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Are Hobo Spiders of Medical Concern?

The hobo spider is one of Utah’s most feared indoor pests. The question is, why are people so concerned about them?  One reason is that people believe hobo spiders have a flesh-eating (dermonecrotic) bite.  New research, however, suggests that the idea that hobo spiders cause dermonecrotic lesions is based on circumstantial evidence.  So, where did this belief originate and should we still be concerned about hobos?

Hobo spiders (Tegenaria agrestis) were introduced into the Pacific Northwest (PNW) from Europe in the early 1900’s.  Since then, these spiders have spread south and eastward into Utah, where large populations now exist.  Physicians began diagnosing patients experiencing dermonecrotic lesions as bite victims of brown recluse spiders (Loxosceles reclusa).  Arachnologists refuted this claim, because the brown recluse’s range does not extend into the PNW.  A survey of PNW homes, however, found hobo spiders and instead of brown recluse spiders.  This finding led to the belief that hobos were actually responsible for the necrotic lesions.  As a result, the hobo spider became known as the “aggressive house spider,” due in part to the fact that the hobos specific name is “agrestis.” In reality “agrestis” means “of the field,” which describes its natural European grassland habitat.

In 1987, Darwin Vest published research in which he forced male hobo spider envenomation (bites) of New Zealand white rabbits.  In his study, the bites resulted in dermonecrotic lesions.  This result, coupled with numerous unverified reports of hobo spider bites causing dermonecrotic lesions, and one verified bite leading to necrosis on a woman suffering from phlebitis (a disease that predisposes patients to necrotic ulcers) are why hobo spiders are considered medically significant.

The rabbit study sounds like strong evidence; however this study was anecdotal and had a small sample size.  Interpreting the results from rabbit assays in the context of human reaction to hobo venom is invalid.  One cannot assume that humans will react similarly to hobo envenomation as rabbits.  The only conclusion that can be drawn from the study is that male hobo spider venom may have greater effects on New Zealand white rabbits than do female hobos.  To this day, no one has replicated the results of Vest’s study, which begs the question, why hasn’t this experiment, one of relatively simple design, been replicated with similar results?

Just because people have necrotic lesions and happen to have a hobo spider in their home does not implicate the spider.  In order to have a verified bite one must actually see the spider biting, catch the spider, and then have it identified by a qualified arachnologist.  Except for the phlebitis patient mentioned earlier, this has never happened.  Many residents of northern Utah have had hobo spiders in their home at some point.  Given our huge hobo population and their frequency in homes, at least one verified bite leading to skin necrosis should have been reported by now.  There are over 40 causes of necrotic skin lesions (viruses, fungi, bacteria, etc.); bites from arthropods are very low on the list.

In Europe, hobo spiders are not considered medically significant.  Since U.S. hobos originated in Europe, how can they be considered medically significant here and not there?  To answer this, researchers collected spiders from England, Switzerland, and the state of Washington.  They analyzed and compared the chemical components of their venom and found that hobo venom composition between the U.S. and England populations was remarkably similar.  In both populations, male hobos spiders produced 3 to 4 times the amount of venom as females, whereas the venom in males was less potent than females.  The venom of Swiss hobo spiders was different from the U.S. and English populations, suggesting that the latter two have been reproductively isolated from the Swiss population.  One major difference between U.S. and European populations is in their habitat.  In Europe, hobos live outdoors in fields and disturbed sites, while U.S. populations frequently occur indoors.  This closeness of U.S. hobos to humans may result in a greater frequency of bites than occur in Europe where hobos rarely interact with humans.  It is important to note that in Europe, the giant house spider displaces hobos as the major house-dwelling spider.  A close relative of the hobo, it too has been introduced into the PNW and may make its way into Utah.  The giant house spider is not considered medically significant.

With over 40 causes of necrotic skin lesions it seems unreasonable to implicate hobo spiders as a primary cause.  Doctors’ misdiagnoses of necrotic lesions as hobo spider bites may preclude the finding of a serious medical problem.  A lesion diagnosed as a hobo spider bite that is instead caused by bacteria, fungi, or a medical condition could lead to dire consequences.  The belief that hobo spiders cause dermonecrotic bites is based on shaky, circumstantial evidence.  Decide for yourself how to interpret the information above.  If a spider has bitten you, please send it to the Utah Plant Pest Diagnostic Lab for diagnosis. 

-Ryan Davis, Arthropod Diagnostician