Table 9.

Differentiating cutaneous anthrax from spider bites

Feature	Anthrax	Spider Bitea
Onset	Usually just appears, patient notices lesion and edema; particularly in the event of bioterrorism, the patient is unaware of a precipitating event	Sudden onset associated with pain. Often the patient reports seeing a spider in the area or the event occurred when dressing in the morning
Pain	Lesion is painless	Significant pain particularly at onset
Lesion	Regular and well-demarcated lesion that has raised borders and an area of central necrosis	Irregular and poorly demarcated lesion with red, white, and blue sign of the periphery of the lesion vasodilated (red) with an area of vasoconstriction (white) immediately surrounding the necrotic region (blue)
Edema	Significant	Minimal

^aLoxosceles species of spiders.