My dad’s hospitalized with a big bump on his arm – could it be a spider bite?

SDStaff Doug replies:

It may seem strange to you, after watching all those TV shows and movies where doctors or forensic investigators solve mysterious cases from the tiniest of clues or symptoms, but in reality there’s one major class of symptom that is in fact nearly impossible to diagnose accurately: mystery “bites.” The best-trained doctors in the world can’t diagnose the source of a “bite” based solely on the symptoms. In fact, most doctors are never trained — in any manner whatsoever — regarding the world’s venomous animals, and accordingly it’s more common for a doctor to misdiagnose such cases than to get them right. The most glaring example is the brown recluse spider, which in the U.S. is found exclusively east of the Rockies — yet doctors on the west coast, where the spider doesn’t dwell, routinely diagnose necrotic wounds as brown recluse bites, simply because the patients assume they were bitten, and the doctors don’t know enough to question their assertions.

Your case is a perfect example — even though your father said he had no idea where the lump came from, you, or at any rate those “most involved,” assumed it was a spider bite. Why? No spider in the world feeds on mammalian blood. Even if one allows for an occasional bite in self-defense, 99.9% of the world’s spider species have venom that’s essentially harmless to humans. Of the handful of dangerous species, nearly all have venom that either kills you within a matter of hours or completely goes away within the same period. That’s the kicker, really. The venom of virtually every known biting insect, spider, scorpion, centipede, or other arthropod is completely gone from your system within hours. Any symptoms after that are secondary, usually involving either allergy, histamine response, or infection (and, in the case of vectors like mosquitoes or ticks, another organism or virus that was injected into the victim). At that point, it absolutely — and quite literally — doesn’t matter what the source of the injury was; all that one needs to worry about is treating the symptoms. If you suggested to the doctor that a spider was involved when none was seen, you’ve probably misled them and confused the issue for no good reason. If the doctor suggested it, SHAME ON THEM!

The bottom line: even if your father had the one-in-a-million misfortune of being bitten by a dangerous spider, it has no bearing whatsoever on the treatment he should receive. It probably wasn’t a spider bite, though, and any doctor who assumes to the contrary when a patient can’t produce the offending critter is choosing the least likely explanation. Not only is it lazy, it’s sloppy and potentially dangerous, because many other conditions, some possibly life-threatening, can produce similar symptoms. If doctors wave something off as a “spider bite” they may fail to recognize what it really is and not treat it properly.

That said, there’s one exceptional spider — the aforementioned brown recluse — whose venom does linger in the body, sometimes causing necrosis at the wound site. This can spread and become quite serious. The brown recluse is the only species known to produce this effect (there’s no proof that hobo spiders or any others can induce necrosis, only urban legends), and it’s indeed something to be concerned about. The problem is, there are lots of other things that cause necrotic wounds with symptoms identical to those produced by brown recluse venom. So even a necrotic wound (which is pretty dramatic — black, gangrenous flesh is unmistakable) can’t be blamed on a spider bite unless the victim actually observed the spider in the act of biting. I lived for four years in an apartment in Kansas that I shared with dozens of brown recluses, was only bitten once, and developed no necrosis. A recluse bite is literally a one-in-100-million thing — more people win the Lotto each year than are bitten by recluses. The odds are greater that your father got stuck with a splinter contaminated with bacteria than that he was bitten by a spider or anything else.

We’d all be better off if every doctor in the U.S. memorized this phrase: “You can’t diagnose a bite from the symptoms.” If they’d stop blaming every mysterious wound on spiders, a lot more people would get appropriate treatment.

For more information on brown recluse bites and the things that can be confused with them, see this page from the University of California Riverside.

SDStaff Doug, Straight Dope Science Advisory Board

Send questions to Cecil via cecil@straightdope.com.

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