Spiders are carnivores and use venom to paralyze their prey. They are generally not a threat to humans as their fangs are too small to penetrate human skin, and the amount of venom injected is too small to produce toxicity. Thus, reactions resulting from a spider bite are typically limited to a localized reaction. Fortunately, most bites only require supportive medical therapy.
Loxosceles
Loxosceles are present worldwide, but L reclusa (the “brown recluse spider”) accounts for a significant number of envenomations in the United States. The AAPCC’s 2012 data notes 1,365 cases of exposure to the brown recluse spider with 510 of those victims seeking medical care.3 In many instances, clinicians attribute necrotic bites to the brown recluse spider, however, confirmation is often lacking. Loxosceles are nocturnal, and they are found both indoors and outdoors—mostly in dark and dry areas such as basements, closets, and woodpiles. These spiders are shy, but may bite when threatened. Their venom contains enzymes, including hyaluronidase and sphingomyelinase. Though rare, wounds can become necrotic due to neutrophil activation, platelet aggregation, and thrombosis.25 The most common reaction to a Loxosceles bite is a mild painless erythematous lesion that becomes firm and generally heals over several days to weeks. In severe reactions, erythema, edema, and pruritus initially develop, followed within 24 to 72 hours by a hemorrhagic bulla surrounded by blanched skin. This leads to the “red, white, and blue sign” (ie, erythema, blanching, and ecchymosis). Infrequently, the ecchymotic area becomes necrotic and ulcerates in 3 to 5 days. The differential diagnoses should include necrotizing fasciitis, erythema chronicum migrans (from Borrelia-infected tick bites), and anthrax. Ulcerated lesions may result in significant cosmetic defect. Healing may take up to 2 weeks, and skin grafting is occasionally required.26
Treatment begins with the usual supportive measures, including analgesia, ice, elevation, and a light compression dressing. Antibiotics are not indicated, unless there are signs of secondary infection. Serial evaluation for wound checks should be arranged. If ulceration develops, surgical debridement may be required. The vast majority of bites heal with supportive care alone, and aggressive medical therapy is usually not warranted.27Patients with systemic manifestations should be admitted to the hospital for further care. There is no evidence-based literature to guide therapy. Many therapies have been tried with variable results and there remains no definitive standard of care.
Treatment regimens include antihistamines, antivenin, colchicine, dapsone, hyperbaric oxygen, cyproheptadine, surgical excision, and steroids.28 Dapsone continues to be widely advocated worldwide despite its known adverse effects—most notably hemolysis and methemoglobinemia. Antivenin administration has shown some promise in animal models, but its efficacy in humans is still unclear.29
Tegenaria
The Tegenaria agrestis or hobo spider is a native of Europe and central Asia and is only found in the northwest part of the United States. It is considered aggressive and tends to bite even with only mild provocation. The clinical presentation, inclusive of systemic reactions, is similar to that of the brown recluse spider. Similarly, there is no proven treatment. Surgical wound resection and skin grafting should be considered and is at times required.
Latrodectus.
Latrodectus, also known as widow spiders, are found worldwide. Five species are commonly found in the United States, but the black widow is the most well known. Only three of the species are actually black. Other varieties are typically brown or red. However, almost all Latrodectus spiders have a characteristic orange-red hourglass-shaped marking (Figure 3). Widow spiders aggressively defend their webs, and are most often found in woodpiles, basements, garages, and sheds. Most bites occur in the warmer months, between April and October.
