Spider Bites, Spider Myths

I am concerned about Dr Benjamin Barankin's recent "Photoclinic" case of an alleged brown recluse spider bite (CONSULTANT, August 2005, page 1038) and the subsequent "Consultations & Comments" discussion of the possibility that it might have been a hobo spider bite instead (CONSULTANT, October 2005, page 1242). I fear that this case and the ensuing discussion are promulgating the commonly held myth that otherwise unexplained skin lesions are often caused by spider bites.^1-3

Spider bite diagnosis requires hard evidence. I am a medical toxicologist with a particular interest in spider bites and lesions blamed on such bites. I am not at all convinced by the patient's clinical history that she had been bitten by a spider--brown recluse (Figure) or otherwise. Simply visiting an area where the brown recluse (Loxosceles reclusa) or hobo spider (Tegenaria agrestis) is indigenous, or entering a room where spiders have previously been observed, is the poorest kind of circumstantial evidence. Did the patient actually observe a spider on her body, or feel a bite, or recover the offending spider for positive identification? If not, then blaming this lesion on a spider bite is pure conjecture.

Differential diagnosis of dermonecrotic lesions. Certainly, a dermonecrotic lesion (or rather, a group of clustered necrotic skin lesions) developed in this patient. Nevertheless, the differential diagnosis of diseases that cause dermonecrotic lesions is extensive and includes various bacterial, viral, and fungal infections; primary dermatitides; vasculitides; thromboembolic phenomena; adverse drug reactions; diabetic and artifactual ulcers; skin cancer; the bites of other arthropods or animals; and even chemical burns. Skin and soft tissue infections with community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) are currently some of the most common causes of lesions attributed to "spider bites."⁴

The lesions of true cutaneous loxoscelism are typically solitary; however, Dr Barankin's patient had multiple lesions. The clustering of these lesions around a healed abdominal scar raises the possibility of a surgical wound complication, such as pyoderma gangrenosum.

In the absence of an eyewitness history of a spider bite, development of a typical lesion, wound culture results, or skin biopsy, I am highly skeptical that this patient was bitten by a spider. Moreover, since the treatment of dermonecrosis from spider bites consists almost entirely of supportive wound care (and occasionally antibiotics if the wound becomes infected), attributing the wound to a spider bite does not change therapy and therefore serves no clinically useful purpose. Most lesions blamed on spiders, even in areas in which the brown recluse is endemic, have on further evaluation been attributed to alternative causes.^5,6

An etymologic accident. In addition, I am concerned about invoking the alternate name for the hobo spider--"aggressive house spider." This common misnomer promotes misleading ideas about the spider's behavior. The hobo spider's species name (agrestis) comes from a Greek word that means "rural" or "of the land, fields, or country"; the words "agriculture" and "agrarian" have the same root. The English word "aggressive," on the other hand, is derived from the Latin word aggressus and not from Greek. Thus, the hobo spider is inappropriately called "aggressive" only because of the coincidence of similar-sounding yet disparate word roots and common popular myth, not because it actually seeks out humans to bite. T agrestis is native to Europe, where it is regarded as essentially harmless to humans and does not have a reputation for causing necrotic skin lesions. It is quite possible that the American hobo spider's reputation for causing necrotic arachnidism is based on misdiagnoses like those that gave the brown recluse spider such an undeserved bad name.