be the immunologic mediators responsible for the reactions.

Skeeter syndrome describes a localized (allergic) reaction to mosquito bites masquerading as cellulitis and accompanied by lethargy, fever and general malaise. Skeeter syndrome usually progresses over the course of hours while cellulitis typically will evolve over the course of several days. Diagnosis can therefore be made clinically by the course of the symptoms and can be confirmed by measuring IgE and IgG antibodies to mosquito saliva antigens.

A mild reaction to an insect bite will probably resolve itself within a day or two without any need for treatment. In some cases, the use of a topical steroid may assist in reducing inflammation and an antipruritic may relieve itchiness. There is a scarcity of quantitative scientific studies into the various treatments for insect bite reactions.

Stinging Insects

Insects that sting are members of the order Hymenoptera of the class Insecta. It is almost exclusively the social Aculeata of the families Apidae, Vespidae and Formicidae that cause significant allergic reactions in human beings. Aculeata from other families can cause painful stings to people, but repeated stings from the same species are so unlikely that allergy to them is practically unheard of. There are three major subgroups: Vespidae, which include the yellow jacket, hornet and wasp; Apidae, which include the honeybee and bumblebee; and the Formicidae ( Figure 57-1 ). An overview of the Hymenoptera is given in Figure 57-2 .

The common stinging insects. Shown clockwise from the top right portion of the figure are a yellow jacket, honeybee, bumblebee, Polistes wasp and two hornets.

(From Reisman RE. Insect stings. N Engl J Med 1994;331:523–7. Copyright 1994 Massachusetts Medical Society. All rights reserved.)

Overview of taxonomy of common Hymenoptera (stinging insects).

The Aculeata share in common a stinging apparatus that originates in the abdomen of the female insect and actually is a modified ovipositor; therefore only female insects can sting. The sting consists of a sac containing venom attached to a barbed stinger. The honeybee’s 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 and finer barbs than the Apidae, and do not commonly autotomize, so vespids can inflict multiple stings ( Figure 57-3 ).

The honeybee stinger on the left has multiple barbs. The yellow jacket stinger on the right is smooth.

Vespidae

The Vespidae are divided into the subfamilies Vespinae (yellow jackets and hornets) and Polistinae (wasps). The Vespinae are split into three genera: Vespula (yellow jackets), Dolichovespula and Vespa (hornets). The common names in general usage can be misleading: in Europe the term ‘wasps’ refers generally to any of the social wasps rather than just to Polistes species ( Table 57-1 ).

TABLE 57-1

Popular Names for Vespids in Europe and the USA

Genus	Europe	USA
Vespula	Wasp	Yellow jacket
Dolichovespula	Wasp	Hornet, white-faced hornet, aerial yellow jacket
Vespa	Hornet	European hornet
Polistes	Paper wasp	Wasp

 

In most parts of the USA and Europe, yellow jackets are the principal cause of allergic reactions, whereas Polistes are more commonly implicated in the Gulf Coast areas of the USA and along the Mediterranean coast of Europe. In Australia yellow jackets were only introduced around 20 years ago, but are already found in every state.

The Vespula (yellow jackets) preferably build their nests underground, but can also be found under the roof and in window shutters. They live mostly in proximity to humans. Their nests are often encountered by children playing in the garden, or are disturbed by lawn mowing, gardening or other outdoor activities. Yellow jackets are scavengers and are often encountered at outdoor events when food and drinks abound. Most stings occur in summer.

Vespa include the European hornet ( V. crabro ) which also exists in significant numbers in eastern North America, and the Asian hornet ( V. orientalis ). Hornets nest in shrubs, trees and birds’ nest-boxes. Near their nests, hornets are very aggressive. However, they are attracted much less to meat and sweet foodstuffs than the Vespula . Therefore hornet stings are rare.

The common paper wasps ( Polistes ) build honeycomb nests in shrubs or under the eaves of houses. Their nests are generally limited to a single layer of open cells (or comb) with minimal outer covering. They are important predators, preying on agricultural and horticultural pests. Important Polistes species in Europe are P. dominula and P. gallicus , whereas in North America other species such as P. annularis, P. apachus, P. exclamans, P. fuscatus and P. metricus are dominant. In the last decades, P. dominula has increasingly spread across the North American continent and central and northern parts of Europe. The coloring of wasps varies greatly: they can be brown, black, red or striped. The European species, the Mediterranean wasp ( P. dominula ), is difficult to distinguish from a yellow jacket because it has the same bright yellow and black stripes. Outwardly they are distinguishable by differences at the junction of thorax and abdomen: the waist becomes thicker more rapidly in the Vespinae compared to the Polistinae and they have characteristic dangling legs when in flight ( Figure 57-4 ). Polistes are more prevalent early in the summer season. In some areas of the USA, such as Texas, they are the most frequent cause of sting reactions.

Polistes compared to Vespula: thicker waist and dangling legs when in flight.

(From: http://crawford.tardigrade.net/bugs/BugofMonth16.html .)

Cross-Reactivity

A common problem of in vivo and in vitro diagnosis of insect venom allergy using venom extracts is that patients may have double positive test results for honeybee venom (HBV) and yellow jacket venom (YJV). This double positivity may reflect true double sensitization to HBV and YJV, or may be based on IgE cross-reactivity.

Cross-reactivity may be based on IgE reactivity to homologous single venom allergens present in venoms of different families or on IgE reactivity to cross-reactive carbohydrate determinants (CCD). Causative for the latter are IgE antibodies that are directed against an alpha 1,3-linked fucose residue of the N -glycan core established by insects and plants. Most HBV and YJV allergens are glycoproteins with one or more such carbohydrate structures ( Table 57-2 ). CCD-specific IgE antibodies have been reported to be responsible for more than 50% of double sensitizations to HBV and YJV. The clinical relevance of CCD-reactive IgE antibodies in the case of insect venom allergy appears to be low or nonexistent. Polistes species seem to lack the alpha 1,3-linked fucose residue that is responsible for IgE reactivity to CCDs.

Recombinant allergens lack CCDs, allowing for a more precise distinction between true double sensitization and cross-reactivity between different venoms. By using CCD-free, correctly folded Ves v 2.0101 and Ves v 2.0201, it could also be demonstrated that hyaluronidases – contrary to previous assumptions – do not play a significant role as major allergens of YJV.

Vespid venoms have been extensively analyzed. Venom allergens of diverse Vespidae species such as the white-faced hornet ( Dolichovespula maculata ) or the European hornet ( V. crabro ) are fairly similar to those of the yellow jacket, allowing V. crabro allergic patients to be treated with yellow jacket venom.

The IgE cross-reactivity between European and American Polistes species is described as low because they belong to different subgenera. In contrast, cross-reactivity between Polistinae and Vespinae ( Vespula , Dolichovespula and Vespa ) venoms and purified venom proteins is frequently observed, especially for Vespula and both American and European Polistes venoms. Polistes VIT is not necessary. The converse is also true: half of the people who have had Polistes sting reactions have positive skin tests to yellow jacket or hornet venom and require treatment with Polistes venom only.

Within the Apidae there is cross-reactivity in that people initially sensitized to honeybee venom then react to bumblebee venom due to cross-reactive allergens. Honeybee venom should be effective immunotherapy. Bumblebees have particular importance for pollination industry workers; it has turned out that some of these patients need to be treated with specific bumblebee venom.

Little is known of immunologic cross-reactivity between fire ant and vespid venom.

Sol i 1 shares sequences with the venom phospholipases of Vespula maculifrons.

Development of Insect Sting Allergy

Hymenoptera stings are frequently encountered in the population. The younger the child, the more often they are (re)stung; in contrast, prevalence of systemic reactions to field stings was significantly lower in preschool (3.4%) and school-age children (4.3%) compared with adolescents (15.6%).

No immunologic criterion, such as skin test reactivity or titers of serum venom-specific IgE or IgG, distinguishes or identifies sting reactors from nonreactors.

The chance of developing an allergy increases with sting frequency. In general, no time relationship exists between the last uneventful sting and the subsequent sting that leads to an allergic reaction although multiple stings or repeated stings in close temporal proximity (only weeks apart) have been associated with a greater risk of developing an allergic reaction. A confusing observation is the occurrence of initial insect sting anaphylaxis after the first known insect sting, primarily in children, raising the issue of the cause of sensitization or the pathogenesis of this initial reaction.

Normal Reaction

The usual reaction to an insect sting consists of localized pain, slight swelling, and erythema at the site of the sting. This reaction usually subsides within several hours. Little treatment is needed other than analgesics and cold compresses.

Large Local Reactions

More extensive local reactions are common. Large local reactions are defined as reactions extending from the sting site over a large area, often peaking at 24 to 48 hours and taking 5 to 10 days to resolve. For example, the swelling from a sting on the finger may extend to the wrist or elbow. Fatigue and nausea may develop in addition. For a general clinician it might be difficult to distinguish large local reactions from cellulitis, resulting in misdiagnosis and unnecessary antibiotic treatment.

The cause of these large local reactions has not been established, but it is thought to be an IgE-mediated late-phase reaction.

Toxic Reactions

Large numbers of simultaneous stings (50–100) may result in a toxic reaction due to the vasoactive properties of the venom. Symptoms can have the same clinical characteristics as anaphylaxis, including nausea, vomiting, diarrhea, headache, vertigo, syncope, convulsions and fever. Hemolysis, cardiac complications, renal failure and rhabdomyolysis have also been described.

As insect venom is highly sensitizing, most patients will develop specific IgE, making the differentiation from an allergic reaction difficult.

Unusual Reactions

There have been rare reports of vasculitis, nephrosis, neuritis, encephalitis and serum sickness occurring in a temporal relationship to insect stings. The symptoms usually start several days to several weeks after the sting and may last for a long period. Serum sickness, characterized by urticaria, joint pain and fever, may occur approximately 7 to 10 days after an insect sting.

Some patients develop cold-induced urticaria days to weeks after Hymenoptera stings, not necessarily accompanied by an allergic reaction to the sting.