The Spider Research Project

Necrotic Arachnidism

Necrotic arachnidism is a syndrome caused by the venom of certain spiders: Tegenaria agrestis (the hobo spider), Loxesceles reclusa (the brown recluse) and other Loxesceles species, and Cheiracanthrium spp. (yellow sac spiders). Latrodectus spp. (widow spiders) are also venomous, particularly the black widow spider, but cause a different set of symptoms. Recently, the Centers for Disease Control and Prevention (CDCP) reported that hobo spider bites are increasing in the Pacific Northwest. Also known as the aggressive house spider, the hobo spider causes a local reaction similar to that of the brown recluse; indeed, bites are often attributed to the brown recluse. But according to the CDCP, the brown recluse and other Loxesceles species are not found in the Pacific Northwest.

The hobo spider is large, brown with gray markings, and fast moving; it bites if provoked, according to the CDCP, especially the male during midsummer through fall when it wanders in search of a mate. It is native to Europe, was introduced into the Seattle area during 1920 to 1930, and has now spread as far as central Utah and the Alaskan panhandle. It builds funnel-shaped webs in dark, moist areas such as under wood piles, in crawl spaces, and along foundations. The spider doesn't climb, so is rarely found above basement or ground level.

The hobo spider bite is usually painless at first, according to the CDCP, but a hot, swollen, erythematous lesion with central blistering develops at the site of the bite, accompanied by nausea and severe headaches. Lesions are characterized by induration (usually within 30 minutes), blistering (15-35 hours), and ulceration or necrosis with skin sloughing (within a week). Lesions usually heal in 45 days, but can require up to 3 years. Systemic symptoms include a severe headache (developing within a half hour or up to 10 hours after the bite, and lasting a week), nausea, weakness, fatigue, dizziness, temporary memory loss, and vision impairment. Serious side effects include aplastic anemia, intractable vomiting, and profuse watery diarrhea.

The CDCP report included descriptions of three patients bitten by the hobo spider. Case 1 (a 10-year-old boy) developed two lesions -- hot, swollen, red, and blistered -- within 48 hours of having been bitten. Systemic symptoms included fever, nausea, and severe headaches. In seven days, necrosis and skin sloughing occurred. Migraine-like headaches persisted for 4 months. Case 2 (a 42-year-old woman) had a severe headache, nausea, and dizziness within 3 hours of experiencing a bite, followed by the appearance of an erythematous lesion with a vesicular center. The vesicle ruptured the next day, leaving an open ulcer that expanded over the next 6 to 7 months. The ulcer finally healed (with scarring) 13 months after the bite. A venogram taken 10 months after the bite showed deep venous thrombosis, which did not respond to standard treatment. The patient remains unable to work, where she has to stand or walk. Case 3 (a 56-year-old woman) developed aplastic anemia, pancytopenia, and thrombocytopenia 2 weeks after a hobo spider bite, and she died 2 weeks later from internal hemorrhage.

According to the CDCP, venomous spider bites are not reportable in the United States, and so estimates are unreliable concerning the incidence and severity of bites. The best way to treat hobo spider bites is not well defined. Supportive care is indicated. Treatments for the patients described in the CDCP report included ice and heat applied to the site of the bite, diphenhydramine for allergic response, and antibiotics for secondary infection. Corticosteroids may help hematologic abnormalities, and surgical repair may be required after the necrotizing process is complete. The CDCP report emphasizes the need for physicians in the northwestern United States to recognize that the hobo spider is a more likely cause of necrotic arachnidism than the brown recluse. (MMWR. 1996;45:433-436. CDC. JAMA. 1996;275:1870-1871.)

© 1996 VirSci Corporation. All rights reserved. As seen on PharmInfoNet (http://pharminfo.com)

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