The Hymenoptera order includes over 150,000 different species of flying and non-flying insects. Some species within this order have modified ovipositors (female reproductive organ) which can also act as stingers. Hymenopteran stings only cause local inflammation in most people. However, they also account for the largest percentage of envenomation-related deaths in the United States. Most deaths result from immediate hypersensitivity reactions and anaphylaxis. This review article concentrates on the most common and clinically relevant species: Apidae (bees), Vespidae (wasps, yellow jackets, hornets), and Formicidae (specifically, fire ants). All 3 of these species inject venom, via modified ovipositors in their abdomen (stingers), into prey or as a defense mechanism.
The Apidae family (honey bees, bumble bees, among others) are usually not aggressive and only sting when threatened or provoked. However, there is a subset of “Killer Bees” or “Africanized bees,” that are very defensive, often aggressive, and tend to swarm. These Killer Bees were introduced in Brazil in the 1950s, but have since spread to other areas in South and North America (Southwestern United States). They are an invasive species and account for many deaths associated with bee stings. The Apidae family use barbed stingers, which often remain attached in the skin after a single sting.
The Vespidae family (wasps, yellow jackets, hornets) are known to be more aggressive than their Apidae relatives. They are found in all 50 states. They can be classified as solitary or social wasps. Social wasps, including the yellow-jacket and hornet, commonly make larger nests in the ground (yellow jacket) or shrubbery/trees (hornet). Solitary wasps (mud wasp) usually make smaller nests in areas such as the sides of windows. Unlike the Apidae, Vespidae family does not have a barbed stinger and hence, can sting multiple times.
The Formicidae family includes all ants. Most ants bite with pincer-shaped mandibles; however, similar to the Apidae and Vespidae, some ants have developed the ability to sting with stingers in their abdomens. Fire ants (Solenopsis), are an aggressive species that use their pincer mandibles to latch on to their target and then use their stinger to administer multiple doses of venom. These ant colonies are most commonly found in the southeastern United States but are spreading fast and becoming more common in bordering areas. Other species in the Formicidae family also use stingers but are not found in the United States, so these are not included in this discussion.
Hymenoptera stings can happen in any age group, and the insects are not selective to a particular gender. Humans are most often stung by either accidental contact with a solitary worker (single sting from a single insect) or because they are near a disrupted nest (multiple stings from multiple insects). Occupations that may increase the risk of exposure to these stings include, but are not limited to, construction workers, landscapers, entomologists, beekeepers, exterminators, among others. Vespidae (specifically hornets and yellow jackets) are likely the culprits in most situations due to their aggressive and territorial nature. Most sting reactions are self-limited, result in a small area of local inflammation, and resolve within a few days. Other local reactions, may be larger, more painful and last longer. These are less common (less than 10%) and termed large local reactions (LLRs) and may require medical intervention. Systemic reactions, are far less common but can be fatal if they occur.
Hymenoptera stings cause reactions by injecting venom via their ovipositors into their target. The venom for Apidae, Vespidae, and Formicidae have some similar characteristics, consisting of a mixture of smaller, low-molecular-weight, proteolytic enzymes (hyaluronidase, proteases, phospholipase, acid phosphatase), lipids, carbohydrates, and also high-molecular-weight proteins which act as allergens. The low-molecular-weight components are responsible for local inflammatory reactions while the high-molecular-weight component is integral to the systemic reaction (in other words, anaphylaxis). When the venom is introduced into the skin, the proteolytic enzymes begin to degrade the surrounding tissue. The release of histamine from mast cells and basophil activation, in response to the venom, causes vasodilation and the ensuing inflammatory response: edema, pain, erythema, and increased warmth. Fire ant venom is primarily made of alkaloids that result in the characteristic sterile pustules associated with these stings. Large, local reactions (LLRs) develop in about 10% of Hymenoptera stings and are believed to be immune IgE-mediated. Anaphylaxis develops in people with preformed antibodies to the high-molecular-weight aspects of these venoms. These reactions, like other anaphylactic reactions to allergens, occur via a systemic IgE-mediated histamine release. The resulting mast cells and basophil activation can cause systemic vasodilation, angioedema, urticaria, hypotension/shock, and death.
Hymenoptera stings are almost always diagnosed clinically. For this reason, it is very important to get a good history. Patients with uncomplicated, local reactions typically present complaining of pain and swelling after a presumed or witnessed sting. Apidae and Vespidae stings usually cause immediate pain. The venom then causes a local reaction within minutes that can last for hours. Symptoms include pain, swelling, pruritis, bleeding. On exam, you may find erythema, edema, induration, increased warmth. In some instances, one may also see a stinger still attached in the skin (Apidae). Stingers are usually still attached to the venom sac and so should be removed from the skin by scraping, for example, with a credit card, not by squeezing or with tweezers). Fire ant stings also cause pain immediately with an associated burning/itching sensation which lasts a few minutes, then a wheal with surrounding erythema forms. Patients can also develop a sterile pustule within 24 hours. Most Hymenoptera sting reactions are self-limited, result in a small area of local inflammation, and resolve within a few days. LLRs may be larger (greater than 10 cm), more painful, and persist for a longer duration. The exaggerated response to the venom is likely secondary to an IgE mediated mechanism. These reactions typically worsen over 48 hours and then resolve within 7 to 10 days.
Systemic reactions often present as severe anaphylaxis, are rapid in onset, and life-threatening. Patients may have a history of anaphylaxis or a similar systemic reaction in the past secondary to insect stings. Patients present in extremis with rapidly worsening symptoms, including generalized urticaria, angioedema, flushing, difficulty breathing, wheezing, hypotension/shock.
Evaluate Airway, Breathing, Circulation first.
Uncomplicated Local Reactions
Include a small area of focal edema, induration, increased warmth, and tenderness
Large Local Reactions (LLRs)
Greater than a 10-cm area of erythema, induration, increased warmth, larger, and last longer than uncomplicated local reactions
Systemic Reactions (Anaphylaxis)
Includes generalized urticaria, angioedema/facial swelling, stridor, respiratory distress/wheezing secondary to bronchospasm, abdominal pain, nausea, vomiting, flushing
Uncomplicated, local reactions can be treated with supportive care (ice packs, NSAIDs/APAP for pain, H1/H2 blocker). Within the first few minutes after the sting, the stinger should be removed via scraping with a credit card rather than squeezing/tweezing to avoid further venom exposure.
Large local reactions should also be treated with supportive care along with glucocorticoids (usually a burst course of prednisone 40 to 60 mg per day for 3 to 5 days) to decrease the inflammatory response and improve symptoms.
Systemic reactions (anaphylaxis) are life-threatening and should be managed as such. ABCs first. The airway can be lost within seconds to minutes, so intubate early. As with any anaphylactic reaction, epinephrine, corticosteroids, H1 and H2 antagonists, and intravenous (IV) fluids should be given immediately. Epinephrine 0.3 to 0.5 mg should be given intramuscularly (IM) to the anterolateral thigh. This may be repeated every 5 to 15 minutes. The alpha1-mediated increase in vascular tone, beta1-mediated increase in inotropic/chronotropic cardiac activity, and B2-mediated bronchodilation all help to reverse anaphylaxis. Corticosteroids (prednisone, methylprednisolone, dexamethasone) act to decrease inflammation and immune response to the antigen. H1 and H2 antagonists block the effects of histamine decreasing pruritis, erythema, and urticaria.
The prognosis for most Hymenoptera stings is very good. Most patients have anywhere from a few minutes to a couple of days of discomfort. For patients with severe systemic reactions, mortality is often high, unless treated promptly. All patients with a history of severe reactions to insect bites should avoid Hymenoptera insects to the best of their ability and carry an epinephrine auto-injecting pen at all times.