A Staff Report from the Straight Dope Science Advisory Board

How likely are poisonous spider bites?

October 18, 2001

Dear Straight Dope:

My household has had a recent rash of giant man-eating spider attacks (as is common in early fall) that only I, recently knighted Sir Mikey the Spiderslayer, have been able to neutralize. Prompted by the insistence of my girlfriend that she had once been bitten by a spider and developed itchy welts and a rash, and that her father had captured the monster which he identified as the poisonous hobo spider, I hit the e-books and searched several sites over as many hours and gathered what information I could about these spiders. I did this because I was skeptical of dangerous spiders around these parts (I live 30 miles east of Seattle) as we dwell thousands of miles away from the range of the brown recluse and the black widow (reportedly the only dangerous spiders in the U.S.). I identified the spiders we saw as giant house spiders or common house spiders, deemed "completely harmless" in all sites from which I culled information.

I also have heard somewhere that "spider bites" — itchy little welts, like bug bites, especially with paired raised bumps — are a myth. My gut feeling still says so, because spiders have a completely different type of poison than fire ants, bees, ticks, gnats, mosquitoes and other potential bite-mes. The bites of the other bugs are designed to cause irritation as means of defense, while spider venom is designed to liquefy prey flesh — thus the necrotic reaction to poisonous spider bites and eventual sloughing of skin. This led me to further doubt my girlfriend's self-diagnosis, as the reaction to severe spider poison includes initial painlessness (she said she had immediate itching), induration (hardening) within 30 minutes, erythema (redness) for up to 12 cm around site, swelling, ulceration, migraines, sloughing of skin, deep rooted necrosis (tissue death) in fatty tissue, and the like, all of which she confirmed negative.

Furthermore, in my own experience handling spiders of many types bare-handed — including wolf spiders, garden spiders, house spiders, cellar spiders, giant monster cat-size human bloodsucker spiders and so forth — I have never been bitten or seen any ill effects worth noting, even though I watched while some of the big ones scraped their fangs against the flesh on top of my fingers. I am only 19 years old and spend most of my time in front of the computer, so my hands aren't calloused at all either. Finally, my girlfriend is an ultra-arachnophobe, which you and I know can cause pretty big gaps in reasoning that aren't easily filled by cool logic.

I have since discovered that my sense of safety was false — the hobo spider does indeed inhabit the Pacific Northwest, as far north as the Alaskan panhandle, south to northern California and east as far as Utah. Okay. But the drama continues. Information I obtained from the University of California (www.ipm.ucdavis.edu/PM G/PESTNOTES/pn7488.html) seemed to indicate that there was no proof of any sort of danger from the hobo spider, citing actual venom tests, and I consider this a reliable source. However, all the other sites I searched, including other respectable medical, CDC and university sites, seemed to agree that the hobo spider, while no confirmed deaths were reported, does have a necrotic bite and could be dangerous. Many of the sites I visited listed confirmed reports of bites from hobo spiders.

To further compound my frustrations, I called my local pest control center, and they didn't even bother to come and identify my living specimen that I captured last night, completely sure that it was not a hobo spider, unless I had had something imported from another state. So my attempt to secure an arachnologist was foiled.

So my question is two-fold: firstly, do spider bites really occur that are merely itchy little welts that any common house spider can inflict, and if so, what other kinds of spiders can cause this; and secondly, what's the deal on the hobo spider? Do they inhabit wetlands like I live in, as my pest-control expert doubted? Are they being driven out by house spider competition? Are they truly poisonous?

SDStaff Doug replies:

I'm friends with Rick Vetter, the person who wrote the above-mentioned Web page. Rick has sent along the following response:

The hobo spider does indeed live in Washington. I am currently doing a study on its distribution with help from lots of folks in the northwestern U.S. and British Columbia. The Web site that says the hobo isn't dangerous is mine, but the reader has misinterpreted what I have written. In regard to hobo bites, new research HAS NOT duplicated the original experiments showing that the hobo spider has a poisonous venom that is capable of causing necrosis. The original research pressed actual spiders into shaved rabbit flesh until the spiders bit, and necrotic wounds developed. However, recent unpublished research involving mechanically-milked hobo venom (thereby eliminating any confounding factors like digestive regurgitant or bacterial infection), got no necrosis whatsoever in rabbits, using hobos from America and Europe.

That said, I've heard of several verified bites by hobos that did indeed cause necrosis. What this means is that the mechanism of the bite is not through the toxicological action of the venom (like it is in widow or recluse spiders) but possibly through some other mechanism such as digestive regurgitant or bacteria present either on the person's skin or the spider's fang that gets inoculated during fang penetration. At that point you are not dealing with a venom situation but one of septic conditions.

The problem is that with EVERY spider that is deemed "dangerous," there is a public overreaction to the situation based on fears of the unknown and arachnophobia. It then gets difficult to separate fact from fiction. Is it possible that hobo spiders can cause the wounds described in the e-mail? Sure, why not? Is it possible that the wound described are caused by something else? Sure, why not? The problem is that doctors and the general public pin the blame on spiders when there is no way to prove nor disprove spider involvement. What happens then is that a lot of wounds that are caused by a multitude of conditions are diagnosed as spider bites and the treatment given is incorrect. Some of these conditions can be debilitating or fatal if not properly treated in the early stages.

I am one of the country's experts on the misdiagnoses of brown recluse bites. Bites from recluse spiders are routinely diagnosed throughout the United States. One problem: the brown recluse doesn't LIVE throughout the United States. These wounds, when it is possible to figure out what they are, can be caused by about 30 different medical maladies including Lyme disease, tularemia, chemical burn, diabetic ulcer, pyoderma gangrenosum, bacterial infections, fungal infections (ringworm), viral infections (herpes), adverse drug reactions and thromboembolic phenomena (that cause microclotting in the blood system and hence, death of tissues) and many others.

I just finished a study on the misdiagnosis of lymphomatoid papulosis, a rare, low-grade cutaneous T-cell lymphoma that is non-fatal and causes either single or multiple eruptions that regress. Although it is non-fatal, people who have lymphomatoid papulosis have a 5 to 20% chance of developing potentially fatal lymphomas like mycosis fungoides. Of 106 lymphomatoid papulosis patients who responded to my survey, 35 were initially given 45 diagnoses that involved insects or arachnids for their lymph-proliferative condition: bites from fleas, ticks, mites, bedbugs, mosquitoes, scabies, etc. Sixteen of the 106 were told they had spider bites, 5 of the 16 were told they had brown recluse bites and 4 of the 5 brown recluse bite victims lived in areas of the country that do not have recluse spiders. The bottom line is that it is impossible to diagnose the source of a wound based solely on the symptoms.

So to answer your questions with all that background given above: yes, the hobo spider lives in Washington; yes, it can possibly cause necrotic wounds; yes, the hobo spider is blamed for wounds it doesn't cause; and yes, hobo spider bites are overdiagnosed. However, recent research shows that the hobo spider venom itself may not cause the wound, which would explain why not all hobo spider bites create wounds. Should you be concerned? Not really. You still have lots more dangerous things in your day-to-day living to worry about than spiders. Also be aware that west of the Cascades, there is the more common "gigantic house spider" which is a close relative of the hobo spider and looks very similar in coloration. The only sure way to distinguish between the two is to compare the genitalia of the spiders. Gigantic house spiders, like virtually all spiders in the world, are not known to be dangerous. Rest easy.

P.S. You might include this link: spiders.ucr.edu/necrotic.html.

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