While all spiders are harmful to their prey, few are dangerous to human beings. Most spiders are small and their fangs are too short to penetrate human skin. Of the 50 species of spiders in the USA that have been known to bite humans, only the genera of Loxosceles, Tegenaria, and Latrodectus have venom that produces significant toxic effect in humans.
Spiders of the genus Loxosceles have earned the names "fiddle back spider” and "violin spider” because of a dark brown marking in the shape of a violin on the head and thorax. The brown recluse (Loxosceles reclusa) is a nonaggressive spider and will seek out shelter in undisturbed places, such as attics and closets. Bites of the brown recluse spider vary from mild, local redness to severe ulcerative necrosis with eschar formation. The bite often appears as a central blister with mottling and a blanched halo with surrounding redness and swelling. Current treatment includes cleansing the bite site and applying cold compresses, mild analgesics to control pain, and possible antibiotics after the bite for the treatment of secondary infection. Warm compresses and strenuous exercise are to be avoided, until the ulceration is healed.
Tegenaria agrestis, the hobo spider, has local effects similar to those caused by the brown recluse spider. These spiders, which build funnel-shaped webs in crawl spaces, basements, and wood piles, are brown with a gray herringbone pattern on the abdomen. The initial bite is often painless and the site usually becomes hard within 30 minutes. A red, swollen area may develop around the site; it can be 15 centimeters in diameter. Blisters often develop during the first 36 hours and may rupture producing pus. In cases of a severe reaction, tissue death and sloughing off of the underlying tissue may occur. The most common systemic effect of a hobo spider is a severe headache that develops within minutes to hours and can last up to one week. Other symptoms include nausea, fatigue, and memory impairment. Rarely death can occur owing to systemic effects, including aplastic anemia. Wearing gloves and other protective clothing, while handling firewood or working in crawl spaces, may help prevent these bites. Recommended treatment for hobo spider bites are the same as that for the brown recluse.
Widow spiders are jet black and often can be identified from their characteristic red "hourglass” marking on the undersides of their abdomens. These spiders are shy creatures that bite only when threatened. Bites of the black widow (Latrodectus mactans) are usually associated with mild redness and sweating. Urticaria and cyanosis may also occur at the bite site. Black widow venom interferes with chemicals at nerve endings which may produce agonizing abdominal pain and muscle spasm. Other signs and symptoms include headache, paresthesias, nausea, vomiting, hypertension, and sometimes paralysis. Current treatment for a black widow bite includes intravenous calcium gluconate, analgesics, and L mactans antivenin for severe cases.
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