Tegenabiism

Some agelenid spiders in the genus Tegenarict occur in close association with humans. Upon entering homes, they may become established in basements and other relatively dark, damp locations, where they construct sheet webs with a funnel-like retreat characteristic of the Agelenidae. In Europe, these species are called house spiders. One species of particular medical importance in

North America is the hobo spider, Tegenarict agrestis, which was introduced from Europe to the Pacific Coast of the United States some 70 years ago. It now occurs widely in the northwestern United States and adjacent Canada, where it is recognized as a common cause of lesions similar to those induced by the brown recluse spider,

Hobo spider (Tegenaria agrestis)

This species was first reported at Seattle (USA) in 1930 but did not become common in the Pacific Northwest until the 1960s. It was not until the 1980s, however, that it was confirmed as the cause of bites previously attributed to Loxosceles reclusa. The hobo spider is now well established in Washington, Oregon, and Idaho, with its distribution extending from Montana, Wyoming, and Utah northward into British Columbia and Alberta to the Alaskan panhandle.

Adults of this spider are 12—18 mm long and brownish, with long, unmarked legs and a herringbone pattern or series of dark chevron-like markings on the dorsal aspect of the abdomen (Fig. 22.10). It occurs in funnel-like webs (Fig. 22.9), particularly in basements and cellars, window wells of homes, and crawl spaces, around house foundations, in wood piles, under rocks and wood used in landscaping, and other suitable sites at ground level. The males tend to wander at night in search of females, at which time they enter homes and are more likely to be encountered than females. This occurs primarily from midsummer through fall. In Europe, T. agrestisis found more typically outdoors, apparently being unable to compete well indoors with its close relative T. gigantea. The latter is less prone to bite when disturbed and has a milder venom. This probably explains why few cases of T. agrestis bites are

FIGURE 22.9 Female hobo spider, Tegenctria agrestis(Agelenidae). in entrance to funnel-web retreat. (From Akre and Catts, 1992, courtesy of Washington State University Cooperative Extension.)
figure 22.10 Male hobo spider, Tegenaria agrestis (Agelcnidae). (From Akre and Catts, 1992, courtesy of Washington State University Cooperative Extension.)

reported among Europeans. Similarly, where T. gigantea has become established within the range of T. agrestis in North America, the incidence of T. agrestis bites tends to be diminished.

The venom of T. agrestis is similar to that of Loxosceies species in that it is cytolytic, causing necrosis of tissues and slow-healing, ulcerated lesions (Fig. 22.11). The venom of males and juveniles appears to be more potent than that of the adult females. The bite generally is not painful and may result initially in a slight prickling sensation or go unnoticed altogether. Within 30 min, the bite site becomes numb and indurated, surrounded by an expanding reddened area (5-15 cm diameter). This is followed by the formation of pus-filled blisters during the next 1-2 days. The blisters break about a day later, causing the wound to ooze and develop an encrusted, crater-like skin lesion. As

FIGURE 22.11 Lesion on human finger caused by bite of hobo spider, Tegenaria agrestis (Agelenidae). (From Akre and Catts, 1992, courtesy of Washington State University Cooperative Extension.)

in cases of Loxosceies envenomation, a black scablike eschar forms over the wound, and sloughing of dead tissue can result in disfiguring scars. Healing may take 1—2 weeks to several months. In cases in which significant fatty tissue is involved, the wound may become extensive and deep, requiring as long as 2—3 years to heal. Treatment is similar to that recommended for brown recluse spider bites.

Systemic effects occur in approximately 45% of bite cases caused by the hobo spider, about one-third of which may require hospitalization. The earliest indication is usually severe headaches within the first 10 hr which are not relieved by aspirin. These headaches may persist for several days or up to a week. Other associated problems include general muscular weakness, fatigue, sweating, dizziness, nausea, impaired vision, disorientation, and temporary memory loss. Without appropriate medical treatment, severe systemic reactions may result in intractable vomiting, profuse secretory diarrhea, destruction of bone-marrow tissue, anemia, pancytopenia, low platelet counts, and internal hemorrhaging. Although rare, systemic reactions can be fatal (Anonymous, 1996).

For additional information on the hobo spider and its medical importance, see Vest (1987), Akre and Myhre (1991), and Anonymous (1996).

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