Question of the Week
July 18, 2011
How do I tell the difference between cutaneous anthrax and a brown recluse spider bite?
Even though it has not been in the news lately, acts of bioterrorism could still occur and clinicians must maintain awareness for biologic diseases such as anthrax. The majority of naturally occurring cases of anthrax is the cutaneous form. A spore from Bacillus anthracis contaminates an area of unintact skin, usually on the face, neck or extremities. The lesion starts as a painless papule that may be pruritic and then develops a vesicle with possible satellite vesicles. Edema surrounds the lesion. The vesicle eventually ruptures and develops a black eschar (hence the derivation of the species name from the Greek work for coal, anthrakis).
Since Tennessee is endemic for Loxosceles reclusa, necrotic arachnidism may occur in patients. Brown recluse spider bites may initially have an area of erythema that is pruritic followed by the development of an extremely painful area which is usually purplish in color, surrounded by grayish, ischemic looking tissue, surrounded by an erythematous rim. Vesicles or bullae that are clear or hemorrhagic may develop at the bite site. This tissue eventually mummifies and develops a black eschar.
So the key difference: pain. Cutaneous anthrax is painless despite the appearance of the wound. Brown recluse spider bites are quite painful.
A good review on the recognition and management of anthrax is the New England Journal of Medicine, 2001:345:1621-1626 (correction of a Table is listed in N Engl J Med 2002;346:634.)
This question prepared by: Saralyn Williams, MD Medical Toxicologist