Public Health Importance

Most encounters with venomous arthropods involve stings from ants, wasps, and bees. Probably several million people are stung annually by these insects; most of them do not require professional medical treatment. Some data on sting frequency are available on fire ants. Studies in the southeastern United States have indicated that 30-60% of people are stung by fire ants each year, but only 1-5% of these cases require medical treatment (Lofgren, 1986; Schmidt, 1986b). This rate is higher than that for all other hymenopterans. Stinging hymenopterans also pose a hazard during natural catastrophes such as forest fires and hurricanes. Insect stings, primarily due to yellowjackets, were the single most common cause of nonfatal injuries following Hurricane Hugo, which hit the southeastern coast of the United States in 1993 (Brewer et al., 1994).

A total of40—50 deaths due to Hymenoptera stings are usually reported in the United States each year (Schmidt, 1986c). However, it is suggested that these figures are very conservative and that a more accurate figure is probably closer to 200 (Akre and MacDonald, 1986). More people than this die each year as a result of allergic reactions to penicillin or from lightning strikes (Camazine, 1988). Honey bees are believed to cause about half of the annually reported human deaths due to hymenopteran stings in the United States (Schmidt, 1992). This figure may be somewhat misleading because the general public and medical profession do not reliably differentiate between honey bees and other types of bees and wasps. Some deaths attributed to bees probably are due to stings from yellowjackets and paper wasps.

The stings of most social hymenopterans cause intense pain to humans, with reactions to various species differing primarily in intensity or duration. Comparative scales for ranking the severity of pain caused by aculeate hymenopterans have been proposed by Starr (1985) and Schmidt (1986c, 1990b). The intensity of pain caused by most social bees and wasps is similar, and the responses to stings of only a few species, such as the fire and harvester ants, are diagnostic. Also, there exists great variation in responses among individuals to stings by the same species. The location of the sting on the body (e.g., finger vs neck) also may influence individual reactions.

Although most ant stings are painful, those of Parapon-em and Pogonomyrmex species are especially noteworthy. The large ponerine Pamponem clavata of Central and South America injects venom that produces intense, debilitating pain lasting several hours. This sting is considered to be the most painful of all Hymenoptera (Schmidt, 1986b, 1990b). The affected area can expand 20-30 cm within an hour of the sting. The pain induced by stings of P. clavata serves as a comparative standard for stings delivered by other stinging species (Starr, 1985). Although less severe than that from Pamponem, the pain caused by stings from harvester ants (Pogonomyrmex spp.) is extremely intense and can last up to several hours. It has been likened to "turning a screw" into the flesh around the sting site and also causing a sensation that has been described as "chilling." The sting is unique in that it induces piloerection (elevation of the hairs) and sweating around the sting site for 4—8 hr (Schmidt, 1986c). The venoms of some harvester ants are more toxic to mice than any other tested insect venom and 8—10 times more toxic than honey bee venom; they are almost as toxic per unit volume as the most venomous snakes (Schmidt and Blum, 1978).

Victims of fire ant stings usually experience a temporary burning sensation and discomfort, with some swelling around the sting site. In most cases a characteristic vesicle, 3—5 mm in diameter, containing a clear fluid develops within 6—24 hr at each sting site. The fluid becomes cloudy, forming a sterile white pustule (Fig. 19.28). These pustules usually disappear within a few days. They are replaced by a discolored lesion that results from tissue necrosis caused by the alkaloids in the venom

FIGURE 19.30 Yellowjacket (Vespula sp.) sting on human thigh, showing central puncture mark an<i puffy swelling at sting site, 10 min after sting. (Photo by E. J. Hansens.)

triggered by this immunological response. In some cases this can result in anaphylactic shock and death. Such lethai reactions have been reported for stings of vespines, paper wasps, honey bees, and fire ants.

Deaths from hymenopterous stings are usually due to respiratory failure (70%). Other reported causes of mortality are anaphylaxis (15%), cardiovascular collapse (9%), and neurological complications (6%) (Schmidt, 1986b). Death from stings is usually very rapid, often within 20 min of the sting. Nearly 60% of deaths occur in less than 1 hr following en venom ization. Therefore, it is imperative in cases of severe allergic responses that treatment be administered as soon as possible after the sting occurs. A few deaths attributed to toxic reactions have been reported in humans and animals receiving hundreds

FIGURE 19.31 Swelling of human foot as result of yellowjacket (VespuU sp.) sting. (Photo by R. D. Akre.)

Ants, Wnsps: and Bees (Hymenoptera)

to thousands of stings from the Africanized honey bee or yellowjackets. The victims in these cases usually are either young children or people restrained in some way hi close proximity to a disturbed colony.

Although relatively few people are killed by hy-menopteran stings, many people suffer some degree of sting hypersensitivity. Estimates of the incidence of hypersensitivity reactions, from mild to severe systemic responses, vary from 0.15 to 4% in the United States (Schmidt, 1992). An incidence of 2% in this case represents about 5 million people. Fewer than 1% of fire ant sting victims experience anaphylactic shock when stung, but this may be increasing as more people are stung by these aggressive ants. Vespid wasps cause twice as many allergic reactions as honey bees. However, the importance of the species involved differs with the region. In the United States, for example, yellowjackets cause more serious reactions than do honey bees in the Pacific Northwest and Washington, D.C. area. Paper wasps are more important in Texas and the southwestern states, whereas fire ants cause more allergic reactions and deaths than do honey bees in the southeastern states (Camazirte, 1988). Hypersensitive people often suffer great physical stress and illness, while others may experience apprehension and other psychological effects by not knowing when or how serious a subsequent sting might be (Reisman, 1994b).

A unique type of allergy has been associated with honey bees. Family members of beekeepers can develop an intense allergy to constituents of honey bee venom while laundering venom-impregnated garments. Most of these people do not realize they are allergic because they have had little or no direct contact with the bees themselves.

Several approaches can be taken in the treatment of sting victims. Embedded stings should be removed as soon as possible to lessen the sting reaction. Conventional advice on sting removal has emphasized that the sting apparatus should be scraped off, not pinched off, as the latter was thought to squeeze more venom into the sting site. Experimental evidence on honey bee stings contradicts this advice and emphasizes the importance of immediate removal of the embedded sting apparatus in order to minimize the sting reaction (Visscher et »/., 1996).

The sting site should be washed with soap and water to minimize the possibility of secondary infection. Apply ice packs or a paste made by mixing baking soda or meat tenderizer with water to reduce the amount of venom absorbed, pain, and local swelling. Baking soda apparently helps to neutralize the acidic components of the venom. Meat tenderizers generally contain a proteolytic enzyme (e.g., papain) that breaks down venom proteins. Other home remedies are wet salt, moistened tobacco, 10% ammonia solution, wet aspirin, and commercial sting relievers that help to reduce swelling and pain when applied to the sting sites (Weathersby, 1984). Plastering mud directly on the sting site also works well. Most of these remedies are thought to relieve the effects of the sting by osmotic removal of the venom. The more immediately they are applied, the more effective they are in reducing the intensity of the reaction. Antihistamines are helpful in lessening the swelling in mildly allergic reactions.

People with suspected allergic responses can be tested by means of a skin test involving subcutaneous injection of diluted venom, by a radioallergosorbent test that requires the taking of a blood sample to combine with the venom to detect reactions, or by a blood test using a histamine leukocyte release assay (Goddard, 1996; Piek, 1986a; Reisman, 1994b; Schmidt et al., 1984). All tests should be done in an allergy clinic where immediate medical treatment is available should adverse reactions to the tests occur.

If a serious systemic reaction occurs, a physician's help should be sought immediately. The prompt injection of epinephrine (adrenaline) is the initial step to combat a life-threatening anaphylactic reaction. Most physicians recommend that persons with demonstrated hypersensitivity wear an identification tag and carry a small emergency sting kit containing antihistamines and a syringe of epinephrine. Such kits are available with a doctor's prescription for a modest cost. Hypersensitive people at high risk of a fatal reaction should consider desensitization therapy. These individuals can be desensitized by a series of injections using attenuated doses of the appropriate venom. Such treatment gradually builds up the individual's tolerance to the venom and helps to prevent subsequent systemic reactions. Desensitization programs can be expensive and sometimes require many years.

Additional information about hymenopteran venoms, clinical aspects of stings, allergic reactions, and treatment of sting victims is provided by Piek (1986b), Schmidt (1992), Charpin et al. (1994), Levine and Lockey (1995), Meier (1995), and Mosbech (1995).

Comparatively little is known about the effects of hymenopteran stings on farm and game animals except for stings by fire ants. Imported fire ants Solenopsis invieta and S. riehteri commonly attack and kill newborn game animals such as rabbits, deer, quail, and other ground-nesting birds (Lofgren, 1986). Even the native fire ant in the United States, S. xyloni, is known to attack and kill newly hatched poultry. Deaths have been reported only rarely, however, among other newborn farm animals. A few accounts of animal deaths have been attributed to stings of the Africanized honey bee, usually involving dogs (Fig. 19.27) or livestock either restrained or enclosed near a hive. The German yellowjacket can injure the teats of milking cows by biting (Braverman et al., 1991), and the resulting lesions may be associated with outbreaks of mastitis in Israel (Shwimmer et al., 1995). Yellowjackets also have been observed cutting flesh from wounds of horses.

Few published data are available on allergic sting reactions in nonhuman animals. Severe systemic, allergic reactions to stings of bees and wasps have been reported in dogs. Fire ant stings in dogs do not develop the pustules so characteristic of humans and there have been no reports of anaphylaxis in dogs due to fire ant stings (Rakich et al., 1993). Some dogs, when stung on the face by bees or wasps, develop an enormous swelling of the head that may persist for a few days to a week.

Some ant species are intermediate hosts for parasitic helminths of vertebrates. Formica species are intermediate hosts for the lancet fluke (Dicrocoelium dendriticum), which infests the bile ducts of cattle, sheep, pigs, goats, horses, dogs, and, occasionally, humans. Pavement ant workers and Pheidole ants also serve as intermediate hosts of the poultry tapeworms Raillietina tetragona and R. echinobothrida (Harwood and James, 1979; Olsen, 1974).

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