Venomous Spiders of the United States

The United States has five groups of spiders that can cause serious injury. The Black Widow and Yellow Sac spider are found throughout the country, although the Yellow Sac’s range has yet to be mapped precisely.
The Hobo Spider has expanded its range in the Pacific Northwest, while the Brown Recluse is found in the South and lower Midwest. Other Recluses are found in the Southwest.

Table 1. Conditions which can cause necrotic wounds, have been misdiagnosed as or could be confused with "brown recluse spider bite" (International Journal of Dermatology, volume 37, pages 73,115-117,120, July 1998)

erythema chronicum migrans (Lyme disease)	Stevens-Johnson syndrome
toxic epidermal necrolysis (Lyells syndrome)	G.C. arthritis dermatitis
erythema multiforme	infected herpes simplex
erythema nodosum	chronic herpes simplex
purpura fulminans	diabetic ulcer
bed sores	poison ivy/oak infection
focal vasiculitis	thromboembolic phenomena
periarteritis nodosa	lymphomatoid papulosis
pyoderma gangrenosum	sporotrichosis
warfarin poisoning	Rocky Mountain Spotted Fever
bite of the pajahuello (Ornithodoros coriaceus, an Argasid tick)