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.
Of the 13 species of Loxosceles in the United States, at least 5 have been associated with necrotic arachnidism. Loxosceles reclusa, or the brown recluse spider, is the spider most commonly responsible for this injury.
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.
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.
Brown recluse spider bites can cause significant cutaneous injury with tissue loss and necrosis. Less frequently, more severe reactions develop, including systemic hemolysis, coagulopathy, renal failure, and, rarely, death.
Brown recluse venom, like many of the other brown spider venoms, is cytotoxic and hemolytic. It contains at least 8 components, including enzymes such as hyaluronidase, deoxyribonuclease, ribonuclease, alkaline phosphatase, and lipase. Sphingomyelinase D is thought to be the protein component responsible for most of the tissue destruction and hemolysis caused by brown recluse spider envenomation. The intense inflammatory response mediated by arachidonic acid, prostaglandins, and chemotactic infiltration of neutrophils is amplified further by an intrinsic vascular cascade involving the mediator C-reactive protein and complement activation. Laboratory studies have shown a decrease in hemolysis from brown recluse venom in the presence of complement inhibitors.  These and other factors contribute to the local and systemic reactions of necrotic arachnidism.
Although numerous cases of cutaneous and viscerocutaneous reactions have been attributed to spiders of the genus Loxosceles, confirming the identity of the envenomating arachnid is difficult and rarely accomplished.
Although various species of Loxosceles are found throughout the world, L reclusa is found in the United States from the East to the West Coast, with predominance in the south. Recently, reports of persons with "spider bites" presenting to emergency departments have reached near urban legend proportions, prompting many physicians to question the diagnosis of a brown recluse bite in nonendemic areas. [2, 3, 4] The list of conditions that can present in a similar fashion to that of a brown recluse spider envenomation is extensive. A more likely explanation for this epidemic of spider bites is in fact community-acquired methicillin-resistant Staphylococcus aureus (MRSA) skin infections.  The 2014 Annual Report of the American Association of Poison Control Centers recorded 1330 individual exposures to brown recluse spiders that year. 
Systemic involvement, although uncommon, occurs more frequently in children than in adults. 
See the distribution map below.
Data regarding mortality rates are not reliable because diagnostic tests to detect brown recluse venom in tissue are not readily available.
Although deaths have been attributed to presumed brown recluse envenomation, severe outcomes are rare.  Typical cases involve only local soft tissue destruction. The 2014 Annual Report of the American Association of Poison Control Centers recorded 275 minor outcomes, 218 moderate outcomes, 11 major outcomes, and no deaths. 
In South America, the more potent venom of the species Loxosceles laeta is responsible for several deaths each year.
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