In the United States, reports of severe envenomations by brown spiders began to appear in the late 1800s, and today, in endemic areas, brown spiders continue to be of significant clinical concern. See the current distribution map below.
Complete distribution range of wild and domestic Loxosceles reclusa (brown recluse spider). Courtesy of Wikimedia Commons (By ReliefUSA_map.gif: Public domain, U.S. government derivative work: Bob the Wikipedian).
Of the 13 species of Loxosceles in the United States, at least five have been associated with necrotic arachnidism. Loxosceles reclusa, or the brown recluse spider, is the spider most commonly responsible for this injury.
Typical appearance of a male brown recluse spider. Photo contributed by Michael Cardwell, Victorville, Calif.
Spider envenomations, brown recluse. Close-up image of dorsal violin-shaped pattern. Photo contributed by Michael Cardwell, Victorville, Calif.
Dermonecrotic arachnidism refers to the local skin and tissue injury noted with this envenomation. Loxoscelism is the term used to describe the systemic clinical syndrome caused by envenomation from the brown spiders.
Dermonecrotic arachnidism represents a local cutaneous injury with tissue loss and necrosis.
See Arthropod Envenomation: From Benign Bites to Serious Stings and Venomous Spider Bites: Keys to Diagnosis and Treatment, Critical Images slideshows, for help identifying and treating various envenomations.