| The Hobo Spider |
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| The Hobo Spider |
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Modified from: Oregon Health Division Center for Disease Prevention and Epidemiology CD Summary Vol. 44, No. 22, October 31, 1995. Graphics Source: Washington State University Cooperative Extension Service Bulletin EB1548: Insect Answers: Spiders. Revised 1992.
Although not a major medical issue, spider bites are a concern. Many references incorrectly state that in North America, only two types of spiders have poisonous bites. These well-recognized threats are the widow spiders (Latrodectus spp., including the notorious black widow L. mactans) and the brown spiders (Loxesceles spp., notably Lox. reclusa, the brown recluse, aka violin- or fiddle-backed spider).
Good figures for how common bites are do not exist. In 1994, the Oregon Poison Center recorded 375 inquiries concerning spider bites (not including information only calls). Of those calls where a specific spider was mentioned, 49 concerned black widow bites, 4 concerned tarantulas (probably angry pets), and 41 were nominally about brown recluse bites. Recluse spiders, however, do not live in Oregon or anywhere near it.
Most, if not all, of these reports reflect a common misidentification. In fact, a third important venomous spider, the Hobo spider, occurs in the United States. It lives throughout the Pacific Northwest. As its range has extended in recent decades, reports of spider bites leading to significant injuries in both humans and animals have increased. As with recluse spider bites, these bites can cause moderate to severe skin lesions that may take months to heal, often resulting in significant scarring.
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| Case Histories |
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Case #1 [all case reports from reference (3)]. A 15-year-old boy was bitten on the back of the leg while asleep in Cheney, Washington. At the site of the bite, a circular area of whitened skin and blisters formed. Seven to 10 days later he became nauseated, and the lesion became necrotic with sloughing of affected skin. The lesion was diagnosed as due to a spider bite. The patient was given antibiotics, because a local infection was suspected. Topical hydrocortisone was used after 14 days. In one month the lesion was healing, but still red.
Case #2. A 55-year-old woman was bitten by a spider on the cheek while cleaning under her bed in Ocean Shores, Washington. The bite was accompanied by a burning sensation. Within 48 hours, she developed persistent headache (despite aspirin), blurred vision, fever, severe localized and some generalized itchiness, and malaise. Blisters developed at the site of the bite, followed by tissue sloughing of ~3 cm in diameter. The lesion was treated with topical corticosteroids. The patient was also given corticosteroid injections on day 2 and day 5. After four days, the lesion was surrounded by a red area; her whole cheek was swollen. By day 7 the lesion had enlarged to 3.5 cm and began to blister again. It continued to enlarge. After one month a secondary infection was diagnosed. Her physician prescribed antibiotics, oral and topical corticosteroids. Swelling and an eventual scab resolved after two months.
Case #3. A 2-year-old girl was bitten on the stomach after gathering firewood from an outdoor wood pile near White City, Oregon. The bite became red with a white center, and the girl developed fever and vomiting. (Exactly when this happened is not clear from the report.) The child´s mother called the Oregon Poison Center, and was told that the symptoms fit those of a brown recluse bite. A physician then diagnosed the wound as a "brown recluse" bite.
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| The real culprit |
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Who is The Real Culprit?
Although these wounds and many others are usually blamed on Loxosceles spp., in the Pacific Northwest spider bites that lead to local ulceration and necrosis are caused by Tegenaria agrestis, the Hobo spider (formerly known as the aggressive house spider). Recluse spiders, on the other hand, have not become established beyond the American southeast and southern midwest, although there are reports of some populations now in southern California. According to Washington State University studies, reports of poisonous spider bites in Washington, Oregon and Idaho, usually diagnosed as "brown recluse" bite, come only from areas where populations of hobo spiders are well established. Furthermore, on-site investigations routinely fail to discover recluse spiders, while hobo spiders are common.
T. agrestis belongs to the family Agelenidae--the funnel-web spiders. These arachnids build funnel-shaped webs in dark, moist areas, often in basements, window wells, wood piles, and around the perimeter of homes.(5) They rarely climb vertical surfaces, and are rarely found above basements or ground level. T. agrestis is a fast runner, attaining speeds up to 2 mph.(3)
T. agrestis is now one of the common spiders found in and around houses in many parts of the Pacific Northwest. Like the brown recluse, the hobo spider is large (up to 4-5 cm in diameter, including legs) and brown, but it is much more likely to attack when provoked or threatened. The hobo spider was apparently introduced from Europe earlier this century, and was first reported in Seattle in 1936. By the 1960´s it had become well established in many parts of Washington, Oregon, and Idaho. Only recently, however, largely through the efforts of WSU entomologists, has T. agrestis been appreciated as a danger to human health.
Hobo spiders have a two-year life cycle. Sexually mature spiders are abundant from mid-summer through fall. Males are more likely to wander in search of females during this period, especially in fall months. Males are somewhat more venomous than females. Bites commonly occur when a person picks up firewood with a spider on it or when a spider finds its way into clothing or bedding.
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| Clinical Course |
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The effects of hobo spider bites are similar to those of the brown recluse spider. The actual bite is not always painful, but a small, hard area typically appears within 30 minutes, surrounded by an expanding red welt that may reach 5-15 cm in diameter. Blisters form within 15-35 hours, which eventually crust over the cratered wound. A scab can develop over necrotic tissue that eventually sloughs off. In some cases, tissue loss is so severe that surgical removal of damaged tissue and repair are needed. The fully developed lesion may reach 3 cm or more in diameter. Lesions may take several months to heal, and frequently leave a permanent scar.
The occurrence of systemic illness is variable. However, the most common symptom is a severe headache, sometimes occurring within 10 hours (sometimes ~30 minutes) that does not respond to aspirin. The headaches have been compared to migraines and may persist for a week, sometimes accompanied by nausea, weakness, fatigue, temporary loss of memory, and vision impairment. No deaths due to T. agrestis bites have been reported.
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| Treatment |
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Optimal treatment for suspected spider bites is not well defined; identification of the offending creature is often a guess. Patients are encouraged to seek medical attention and, if possible, submit spiders for identification (contact Oregon State University's Extension Entomology service at 541/737-3151). Clinicians may wish to consult with the Oregon Poison Center (503/494-8968 in Portland; elsewhere 800/452-7165) for current recommendations (also see Oregon Health Division Center for Disease Prevention and Epidemiology CD Summaries, and "Necrotic Arachnidism - Pacific Northwest, 1988-1996, MNWR v. 45 No. 21, May 31, 1996).
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| References |
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Wallace JF. Disorders caused by venoms, bites, and stings. In: Wilson JD, Braunwald E, Isselbacher KJ, Petersdorf RG, Martin JB, ed. Harrison´s Principles of Internal Medicine. 12th ed. New York: McGraw-Hill, 1991: 2189.
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Akre RD, Myhre EA. The great spider whodunit. Pest Control Technology 1994;22:44-50.
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Akre RD, Myhre EA. Biology and medical importance of the aggressive house spider, Tegenaria agrestis, in the Pacific Northwest (Arachnida: Araneae: Agelenidae). Melanderia 1991;47:1-30.
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Vest DK. Necrotic arachnidism in the Northwest United States and its probable relationship to Tegenaria agrestis (Walckenaer) spiders. Toxicon 1987;25:175-184.
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Akre RD, Catts EP. Spiders. Bulletin EB 1548, 8 pp. Pullman, Washington: Washington State University Cooperative Extension, 1990.
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Exline H. New and little known species of Tegenaria (Araneida, Agelenidae). Psyche 1936;43:21.
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