Arthropod Bites and Stings at a Glance
- Many arthropod species are capable of inflicting bites and stings on humans.
- Reactions to arthropod assaults can range from mild to life threatening.
- Arthropods also serve as vectors for systemic diseases.
- The terrestrial arthropods of medical importance include the orders Arachnida (arachnids), Chilopoda (centipedes), Diplopoda (millipedes), and Insecta (insects).
Arthropod bites and stings are a significant cause of morbidity worldwide. Although many arthropod attacks produce only mild, transient cutaneous changes, more severe local and systemic sequelae can occur, including potentially fatal toxic and anaphylactic reactions. Arthropods also serve as vectors for numerous systemic diseases. The medically significant classes of nonaquatic arthropods are Arachnida, Chilopoda, Diplopoda, and Insecta (Fig. 210-1).1
Pictorial key to groups of human ectoparasites. (Used with permission from C.J. Stojanovich and H.G. Scott, US Department of Health, Education, and Welfare, Public Health Service.)
Many arthropod bites produce a similar histologic reaction pattern. In the acute phase, there is a superficial and deep, perivascular and interstitial inflammatory infiltrate, which is characteristically wedge shaped. The infiltrate is usually mixed in composition with an abundance of lymphocytes and eosinophils, although neutrophils and histiocytes can also be seen. Neutrophils may predominate in reactions to fleas, mosquitoes, fire ants, and brown recluse spiders. Over the most prominent superficial infiltrates, spongiosis can be seen, sometimes with progression to vesicle formation or epidermal necrosis. Older, excoriated areas are usually altered by the effects of scratching, with the development of parakeratosis, serum exudates, and a dermal infiltrate with neutrophils and more abundant lymphocytes. Although not commonly seen on histology, insects or insect parts, including burrowed scabies mites, eggs, feces, or the retained mouthparts of ticks, may be visible. Chronic lesions, which most often result when arthropod parts are retained in the skin, may have a pseudolymphomatous appearance (see Chapter 146).
The morbidity from arthropod bites and stings varies with the species inflicting the injury. Although species-specific clinical findings and treatment will be discussed in further detail, there are several applicable general treatment principles. Local wound care is essential following arthropod assault. Wounds should be cleansed; any remaining arthropod parts, including stingers, should be removed expeditiously. Patient discomfort should be addressed and can involve a variety of treatment modalities, including the use of ice packs, application of topical corticosteroids and antipruritics, injection of local anesthetics, and, less frequently, the use of systemic analgesics. Supportive measures for systemic toxic and allergic reactions, including anaphylaxis, should be instituted when necessary. Secondary infection should be treated with appropriate antibiotics. Bites from several terrestrial arthropods species may require tetanus prophylaxis. Severe envenomation from particular species, such as the black widow spider, may require antivenom administration. Documented hypersensitivity to some species can be treated with desensitization immunotherapy. Awareness of the potential arthropod-borne illnesses spread by each species is also ...