Arthropod bites and stings

The host reaction largely depends on whether the bite or the sting is defensive, toxic, or for feeding purposes. The highly visible, buzzing yellow bee or wasp stings primarily in defense of the hive. The venom causes rapid onset of severe pain for immediate negative feedback and defense.

The bite of spiders, on the other hand, contains toxins and proteolytic, digestive enzymes; thus spiders subjugate and then liquify their prey for food. Unfortunately, these venoms are often quite painful and destructive when injected into humans.

The bites of ectoparasites that require blood for nutrition or reproduction, such as the mosquito, are almost always painless and asymptomatic until allergic or irritant reactions take place. The salivary products are often allergenic and cause itching and redness.

The severity of bug bite reactions depends most on the immunologic status of the patient. In general, a severely symptomatic eruption may result from only a few bites in an immunologically sensitized person, while many bites may produce no symptoms at all in a person with an acquired immunologic tolerance.

In clinical settings and in experimental models, continuous exposure to biting arthropods will produce five stages of reactivity. Confusing clinical reactions may emerge because the immunologic reaction may range from nonreactive to severe and even tolerant, depending on the patient’s immune status.

The best treatment for stings is prevention. Exposure to areas of wildflowers, dandelions, or clover fields should be avoided. Always wear shoes. Brightly colored clothing, flowery designs, and the use of colognes, perfumes, and scented soaps should be minimized. When confronted by agitated bees or wasps, avoid rapid movements and either stand still or withdraw very slowly to prevent further aggravation.

The venom-containing barbed stinger, if still present at the site of the sting, should be removed by gently scraping the skin horizontally with a dull knife or credit card (Fig. 34-1). Stinger removal with forceps compresses the venom gland, forcing more venom into the skin, and should be avoided. Symptomatic care with rest, elevation, and ice to the area are helpful. Antihistamines may also be useful. Early signs of systemic toxicity or allergic reactions should be noted.

The fire ant, accidentally introduced into the United States in the 1930s from Brazil, now occupies most of the Southeast, limited only by cold winter weather in the north and the deserts of the Southwest. When the ants are provoked, they may attack en masse, administering up to several thousand stings to a victim. The sting is actually initiated by the ant biting the flesh and then pivoting slightly and repeatedly stinging the victim to form an annular eruption of sterile pustules and sting reaction sites. The sting site develops a large wheal and flare punctuated by a small sterile pustule, which ulcerates and potentially scars. The large erythematous wheal will then gradually desquamate over the next couple of days, and the sting site resolves normally. The unusual fire ant venom, a piperidine alkaloid, has become a leading cause of anaphylactic reaction to venomous animals in the United States. Treatment of the ant stings is symptomatic. Select patients with severe local or generalized sting reactions may be considered for immunotherapy.

Nguyen SA, Napoli DC: Natural history of large local and generalized cutaneous reactions to imported fire ant stings in children, Ann Allergy Asthma Immunol 94:387–390, 2005.

The brown recluse (Loxosceles reclusa) and the female black widow (Latrodectus mactans) (Fig. 34-2) are the most serious spiders in the United States. The hobo spider (Tegenaria agrestis) in the Pacific Northwest has been reported to produce dermonecrotic arachnidism, a serious necrotic reaction in the skin induced by the venom. However, whether or not this spider produces dermonecrotic arachnidism is controversial; some authorities believe that the reported bites have come from other causes. As many as 16 other species may bite humans, causing a small amount of pain or even necrosis at the site, but these reactions are transient, self-limiting, and require little attention. Most species are either too small or the biting mouthparts will not penetrate the human skin and, therefore, inject an insignificant amount of venom.

The female black widow spider is 10 to 15 mm long and has a black, globose abdomen, often with a red hourglass figure on the ventral surface. This spider is found in all 48 contiguous states. The venom is a potent neurotoxin.

Initially, the bite may sting slightly and then begins to burn. The skin changes are usually minimal, but the systemic symptoms of pain and muscle cramping may become intense. Diaphoresis, dizziness, anxiety, salivation, and other neurotoxic symptoms may ensue. Treatment of this neurotoxin should be prompt. Antisera are available and afford prompt relief, especially in the very young, old, or those with a history of heart disease or hypertension. Calcium gluconate, muscle relaxants, and pain medications are also used.

Elston DM: What’s eating you? Latrodectus mactans (the black widow spider), Cutis 69:257–258, 2002.