Throughout the United States, dermonecrotic wounds of uncertain cause are often attributed to the brown recluse spider, Loxosceles reclusa. Many such diagnoses occur in parts of the country where the spider is not native and its populations are not known. The brown recluse has a fairly limited range (see map, Figure 161-2). Even inside endemic areas, these bites are very uncommon. Unfortunately, this spider has gotten a lot of press and has thereby inspired a lot of fear in the general population.

Figure 161-2 Reported distribution of Loxosceles species in the United States. (Adapted from Furbee RB, Kao LW, Ibrahim D: Brown recluse spider envenomation. Clin Lab Med 26:211-226, 2006.)

Necrotic wounds can be caused by many different factors. Today, patients frequently present with CA-MRSA skin infections that they have self-diagnosed as a spider bite. Misdiagnosing a necrotic wound as a brown recluse spider bite can lead to delays in appropriate care, with adverse outcomes. In general, spider bites have been overdiagnosed as the cause of necrotic lesions. In addition to CA-MRSA, other important misdiagnosed conditions include neoplasms, Lyme disease (see Chapter 180), cutaneous anthrax, gonococcemia, and perhaps the worst case scenario—necrotizing fasciitis.

Corroborative evidence should be sought before attributing the cause to a spider or other arthropod bite. Although general wound care may be sufficient for most similar wounds, it will be ineffective for conditions such as CA-MRSA, neoplasms, Lyme disease, cutaneous anthrax, gonococcemia, and necrotizing fasciitis, in which case a delay in treatment can have grave consequences.

Loxosceles reclusa has several distinguishing characteristics that may not be visible without magnification. The dark-colored violin-shaped markings on the dorsal aspect of the cephalothorax (hence the name “fiddleback”) (Figure 161-3) may not always be visible because of the variable color of the spider. A helpful identifying feature is the presence of six eyes arranged in three pairs (dyads) as opposed to the more common eight eyes found in most spiders.

Figure 161-3 L. reclusa displays classic violin markings on the cephalothorax. (Adapted from Furbee RB, Kao LW, Ibrahim D: Brown recluse spider envenomation. Clin Lab Med 26:211-226, 2006.)

The current mainstay of therapy is supportive care. Even in the rare case of a confirmed bite, treatment should be expectant. Despite multiple trials, early surgical excision, electric shock, steroids, hyperbaric oxygen therapy, colchicine, antihistamines, vasodilator drugs, anticoagulants, prophylactic antibiotics, and dapsone remain unproven therapies for brown recluse envenomation. All have variable degrees of risk. Antivenin and specific Fab fragments have been shown to be of some benefit but are not available commercially. Furthermore, given the inaccuracy of brown recluse bite diagnosis, much better documentation of the cause than is usually available would be required before assuming the risk of administration of these agents.

In areas of high Loxosceles density, the public should be educated about avoidance of bites (inspecting clothing and bed linen) and should be reminded to bring the suspected organism for identification, even if crushed. Should a person find a spider on himself or herself, it should be brushed off, not crushed.

Like the brown recluse, the maligned black widow spider (most commonly Latrodectus mactans) is a shy creature that bites only when provoked. Most of the 26 species of widow spiders are jet black and often can be identified from their characteristic red “hourglass” marking on the undersides of their abdomens. Because webs can be found around outdoor toilet seats, bites may occur on or near the genitalia. These bites may be painful but are usually associated with only mild dermatologic manifestations. Black widow venom causes depletion of acetylcholine at motor nerve endings and release of catecholamine at adrenergic nerve endings. Consequently, black widow bites may produce agonizing abdominal pain and muscle spasm, which may mimic acute abdomen. Other signs and symptoms include headache, paresthesias, nausea, vomiting, hypertension, and sometimes paralysis; fortunately, death is not common. Black widow bites may be misdiagnosed as drug withdrawal, appendicitis, meningitis, or tetanus, to name a few conditions. Treatment is supportive, with intravenous opioids for the pain and a muscle relaxant for the spasms. Wound care is usually minimal. At one time, calcium gluconate was recommended, but it has now been shown to be ineffective.

Equine-derived black widow antivenin is the only nonsnake antivenin approved in the United States. It is highly effective, even when given late. However, it has also been reported to cause hypersensitivity reactions. In one case series of 163 bite patients, 1 of the 58 treated patients died of bronchospasm. Therefore some experts will never use antivenin, whereas others may give it only for severe cases, when the patient is very old or very young.

With the increasing trend toward having exotic pets, a practitioner may find a patient presenting with a tarantula bite. While dramatic in size and appearance, these patient spiders are not likely to bite, and their envenomation is generally no worse than that of a hymenoptera sting. Several species do have irritant hairs on their abdomen, which they may fling outward, causing cutaneous or ophthalmic irritation.