It is estimated that nearly 40,000 unique species of spiders exist worldwide. However, despite the large number of spiders, human toxicity is rare for several reasons. First, spiders are frequently too small to be able to inject venom through human skin. Second, the small quantity of venom in most spiders, along with the lack of physiologic activity between most spider venom and humans, also contributes to toxicity being somewhat rare.2
In the absence of a clear history of a spider bite, it is imperative that the clinician consider alternative etiologies, as numerous medical conditions are frequently misdiagnosed as spider bites. Some conditions that have been incorrectly diagnosed as loxoscelism include various infections (bacterial, fungal, parasitic, and viral), necrotizing vasculitis, vascular occlusive disease, and a host of various systemic diseases with cutaneous manifestations (eg, pyoderma gangrenosum, pemphigus).2
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