Necrotic Skin Lesions: Spider Bite—or Something Else?

We use the term "idiopathic dermonecrotic skin lesion" for lesions ascribed to spider bites that defy easy diagnosis. Empiric antibiotic therapy is often indicated because many of these lesions are, in fact, caused by bacterial infection.^15,16Patients whose condition does not improve with this treatment, or whose lesions are not consistent with infection, may require further workup. Dermatology referral may be indicated in some cases.

Bacterial cellulitis. Most patients with alleged spider bites have bacterial cellulitis, which is often caused by community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA).^17-21 This bacterium is an emerging cause of skin and soft issue infections worldwide.

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The classic CA-MRSA lesion appears as an area of cellulitis around a darker purpuric center with overlying dermonecrosis (Figure 7). The central necrosis of the CA-MRSA lesion, unlike that of the brown recluse spider bite, is usually slightly raised from the surrounding skin, because of edema or an underlying abscess. Patients with a CA-MRSA infection who claim to have a spider bite rarely report feeling a bite or seeing a spider and often admit that a friend or family member suggested this was the cause.

 CA-MRSA strains are generally resistant to the b-lactam drugs (eg, oral cephalexin) used to treat skin and soft tissue infections. Effective antibiotic choices for CA-MRSA include vancomycin(Drug information on vancomycin), daptomycin, tigecycline, and linezolid, when intravenous administration is indicated, or clindamycin(Drug information on clindamycin), doxycycline, linezolid(Drug information on linezolid), minocycline, and trimethoprim(Drug information on trimethoprim)/sulfamethoxazole (TMP/SMX), when oral therapy will suffice.²²

Many of these drugs choices are rather expensive. In our experience, most patients with these lesions—frequently, parenteral drug abusers and recent prison inmates and their close contacts--have very limited financial resources. Fortunately, nearly all CA-MRSA strains reported to date are susceptible to TMP/SMX and clindamycin, which are relatively inexpensive options. Clindamycin, however, is not used as empiric monotherapy because many strains have inducible clindamycin resistance.

When outpatient oral therapy is deemed adequate, we treat presumed CA-MRSA infection with 2 drugs: TMP/SMX (1 double-strength tablet twice daily) and either clindamycin (300 mg 4 times daily) or doxycycline(Drug information on doxycycline) (100 mg bid). Treatment duration is 7 to 10 days, and generic formulations can be prescribed to reduce costs. Incision and drainage of any concurrent abscesses is also indicated.

EXNOERATING THE OFT-ACCUSED BROWN RECLUSE
The brown recluse—the spider most commonly implicated in alleged bites—lives and breeds predominantly in the south central United States. Yet complaints of brown recluse spider bites are ubiquitous.^10-12 In one study, a total of 216 cases of brown recluse spider bites were diagnosed in California, Oregon, Washington, and Colorado during a 41-month period, yet only 17 Loxosceles reclusa specimens have ever been verified in these states.¹⁰ Because public reporting of spider bite diagnoses is not required, the number of misdiagnoses noted in this study is likely a gross underestimate of the actual total.

When confronted with evidence of geographic improbability, patients with an alleged brown recluse spider bite commonly respond that the spider was recently imported from an area where it is endemic.¹³ In California, a common assumption is that the bite was caused by a cousin of the brown recluse spider (Loxosceles deserta). Maps of the number of reported spider bite cases in the United States indicate that the highest number of cases are in the most densely populated areas, irrespective of the spider's confirmed habitat.¹⁰ These data suggest that the larger the population of a given area, the higher the incidence of dermo-necrotic lesions of unknown origin, which may be attributed to spider bites.

CLINICAL HIGHLIGHTS
•  Persons who have been bitten by a brown recluse spider have pruritus, burning pain, erythema, and swelling at the bite site within 2 to 6 hours. Over the next several hours or days, a hemorrhagic vesicle appears that ulcerates. Progression of the lesion results in a central bluish necrotic depression with surrounding erythema.

• Treat brown recluse spider bites with standard local wound care. Consider antibiotic therapy only if secondary infection develops.

• The classic lesion that appears after a black widow spider bite consists of a small pallid area surrounded by a rim of erythema. Associated pain can be severe. The signs and symptoms of black widow spider envenomation typically wax and wane over 24 to 48 hours and can last several days.

• For patients with black widow spider bites, intravenous opiate analgesia, with or without benzodiazepines, is usually sufficient. Antivenin greatly shortens the duration of symptoms and prevents their recurrence; however, it is associated with a risk of allergic reactions.

• Consider empiric antibiotic therapy for idiopathic dermonecrotic skin lesions because many of these lesions are caused by bacterial infection. If the patient’s condition does not respond to antibiotics, further workup or referral to a dermatologist may be warranted.