Loxoscelism
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Loxoscelism
A small percentage of the spider species in the world are medically important with the majority causing systemic injury. Spiders of the genus Loxosceles cause necrotic dermatologic injury through a unique enzyme, sphingomyelinase D, found only in one other spider genus and several bacteria.1 The first inklings of association of spiders in general with dermonecrosis were made in the late 19th century in the Western Hemisphere.2, 3 In the early 20th century, suspected association specifically to
Distribution of Loxosceles
There are 11 native and two nonnative species of Loxosceles spiders in North America.6, 7 The brown recluse spider, Loxosceles reclusa, (Fig. 1) is the best known and is responsible for most American envenomations. In subtropical North America, the brown recluse can be found from Texas to northern Georgia. The anecdotal opinion of southeastern American arachnologists, which was corroborated by a recent distribution study,8 is that as one gets closer to the Gulf of Mexico, Loxosceles specimens
Identification of Loxosceles spiders
Many medical publications definitively state that Loxosceles spiders can easily be identified by a violin or fiddle pattern on the dorsal surface of the cephalothorax. Although this is true in principle, lack of expertise causes nonarachnologists (including physicians) to mistake many varied darkened forms on spider bodies as violin patterns.8, 17 Although several Loxosceles species can indeed be readily identified by a violin, using this overly simplified diagnostic identifier will lead to
Other spiders purported to cause necrosis
Over several decades, many additional spiders from a diverse number of spider genera have been implicated in dermonecrosis including orbweavers, wolf spiders, funnel-weaving spiders, etc. Much of this has occurred by the simple implication of spiders by patients presenting with dermonecrosis, but while lacking the definitive proof of a spider caught in the act of biting and properly identified. Many of these events have been reported under the blanket term of necrotic arachnidism, and,
Loxosceles toxin
Loxosceles bites can cause necrosis in humans, guinea pigs, and rabbits,28 but not in mice and rats,29 thereby showing differential mammalian toxicity due to unknown reason. Research has recently characterized the nature and composition of the toxins causing the necrosis. The toxins are the same in constituency in male and female Loxosceles, but the females have more concentrated venom—as much as two times the concentration of that of the male spider.30 Fifteen micrograms of venom will produce
Pathophysiology
In milder cases, Loxosceles bites may simply cause a very mild urticarial reaction.51 In the more severe cases, the initial bite is painless,13 but this is followed over 2 to 8 hours by sharp, penetrating pain that progressively changes into a burning sensation.3, 13 There may be two small puncta at the bite site (Fig. 4).13 The bite area pales52 and the area immediately surrounding the bite becomes red and edematous, with mild to severe pain secondary to vasospasm and ischemia.13 A blister may
Diagnosis
The standard for diagnosis of spider bites is collecting and properly identifying the biting spider responsible for the cutaneous findings.
Other diseases are frequently misdiagnosed as loxoscelism.65 These include staphylococcal or streptococcal infection, herpes simplex, herpes zoster, diabetic ulcer, fungal infection, pyoderma gangrenosum, lymphomatoid papulosis, chemical burn, Toxicodendron dermatitis, squamous cell carcinoma, neoplasia, localized vasculitis, syphilis, Stevens-Johnson
Laboratory diagnosis
The diagnosis of loxoscelism remains a clinical judgment dependent on proof of a Loxosceles spider bite. There is at least one potential venom test that has been developed and tested experimentally, using an ELISA.52 The authors detected venom from rabbit test subjects up to 7 days after injection. Venom was recoverable from plucked hair and skin aspirates, but the greatest yield was with biopsy specimens. There is currently no commercially available assay for humans.
Therapy
The proper treatment of loxoscelism remains controversial. Reported therapies include hyperbaric oxygen, dapsone,52 antihistamines including cyproheptadine,52 antibiotics, dextran, glucocorticosteroids,73 vasodilators,74 heparin, nitroglycerin, electric shock, curettage, surgical excision, and antivenin.52 It seems reasonable at minimum to provide routine first aid: elevation, immobilization, application of ice, local wound care, and tetanus prophylaxis.
Dapsone has been recommended for over two
Conclusions
Loxosceles spiders are the only important cause of skin necrosis from spider bites. Loxosceles spiders are common in much of the tropical and temperate world. Considering how prevalent they are, human bites from Loxosceles spiders are uncommon. When bites occur, they are usually mild, but they can occasionally cause ulceration and rarely systemic symptoms. Other more common diseases mimic spider bites, and, therefore, unless the culprit Loxosceles spider is caught in the act, practitioners must
References (94)
- et al.
The phylogenetic distribution of sphingomyelinase D activity in venoms of haplogyne spiders
Comp Biochem Physiol B Biochem Mol Biol
(2003) Loxoscelism
Am J Med Sci
(1992)- et al.
Wolf spider bites in Sao Paulo, Brazil: a clinical and epidemiological study of 515 cases
Toxicon
(1990) - et al.
Do hobo spider bites cause dermonecrotic injuries?
Ann Emerg Med
(2004) - et al.
Survey for potentially necrotizing spider venoms, with special emphasis on Cheiracanthium mildei
Comp Biochem Physiol C Toxicol Pharmacol
(2005) Letter to the editor
Toxicon
(2001)- et al.
Brown spiders and loxoscelism
Toxicon
(2004) - et al.
Variations in Loxosceles spider venom composition and toxicity contribute to the severity of envenomation
Toxicon
(2005) Preliminary studies on venom from brown recluse spider, Loxosceles reclusa
Toxicon
(1969)- et al.
The venom optimisation hypothesis: a spider injects large venom quantities only into difficult prey types
Toxicon
(2002)