There are three species of related Tegenaria spiders in the Northwest, that commonly occur in and around homes. T. Agrestis, T. Duellica, and T. Domesticus. They have similar markings, colorings, and size, although one of the species, T.Duellica, sometimes called the Giant house spider, can grow to be much larger the other two – when fully grown. The best way to tell which species you have is to catch one and put it under a microscope…. Ha, ha! No, really. Go grab one, I’ll wait.
The Hobo spider, (T.Agrestis) is blamed throughout the US for spider bites that cause tissue necrosis – even in areas it doesn’t exist. Most of the blame is placed by people who do not have a background studying spiders. Medical professionals and nonarachnologists (you and me) alike, commonly diagnose open wounds and other dermatological lesions as being caused by Hobo spiders.
Media outlets, always looking to sell copy and create headlines are all too willing to run with stories of Hobo spider bites that cause loss of limbs or threaten lives, regardless of their validity.
There was an exhaustive study involving pest control associations and public health officials, throughout the range of the spider, along with arachnologists from area universities and museums, as well as the general public, who helped submit thousands upon thousands of spiders. The study results indicated that you are more likely to encounter T. Duellica, the Giant house spider, than the Hobo spider, except in Portland, Oregon. (Sorry Portland.)
Additionally, although there are over 30 possible causative sources for necrotic wounds, including a number of different bacteria, the venom of the spider does not cause necrotic wounds. The venom of the Hobo spider which was introduced from Europe has no history of causing necrotic wounds in Europe.
The rap for Hobo spiders causing wounds started with bad science. A study was done involving rabbits and Hobo spiders. The study was flawed and it drew invalid conclusions, because something causes necrosis in animals does not mean it cause necrosis in humans. Other spiders have proven that is not the case. In hundreds of spider investigations since then, hobo spiders have not been implicated. The usual diagnosis is, ‘I have a skin lesion, I have seen a hobo spider’ and the person or their medical professional makes the diagnosis of the wound being caused by hobo spiders, without verifying that the spider they had seen was in fact a Hobo spider, that the person had really been bitten by a spider, or that there was something in that spider’s venom that would have caused tissue necrosis.
Many of the diagnosis for Hobo spider bites come from areas where the spider doesn’t even occur. In the event that a Hobo spider bite is suspected, a culture of the wound should be taken to make sure that it was the causative agent.
The conclusion that many professionals and arachnologists share is that Hobo spiders can bite humans, the resulting bite can make a small wound, much like other insects bites, and that by itself it will not cause skin lesions. Some have conjectured that the spiders might be able to carry bacteria in their fangs that could be transmitted when a person is bitten – like going to hospital to have them remove a splinter and ending up losing your foot due to MRSA, however this has yet to be proven.
Other findings of the exhaustive study? This area does not have Black Widow or Brown Recluse spiders.
Next week, what you can to do help control spiders around your home – without using pesticides.
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