exerpts from - A CALL FOR PARSIMONY IN SPIDER BITE DIAGNOSES
by:  Richard S. Vetter and P. Kirk Visscher
     Department of Entomology, University of California, Riverside, CA 92521 USA
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Although some spiders listed in the report can have deleterious effects, not all bites lead to serious symptoms, and most may in fact be unremarkable. Typically, spiders bite only in defense, only once, and the bite is rarely felt. Multiple "bites" or an initially painful bite would exonerate most spiders as probable culprits. There are many arthropods which actively seek to bite humans, and these seem more logical candidates for suspicion 115.

     In one study of 600 suspected "spider bite cases" in southern California, 80% were caused by other arthropods, mostly ticks and reduviid bugs 115,116. Loxosceles spiders have been erroneously blamed for a variety of necrotic wounds of unknown origin, which have often turned out to have other etiological agents (Table 1). It has been estimated that 60% of "alleged brown recluse bites" occur in areas where no Loxosceles spiders are known to exist 116. There are many   causative agents of necrotic wounds, many not arthropod in nature, which can be or have been confused for recluse spider bites 74,116,118-120. A call for greater parsimony in spider bite diagnoses has been made more than once  74,88,116,117,121 and excellent guidelines for "verified", "probable" and "possible" spider bite diagnoses have been put forth118.

      Table 1. Conditions which can cause necrotic wounds, have been misdiagnosed as or could be confused with "brown recluse spider bite" (from 73,115-117,120)
 

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)   

     Other causes of necrotic wounds are undoubtedly yet to be discovered. Some may turn out to be due to other spiders. In the late 1800's, some biologists scoffed at the idea that a tiny black widow spider could cause death 50,122, it wasn't until 1957 that the brown recluse spider was proven to be a cause of necrotic lesions in the USA 123, and the hobo spider was implicated in the last 10 years. Even the extremely poisonous Sydney funnelweb spider was not proven to be dangerous until 1927 124. These observations argue that accurate information from physicians can be invaluable in sleuthing out the causes of now-mysterious lesions, but the search for these causes is not helped by facile and unsupported diagnoses of "brown recluse bites".
     In rare events where a victim brings in the "smoking gun" of a spider that was observed to bite, or the crushed corpse inside clothing after feeling a pinch or bite, this information is of great value, and one should seek out a trained arachnologist or entomologist for accurate identification. Even badly damaged spider remains can be examined by an arachnologist if not to confirm an identification at least to preclude an erroneous one.

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