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Necrotic Arachnidism -- Pacific Northwest, 1988-1996 |
| ...5... | The aggressive House Spider:
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AN INFORMATIONAL WEB SITE DEDICATED TO THE DISSEMINATION OF KNOWLEDGE REGARDING THE HOBO SPIDER (Tegenaria agrestis), HOBO SPIDER POISONING, AND OTHER BITING OR STINGING ORGANISMS CAPABLE OF CAUSING SIGNIFICANT REACTIONS IN HUMANS
Text and Photographs by DARWIN K. VEST, Eagle Rock Research, Idaho Falls,
Idaho U.S.A.:
ALL RIGHTS RESERVED
back to the Hobo
Spider Web Site
The hobo spider, Tegenaria agrestis, is a member of the family of spiders known as the Agelenidae or funnel web weavers. Approximately 500 species of funnel web weavers occur worldwide; about 300 of these are found in North America, and about 100 species are native to Europe. Funnel web weavers are characterized by their ability to move rapidly, and by their layered, flat web, which has a funnel-like lair at the rear in which the spider resides and waits for prey. The web is not sticky like that of many spiders, but rather is a trip web, which traps insect prey that is unable to navigate on the surface.
The genus Tegenaria contains those spiders which are commonly known as European house spiders, although not all are European, and some do not live in houses; about 90 species are recognized. Some species of European Tegenaria have extended their range far beyond the European continent: The domestic or lesser house spider, Tegenaria domestica, is now cosmopolitan, and is common in and around human habitations in many areas of the world. In the United States the giant house spider, Tegenaria gigantea, is now found in the Pacific Northwest, and Tegenaria pagana is now established in some southern and western states.
Tegenaria agrestis, the hobo spider, was first described in 1802 by naturalist C.A. Walkenaer, in the Paris based journal Faune Parisienne, (vol. 2, pp. 187), under the name Aranea agrestis, which translates spider of the field. This reference to the field was accurate for the species in its indigenous range (western Europe), where the spider lives in fields, woods and rock quarries. While in Europe the hobo spider is occasionally found in greenhouses, it is largely divorced from the human population on that continent.
The hobo spider was first reported in the U.S. in 1936 by arachnologist Harriet Exline (as Tegenaria magnacava), who published her findings in the journal Psyche, vol. 43(1), pp. 21-26. In the United States, the introduced hobo spider did not seek the same type of habitat favored by its European counterpart: Rather, it became established around human habitations, and is now commonly found around the foundations of houses, and beneath rocks, wood, and ornaments in yards. Recently, U.S. specimens of hobo spider have been found in rural areas, including some heavily farmed areas; this trend may continue, placing some U.S. populations of this spider in habitats similar to those that it occupies in Europe. It is unlikely that this trend will affect populations in urban areas. Urban populations of hobo spider do appear to be decreasing in some areas, such as Seattle, Washington, where the giant house spider, Tegenaria gigantea, has become abundant.
The life cycle of the hobo spider in the United States has not been conclusively established, as controlled laboratory studies on this topic have not been reported. Some arachnologists believe that west coast populations are annuals, completing the entire life cycle in one year. Inland populations were originally reported in the literature to undergo a two year life cycle; however, three distinct stages (small juveniles, medium sized immatures, and adults) can be found during the late summer and early fall in inland areas, suggesting that inland populations may actually have a life cycle of three years. The eggs of the hobo are deposited in one to four egg cases in late September and October. These egg cases are composed of several layers of silk, intermingled with layers of soil and debris. The egg cases are usually attached to the undersides of rocks, wood, or other items found in yards, gardens and vacant lots; each egg case may contain 100 or more eggs. Contrary to some references hobo spiders are not prone to construct egg cases inside living quarters (though they have been found in crawlspaces). The eggs hatch in early to mid-June: The spiderlings emerge, feed, and largely remain beneath the surface during their first season, molting their exoskeleton periodically as they grow. The juvenile spiders then overwinter, and are sometimes found indoors during this period. Assuming the two year scenario, the spiders emerge as adults in late July to early August of the second year. The females build webs and remain stationary, but the males began nocturnal wanderings in search of mates. It is at this juncture that males enter houses (often in large numbers) from outside habitat and from garages and crawl spaces; thus, they come into contact with humans; most bites occur during this time period. The hobo spider season peaks during the first week or two of September in most areas. After mating the males began to die, and are absent by the first week of October. The females construct the egg cases, and most remain in the web until they die, usually expiring by mid-November: Some adult females do enter houses in late September, October, and November.
The behavioral aspects of the hobo spider have been a subject of controversy in the past, primarily due to the the publicized myth of the "aggressive house spider". While the hobo will bite when pressed against skin or tormented, it is no more "aggressive" than other wandering spiders; it does not bite without clear provocation, and certainly does not track people down and attack them, as some people have come to believe. The quick movements of this spider, and the fact that they sometimes run toward individuals when disturbed have fueled such myths, but the idea that such spiders are "attacking" is a misinterpretation. In actuality, these spiders have very poor vision and cannot distinguish objects more than a foot or two away. When disturbed inside a house, particularly when a light has just been turned on, the spiders most often remain stationary. When such spiders finally sense potential danger, they run, sometimes toward a person that they really can't see. Hobo spiders are not good climbers, and are usually found at ground or basement level. They sometimes climb up to a level of about four feet if the surface of the wall, etc. is sufficiently porous. Hobo spiders found in wash basins and bathtubs arrived there by falling down the slick porcelain surface; they did not come up through the drain.
Physically, the adult hobo spider is a moderately large
(12-18 mm body length) brown spider, with long, unmarked legs (legs
included, a typical specimen would fit nicely on a silver dollar). The dorsal
abdomen exhibits a "herringbone" or multiple chevron pattern, which
may be quite obscure in darker individuals. The male
sports two pedipalps (antennae like protuberances) between the two front
legs, which are swollen at the ends, looking somewhat like a pair of boxing
gloves; these "boxing gloves" are actually the male genitalia, not
"fangs" or "poison sacs". The female
generally has a larger abdomen than the male, and does not have swollen
"boxing gloves" at the ends of the pedipalps. Several other spiders
resemble hobo spiders; only an arachnologist, or other specifically trained
person should attempt to make a positive identification of this species: In an
instance where a spider has bitten a person, it is imperative to obtain positive
identication only from a qualified professional arachnologist. 
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Although spider bites are common in many parts of the United States, most domestic spiders are not substantially venomous to man. The best known exceptions are widow spiders (Latrodectus spp., including the black widow L. mactans) and brown spiders (Loxesceles spp., particularly the brown recluse, Lox. reclusa). However, cases of arachnid envenomation from the hobo spider (Tegenaria agrestis) are being reported increasingly in the Pacific Northwest. This report summarizes investigations of three cases of T. agrestis bites among persons in Idaho, Oregon, and Washington; spider bites reported to U.S. poison-control centers during 1994; and emphasizes the need for physicians in the northwestern United States to consider this species as a cause of toxic arachnidism.
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Case Reports
Case 1. On November 23, 1995, a 10-year-old boy residing in suburban Portland, Oregon, was bitten on the lower leg while asleep in bed. Within 48 hours, two swollen and erythematous lesions 3-4 cm in diameter developed around the site of the bite. Both were hot to the touch, with central blistering. Seven days after the bite, necrosis and skin sloughing developed, and his entire leg and ankle were red and edematous. The patient reportedly was febrile and nauseated and had severe headaches. Treatment included oral diphenhydramine hydrochloride and alternating local applications of heat and ice. After 30 days, ecchymotic residua were still visible, but local tenderness was diminished. Migraine-like headaches persisted for 4 months. Pesticide applicators who inspected the house reported that it was infested with T. agrestis spiders.
Case 2. On October 8, 1992, a 42-year-old woman residing in Bingham County, Idaho, who had a history of phlebitis felt a burning sensation on her left ankle while at work at a convenience store. She rolled up the leg of her pants and found a crushed brown spider, subsequently identified as T. agrestis. The pain on her ankle persisted, and within 3 hours she was dizzy and nauseated and had a severe headache. An erythematous lesion with a vesicular center was noted several hours later; by the next day the vesicle had ruptured, leaving an open ulcer with a diameter of approximately 2 mm. During the next 10 weeks the ulcer deepened and expanded to a diameter of approximately 30 mm, circumscribed by a blackish margin. The patient sought medical care on December 26, 1992, and received a course of antibiotics. The ulceration continued to enlarge, and swelling of the leg and toes impaired walking. A venogram in July 1993 indicated deep venous thrombosis, which did not respond to standard therapy. The lesion healed slowly between May and November 1994, but left a cratered scar. The patient remains unable to work in situations requiring standing or walking.
Case 3. In late January 1988, a 56-year-old resident of Spokane, Washington, was bitten by a "bug" on her right thigh. Within 24 hours, she developed a severe headache, nausea, and altered mentation. Although symptoms persisted, she did not seek medical attention until February 16, 1988, when she began to bleed from her ears and other orifices. She was admitted to a hospital with a diagnosis of aplastic anemia, pancytopenia, and thrombocytopenia. An eschar on her leg was consistent with necrosis from a spider bite. Despite transfusion therapy, the patient developed severe internal hemorrhage and died in early March 1988. T. agrestis spiders were abundant along railroad tracks adjacent to the patient's home during an inspection of the patient's neighborhood of residence.
Spider Bites Reported to Poison-Control Centers During 1994
Some persons who suspect they have been bitten by spiders and some physicians who treat spider bites contact poison-control centers for advice or information; most of these centers use a standard coding scheme for classifying calls. In 1994, poison-control center log reports compiled by the American Association of Poison Control Centers listed 9418 spider bites (Table_1) (1). Of these, a disproportionate number (1027 {10.9%}) was reported to poison-control centers in Idaho, Oregon, and Washington, which comprise approximately 4% of the U.S. population. A specific kind of spider was noted for 246 of these bites, including 66 (27%) that were classified as brown recluse bites (there is no coding category for hobo spiders). Adapted from: CD Summary 1995;14(no. 22), Center for Disease Prevention and Epidemiology, Oregon Health Div, Oregon Dept of Human Resources.
Reported by: DK Vest, Idaho Falls, Idaho. WE Keene, PhD, M Heumann, MPH, Center for Disease Prevention and Epidemiology, Oregon Health Div, Oregon Dept of Human Resources; S Kaufman, MD, West Linn Pediatric Clinic, West Linn, Oregon.
Editorial Note: Although envenomating spider bites in the Pacific Northwest often are erroneously attributed to brown recluse spiders, most such bites are caused by hobo spiders (formerly also known as "aggressive house" spiders). In Idaho, Oregon, and Washington, venomous spider bites usually are reported from areas with well-established populations of hobo spiders (2). T. agrestis spiders often are found in the homes of persons with these bites; recluse spiders are never found (3). Lox. reclusa and other Loxosceles species are not found in the Pacific Northwest (Figure_1) (4).
The local effects of T. agrestis envenomation are similar to those of brown recluse bites -- a syndrome described as necrotic arachnidism (5). Although many bites occur without substantial envenomation, the cases described in this report illustrate the possible severe outcomes for hobo spider envenomation. Similar local reactions can result from the bite of yellow sac spiders (Cheiracanthium spp.), which are widely distributed in North America and elsewhere (6).
The bite of the hobo spider usually is initially painless. A small area of induration may appear within 30 minutes, surrounded by an area of expanding erythema that can attain a diameter of 5-15 cm. Blisters develop within 15-35 hours; soon thereafter the blisters can rupture with a serous exudate encrusting the cratered wound. An eschar can develop with underlying necrosis and eventual sloughing of affected tissue. Lesions generally heal within 45 days, but can result in a permanent scar; healing can require up to 3 years if the bite occurred in fatty tissue. The most common systemic symptom is a severe headache -- occurring as soon as 30 minutes after the bite, and usually within 10 hours -- that can persist for a week. Other symptoms can include nausea, weakness, fatigue, temporary memory loss, and vision impairment. Protracted systemic effects, including aplastic anemia, intractable vomiting, or profuse secretory diarrhea, are rare but may be associated with death (7).
Optimal treatment for necrotic spider bites is not well defined (5). Systemic corticosteroid therapy may be of benefit if any substantial hematologic abnormalities are noted other than a moderate leukocytosis. Surgical repair may be necessary in severe cases of ulcerative lesions, but should not be initiated until the primary necrotizing process is completed (5).
T. agrestis is native to Europe and probably was introduced into the Seattle area in the 1920s or early 1930s (8); it subsequently has spread as far as central Utah and the Alaskan panhandle (Figure_1). Hobo spiders build funnel-shaped webs in dark, moist areas, often in wood piles, crawl spaces, or around the perimeters of homes (9); they rarely climb vertical surfaces and are uncommon above basements or ground level. Hobo spiders are moderately large (7-14 mm body length; 27-45 mm leg span) and brown with grey markings. They can move quickly (up to 1 m/second) (2), and can bite if provoked or threatened. Mature spiders are abundant from mid-summer through fall when males, which are more venomous than females, wander in search of females (9).
Practical control strategies should emphasize personal protection rather than attempted eradication of T. agrestis populations. Exposure can be reduced through the use of gloves and other clothing that covers the skin while working in crawl spaces and similar locations and through precaution when retrieving firewood or other items stored in potentially infested areas. Screens on basement and ground-floor windows and insulation strips under doors may reduce the risk for spider infestation.
Venomous spider bites are not reportable in any state, and there are no reliable estimates of the incidence of such bites or how often medical attention is sought for them. The addition of a specific designation for hobo spider envenomations in poison-control center report classifications may provide better information on how frequently these bites occur. Medical references should be updated to acknowledge causes of necrotic arachnidism other than Loxosceles spp.
Litovitz TL, Felberg L, Soloway RA, Ford M, Geller R. 1994 annual report of the American Association of Poison Control Centers. Am J Emerg Med 1995;13:551-97.
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.
Vest DK. Necrotic arachnidism in the Northwest United States and its probable relationship to Tegenaria agrestis (Walckenaer) spiders. Toxicon 1987;25:175-84.
Gertsch WJ, Ennik F. The spider genus Loxosceles in North America, and the West Indies (Araneae, Loxoscelidae). Bulletin of the American Museum of Natural History 1983;175:264-360.
Wasserman GS, Anderson PC. Loxoscelism and necrotic arachnidism. J Toxicol Clin Toxicol 1983;21:451-72.
Edwards RJ. The spider family Clubioninae of the United States, Canada, and Alaska (Araneae: Clubionidae). Bulletin of the Museum of Comparative Zoology 1958;118:365-436.
Vest DK. Protracted reactions following probable hobo spider (Tegenaria agrestis) envenomation {Abstract}. American Arachnology 1993;48:10.
Exline H. New and little known species of Tegenaria (Araneida, Agelenidae). Psyche 1936;43:21.
Akre RD, Catts EP. Spiders. Pullman, Washington:
Washington State University Cooperative Extension, 1990. (WSU report no. EB
1548).
Table_1
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TABLE 1. Reported spider bites to poison-control centers -- United
States and Pacific Northwest, 1994
=======================================================================
United States * Pacific Northwest +
------------------- -----------------------
Type of spider No. (%) No. (%)
-----------------------------------------------------------------------
Black widow 2120 ( 22.5) 139 ( 13.5)
Brown recluse 1835 ( 19.5) 66 ( 6.4)
Tarantula 82 ( 0.9) 41 ( 4.0)
Other/Unknown 5381 ( 57.1) 781 ( 76.1)
Total 9418 (100.0) 1027 (100.0)
-----------------------------------------------------------------------
* A total of 65 reporting poison-control centers that represent 83% of
the U.S. population (1).
+ A total of three reporting poison-control centers that represent 100%
of the population in Idaho, Oregon, and Washington. Source: 1994
Annual Report of the Toxic Exposure Surveillance System for each
state, published by each state's poison-control center.
=======================================================================
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Figure_1
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The aggressive house spider, which was introduced to the Pacific Northwest in the early 1900's has become much more commonplace and is recognized as a danger to human health. It has a venomous bite that produces necrotic lesions (localized areas of dead tissue) and can be very toxic to humans and animals. It is one of the most common spiders found in and around homes in this area. If a circle was drawn around an average specimen of this hairy brown arachnid just touching the tips of its legs, it would measure 1 to 1 3/4 inches in diameter (see figure). There is a chevron-shaped marking on the back of the abdomen. Tegenaria agrestis should be approached with caution, as some will bite with little or no provocation when cornered or threatened. It is a swift runner and is often seen moving on the floor at night while hunting or trapped in buckets, open jars, sinks and bathtubs. It is found year-round in dark, moist areas, such as basements and garages, behind stored items, in window sills and closets. It builds a funnel-shaped web, open at both ends, and sits in the funnel mouth, waiting for prey. Its bite is commonly blamed on the Brown Recluse, which is also known as the Violin or Fiddleback Spider. Contrary to popular belief, the Brown Recluse does not occur in the Pacific Northwest, except for those that are transported here from the south central United States. People are most commonly bitten by the Aggressive House Spider when picking up a piece of firewood or putting on an article of clothing containing the spider. The symptoms of the Brown Recluse bite are similar to that of Tegenaria agrestis. The typical sequence of bite symptoms seen in the Pacific Northwest aggressive house spider cases start with a bite that either goes unnoticed or is felt as a mild pin prick. A large red swollen area develops shortly after the bite. Within 36 hours this may be followed by severe headache, impaired vision, weakness, joint pains, nausea, sweating and blistering at the bite area. The healing process may take up to six months and leave a large disfiguring scar the size of a fifty cent piece. Eliminating the spider's habitat is the best method of controlling it. This means that all tall grass next to foundations should be cut or removed. Cracks or deep cavities in rock walls should be filled. Large cracks left by ill-fitting doors or windows and holes in the wall of a house for pipes or electrical lines should be sealed. Always inspect firewood before bringing it into your home for spiders or their egg sacs. Woodpiles and other debris should be kept away from the house. Rather than lavish use of chemicals, disposal of spiders by stepping on them or by using a vacuum cleaner to remove them and their webs from corners and other hard to reach areas is recommended. The spider should die while entrapped within the vacuum bag, but it might be a good idea to dispose of the bag immediately so the spider cannot escape. If spiders are unusually numerous and always entering your house, the recommended insecticides to kill unwanted spiders include sprays containing Dursban and Knox-Out. Do not exceed the recommended dosage given on the label. (Source: "Insect Answers: Spiders", Bulletin EB1548, Cooperative Extension, Washington State University, Roger D. Akre, PhD., E. Paul Catts, Ph.D., April 1992)
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