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The stinging insects are members of the order Hymenoptera of the class Insecta. These venomous insects possess the capability to sting using a modified ovipositor found on the terminal end of their abdomen. The three medically important groups are the Apoidea (bees—with 20,000 species), Vespoidea (the wasps, hornets, and yellow jackets—with 15,000 species) and Formicidae (the ants—with 15,000 species). The fire ants will be considered separately in this discussion.

The family Apoidae includes the social honeybees, the solitary bees, and bumblebees. Honeybees are herbivorous and live on nectar and pollen. Wasps, hornets, and yellow jackets (Vespoidae) are predaceous carnivores and live on other insects and sweet substances, such as sap and nectar. Feeding cues for bees emanate from flowers among which they forage. The feeding cue for the vespids comes from flesh and the smell of sugars. There is often a great deal of misidentification between bees and their vespid cousins. However, the two groups differ tremendously in their behavior and body type and can be readily identified. Honeybees are social insects and build their nests (hives) in hollow trees or other cavities. Yellow jackets are usually ground dwellers, whereas the hornets and wasps live in shrubs and trees, and are not ground nesting. Unlike bees, vespids can be frequently found near open cans of soft drinks and sweet food and garbage.

The stinger of these insects is another method of identification.^1,2 Honeybees can only sting once; they possess a barbed stinger that stays behind in the victim’s skin after they sting. The stinger and the venom sac are pulled out of the bee’s abdomen and soon after the insect dies. Wasp, hornet, and yellow jacket stingers are not barbed and each insect is capable of delivering multiple venom-injecting stings without dying.³ Vespids are much more aggressive while bees are generally more docile. However, honeybees will vigorously defend their hives against intruders. Typically, people and animals are stung accidentally when they step on bees or otherwise disturb the insects. An exception to this is the aggressive behavior of the more recently introduced Africanized honeybee. These bees attack more readily than their European and North American counterparts, potentially inflicting hundreds of stings.⁴ If the offending specimen causing the sting is not available, learning the circumstances of the stinging incident, looking for the presence of a stinger in a victim, knowing the differences in body types, and understanding the behavioral differences between bees and wasps can be instrumental in correctly identifying the stinging insect.⁵ The taxonomy and relationship of hymenopterans is illustrated in Figure 48-1.

Figure 48-1 Classification of the Hymenoptera.

Toxic dose

Most deaths related to Hymenoptera stings are the result of immediate hypersensitivity reactions causing anaphylaxis. However, death may also occur from severe local reactions, particularly if involving the airways with subsequent respiratory obstruction. Massive envenomation, as seen in swarm attacks, can likewise cause death in nonallergic individuals. In humans, the estimated lethal dose is about 500 stings for adults.⁶ The estimated lethal dose is about 20 stings/kg in most mammals.⁷

It has been estimated that the European honeybee injects 147 μg of venom per sting, the Africanized honeybee injects an average of 94 μg of venom per sting, and most wasps about 17 μg of venom each sting.^8,9 It does not appear that stinging insects can meter their venom like some venomous snakes and spiders; each Hymenoptera sting delivers a relatively standard venom volume. Anaphylactic reactions are not dose-related and death can occur following a single sting.

Toxicokinetics and mechanism of toxicity

Hymenoptera venoms are composed of complex mixtures of allergic proteins, active antigens, and peptides.¹⁰ Both bee and wasp venoms are made up primarily of protein. Bee venom is a complex mixture of biologically active components, primarily consisting of proteins, enzymes, and amines.

Clinical signs

Typically, honeybee stings are manifested as localized edema without a systemic reaction. Unlike venomous spider bites, venom of all Hymenoptera causes some degree of local swelling and pain, and victims know that they have been stung. Generally the small local reaction of erythema, edema, and pain at the site of the sting is a self-limiting, non–IgE-mediated condition, which spontaneously resolves within 24 hours. Occasionally, animals develop more extensive regional reactions. These more severe regional responses involve erythema and local edema and may involve an entire extremity. The regional reaction is thought to occur from local mast cell degranulation and may not develop until up to 24 hours after the envenomation. This reaction is often termed cellulitis; however, infection rarely follows insect stings. Less common reactions from envenomating stings is edema of the oropharynx, which can result in compromise of the airways. Fatalities can result from airway occlusion from stings inside the oral cavity.

Systemic anaphylactic signs caused by insect stings are no different from other anaphylactic reactions. Onset of life-threatening signs occurs rapidly (often within 10 minutes of the sting). Although it is not understood, signs of anaphylactic reaction may vary in severity.¹⁴ Mild anaphylactic signs include urticaria, pruritis, and angioedema.¹⁵ Other non–life-threatening signs include vomiting and diarrhea. More serious signs of anaphylactic reaction include the respiratory and cardiovascular systems.¹⁶ Wheezing, dyspnea, cough, and bronchoconstriction may occur and lead to hypoxia and respiratory arrest. Local upper airway edema can cause congestion of the larynx, epiglottis, and surrounding tissue. The majority of insect sting fatalities are the result of severe respiratory compromise.

The unusual delayed reactions reported include serum sickness, vasculitis, glomerulonephritis, neuropathy, disseminated intravascular coagulation, and arthritis.¹⁷ Direct toxic effects of Hymenoptera venom independent of immune mechanisms are venom volume-dependent reactions. Animal victims of such multiple stings may demonstrate rhabdomyolysis, hemolysis, and acute renal failure from direct tubular toxicity.¹⁵ Myocardial infarction has been documented in victims of insect stings.

Bee and wasp stings typically cause only local redness, erythema, and transient pain in dogs. Urticaria may or may not accompany the swelling. Dogs may cry out when stung, and they may rub their mouth and eyes on the ground. These cutaneous reactions appear quickly and will spontaneously regress.

Potential allergen mediators of anaphylaxis include phospholipase A, hyaluronidase, acid phosphatase, and mellitin.¹ Vespid venoms share much more similarity in allergens with other vespids than with bee venoms. This may explain the cross-sensitivity in allergic reactions seen in people stung by various vespids. Furthermore, cross-sensitivity to both bee and wasp venom has been documented in humans.

The signs of anaphylaxis in dogs include urination, vomiting, defecation, muscular weakness, depressed respiration, and finally seizures.¹⁵ Signs typically are seen within 15 minutes of the sting and if a systemic reaction has not started within the first 30 minutes, it is unlikely to occur. Fatalities typically occur within 60 minutes of the initial sting. Anaphylaxis in cats is manifested by pruritis, salivation, incoordination, and collapse.¹⁵ The signs of anaphylaxis are attributable to antigen-induced IgE release and formation of chemical mediators that target smooth muscle and blood vessels.

Animals receiving massive envenomations (many stings) are usually febrile and visibly depressed. Facial paralysis, ataxia, seizures, and neurological signs may be observed. Dark brown or red urine, bloody feces, bloody or dark brown vomitus may also be seen.¹⁵ A complete blood count may reveal a leukocytosis. Animals may be thrombocytopenic, particularly if disseminated intravascular coagulation is imminent. Granular casts may be detected on urinalysis reflecting renal tubular damage as a result of the nephrotoxic nature of Hymenoptera venom. Acute renal failure can be caused by acute tubular necrosis (the result of hemolysis) or direct renal toxic effects of the venom.¹⁵ Dogs suffering multiple stings may develop a secondary immune-mediated hemolytic anemia.¹⁸ Animals suffering massive envenomations should be hospitalized and monitored closely. Clinical signs of insect stings are listed in Box 48-2.

Minimum database and confirmatory tests

Accurate diagnosis of Hymenoptera stings stems from a history of potential contact with stinging insects and the clinical signs displayed by the victim. It has been reported that certain dog breeds (bull terriers, Staffordshire terriers, and boxers) have a higher incidence of severe reactions to insect stings.¹⁵

The circumstances surrounding the sting episode may reveal clues as to the offending insect. Yellow jackets are attracted to and are frequently found near open sweet soft drink cans. Bees usually are not. Honeybees are more commonly found foraging among flowers. They are often stepped on as they work through clover. Honeybees can sting only once, leaving their stinger behind in the victim. Wasps and other vespids can sting multiple times and do not lose the stinger. Thus if the attacker is not seen or found, the presence or absence of a stinger, the conditions surrounding the sting incident, and knowing something of the different behavior between bees and wasps can be helpful in narrowing the index of suspicion and identifying the stinging insect.⁵

Other diagnostic aids include skin testing, isolation of specific IgE or IgG antibodies, assay for histamine release, and actual sting challenges. Most insect stings cause small, self-limiting local reactions and victims rarely are taken to veterinary hospitals for treatment. Dense fur may mask local clinical signs of Hymenoptera stings. The true incidence of insect stings in companion animals is unknown as many incidents probably go unrecognized.