“From the time I started working at the Utah Plant Pest Diagnostic Lab, it became obvious that hobo spiders were the major indoor pest of concern in Utah,” he said. “The question is: Why are people so concerned with hobo spiders? For one, they are spiders, and most people don’t like spiders. Another reason is that people believe that venom from a hobo spider bite can cause a dermonecrotic or flesh-eating lesion. New research, however, suggests that hobo spiders may not cause dermonecrotic lesions.”
Davis said beginning in the 1960s, people in the Pacific Northwest who experienced dermonectoric lesions began to blame the brown recluse (Loxosceles reclusa) spider. Arachnologists refuted this claim, because the brown recluse’s range does not extend into that region. To prove their point, a survey of homes was conducted. The results indicated that brown recluse spiders were not found, but that hobo spiders were. This coincidence led to the belief that hobo spiders were now responsible for these necrotic bites. That belief coupled with the unfortunate fact that the hobos scientific name is “agrestis,” led to the misnomer of the aggressive house spider. In reality “agrestis” means “of the field,” which describes the spider’s natural European grassland habitat.
With hobo hysteria building, Davis said scientists began to research hobo spider venom. In 1987, Darwin Vest, a researcher studying hobo venom, published results of a study in which he forced hobo spider bites on rabbits. In his study, the bites created dermonecrotic lesions. This immediately elevated the hobo spider to the medically significant list. That study, coupled with innumerable unverified reports of hobo spider bites causing dermonecrotic lesions, and one verified bite leading to necrosis of a woman already suffering from phlebitis (which can cause necrotic ulcers), are why hobo spiders are considered to have necrotic bites.
“Just because people have necrotic lesions and happen to have a hobo spider in their home does not implicate the spider,” said Davis. “In order to have a verified bite, one must actually see the spider biting them, catch the spider and then have it identified by a qualified arachnologist. Except for the instance mentioned above, this hasn’t happened. Additionally, most people in northern Utah probably have hobo spiders in their home at some time during the year. With that, at least one verified bite leading to skin necrosis should have occurred by now, but it hasn’t. There are more than 40 causes of skin necrosis including medical conditions, viruses, fungi, bacteria and others. Bites from arthropods are very low on the list. It has also been suggested that less than 50 percent of hobo spider bites result in the transfer of venom, making over half of the bites harmless anyway.”
Davis said that while the rabbit study suggested evidence for the bites being necrotic, the details are sketchy. Not only is this study anecdotal and with small sample size, but interpreting the results in the context of human reaction to hobo spider venom is not valid. The assumption that humans and rabbits will react similarly to hobo envenomation is not necessarily true.
“To this day, no one has replicated the results of Vest’s study,” said Davis. “This begs the question: Why hasn’t this experiment, one of relatively simple design, been replicated with similar results?”
With over 40 sources of necrotic skin ulcers, it seems difficult to implicate hobo spiders as the main cause, said Davis. Doctors misdiagnosing necrotic lesions as hobo bites may take the focus off the real medical source, which could lead to dire consequences.
“Currently, it is unknown whether hobo spiders have necrotic bites, however we do know that the basis of this belief is based on shaky, circumstantial evidence,” said Davis. “It is also interesting to note that in Europe, hobo spiders are not medically significant; European and U.S. hobo spiders have nearly identical venom.”
Those bitten by a spider can send it to the Utah Plant Pest Diagnostic Lab for diagnosis. Visit utahpests.usu.edu for further information on spiders and other pests.
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