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Unless you saw the spider bite you, that swollen, bite-looking lesion on your arm is probably something else, and blaming it on an innocent 8-legged critter might do more harm than good.

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Let's say you wake up one morning and you have a big, swollen, oozing thing on your arm or leg. Sounds horrible, I know, but bear with me. 

Your friend says it's a spider bite; Doctor Google says 'a spider bite'; and then your actual doctor also says it's a spider bite. So, case closed then: it's a spider bite, right? Well, maybe not - dare I say, probably not - because the truth is spiders don't go out of their way to bite people, and their venoms don't usually cause large, open wounds.

First off, if you were bitten, you'd probably see the spider that bit you. Spiders might seem creepy, but they don't engage in stealth attacks against people. They really have no desire to bite a person that isn't threatening them in some way. 

See, they spend a lot of energy producing their venom which they need for hunting and to protect themselves, so they're not going to waste it by attacking some gigantic creature they clearly cannot eat. 

On rare occasions, spiders will venture onto mattresses so it's possible to roll over one while sleeping which is pretty much the only time you wouldn't know that a spider was involved. 

But even if you did manage to scare a spider into biting you, most spider venoms are pretty harmless to humans. Some spider venoms can cause pain or itchy bumps but only about 25 species are known to cause illness in humans and the type of illness depends on the spider. 

Widow spiders, for example, a group which includes North America's black widows and the Australian red back spider, have potent neurotoxins in their venoms. That's why the bite of a widow spider can cause numbness, agitation and painful muscle contractions, but it doesn't cause necrosis or the death of living tissue - so no open wounds. And that's an important distinction, because only certain spiders can cause that.

Most well-known are the recluse spiders which are common in the southern United States and South America but can be found in other parts of the world. The closely-related six-eyed spiders which live in parts of southern Africa, Central America, and South America, can also cause necrosis.

What these spiders have in common is that their venom contains the toxin sphingomyelinase D which attacks cell membranes, killing cells. As cells die, a blackened lesion can form but it's important to note that this doesn't always happen. Serious tissue death is thought to occur in less than ten percent of cases. Most of the time, people only experience minor symptoms like redness and swelling, and that's likely because so little venom is injected with each bite. 

Still, somehow that small percentage of bad cases has led to some serious paranoia about recluse spiders. Some people seem to think every spider they see is a brown recluse, the most notorious member of the group, even though that's physically impossible. Brown recluse spiders are endemic to the southern United States, which means they're naturally found nowhere else on Earth. So like for me here in Montana, or for anyone who's not in the southern United States, we don't have them. 

And even if you do live in a place that has brown recluses, it might comfort you to know that they're called recluses for a reason. They're less aggressive than other spiders and pretty much only bite when they're trapped. But that doesn't stop "brown recluse bite" from being a frequent diagnosis, even in places where brown recluses don't live. And that's probably because the death of skin tissue happens for all sorts of reasons so it can be difficult to figure out what caused it. If a doctor can't find the cause of a wound, they might be tempted to call it a spider bite because patients like to have a diagnosis, and if their patient has already self-diagnosed something as a spider bite, they might see no reason to disagree. 

But that's a potentially deadly mistake. In all, there are about 40 different medical conditions that have been misdiagnosed as recluse bites, including herpes, Lyme disease, antibiotic-resistant bacterial infections and even skin cancer. And if a person doesn't get swift, appropriate treatment for some of those things, they can become life-threatening. Fortunately, there is a convenient mnemonic device that can tell you and your doctor whether your wound is likely a recluse bite:
you just have to look for signs it's "NOT a RECLUSE". 

If there are numerous lesions, if the occurrence was in an area that doesn't have recluse spiders, or if the timing was between November and March when the spiders aren't active, it's unlikely to be a recluse.  Recluse bites also don't have red centers, they're not elevated, and they're not chronic--that is, they don't persist for more than three months.  They're also not large, typically less than 10 cm wide and finally, recluse bites don't ulcerate early, meaning they don't crust over within the first week, they don't swell, and they're not exudative.  They don't leak pus or other fluids, though, as I noted earlier, a doctor probably can figure out if a bite was from a spider or not by simply asking if the person saw a spider bite them.

So if you do have a suspicious skin lesion, don't automatically blame a spider for it.  Most spiders can't cause anything more than a small, annoying bump and those than can, aren't found everywhere, so odds are, you're blaming an innocent arachnid.

And speaking of innocent arachnids, we also make a podcast here at SciShow and spiders love podcasts.  It's a scientific fact.  Or is it?  Every week on our lightly competitive podcast, SciShow Tangents, we have a segment called Truth or Fail, where one of us presents three "science facts" and only one of them is real and the rest of us on the podcast have to try to figure out which one it is.  We also write and recite epic science poems and end every episode with a fact about butts.  In other words, it's quite a bit of fun and it's quite a bit of science.  So if you like those things, give it a listen.  You can find it on the podcast platform of your choice by searching for SciShow Tangents and our most recent episode was all about spiders, which is why I'm talking about it right now.