How to Know When a Spider Bites and what to Do About It

You spot an angry-looking red welt on your leg, and it seems too big to be a mosquito bite. Must be a spider bite, right?

Not so fast. “People wake up in the morning and find a red mark, and immediately call it a spider bite,” says Rick Vetter, a retired staff research associate and entomologist at the University of California, Riverside. But, he says, most of the skin issues people pin on spiders are actually other types of bug bites or skin issues.

Other researchers back Vetter up on this. “Spider bites are really rare,” says Jonathan Day, PhD, a professor of medical entomology at the University of Florida. He explains that most “spider bites” are more likely mosquito bites that were scratched and became infected. “Spider bite is a catchall grouping whenever there’s a severe skin infection; they’re all lumped in as spider bites,” he says.

Identifying a Spider Bite: What Does One Look Like?

A lot of things. “There’s no one true spider bite,” Vetter says.

Spider Bites Can Look Very Different

Different types of spider bites may provoke different reactions in different people, he says. Even if you’re talking about just one type of spider — say, the brown recluse — its bite could cause a range of reactions: “everything from a little pimplelike bump to a rotting-flesh lesion,” he says.

At the same time, Vetter allows that different types of spider bites do produce distinct reactions. “I’ve had patients contact me saying, ‘This mark on my leg was either from a widow or a recluse,’ but that’s like saying you either got stabbed or trampled to death,” he says. His point: Black widow and brown recluse bites are so different that they could never be mistaken for one another.

But when it comes to common household spiders, hobo spiders, and other domestic varieties, a spider’s bite has some predictable characteristics.

How to Identify Which Spider Bit You

Broadly speaking, a spider’s bite tends to resemble a bee sting: a sharp prick of pain is followed by a red, inflamed skin lump that may hurt or itch but that goes away after a few days. (1) But when it comes to venomous spider bites, there are characteristic signs and symptoms.

Brown recluse bites sting, and they can resemble anything from small blisters to large, rotting-flesh sores, Vetter says. “Its bite causes the collapse of the capillary bed”— also known as skin necrosis (2) — "so people who are obese and have poor support of the capillary cells may have a more massive reaction,” he explains. Along with a wide variety of skin symptoms, brown recluse bites can cause chills, fever, nausea, and other flu-like symptoms common to infections. (3) The brown recluse’s bite is poisonous and can result in coma, kidney failure, or even death.

Because of the severe reaction a brown recluse bite can trigger, these spiders are likely blamed for more harm than they actually cause. The reality is that the brown recluse spider is limited in its geographic range: It’s found in the central and southern United States. (4) The Centers for Disease Control and Prevention (CDC) notes that while venomous spiders are dangerous, they aren’t usually aggressive. (2)

Black widow bites can in some cases cause skin lesions, ranging from small red marks to angry, red, streaky skin patches that are inflamed or contain pus. “But most of the reaction will be on the inside,” Vetter says.

Black widow bites contain potentially deadly amounts of venom and tend to be painful right away. Although that pain starts around the bite site, within an hour, it often spreads to the chest or abdomen, depending on whether the bite occurred on the victim’s upper or lower body. Other symptoms can include everything from headaches, muscle weakness, and difficulty breathing, to seizures, numbness, and painful muscle cramps. (5)

Hobo spiders, wolf spiders, house spiders, and the bites of other domestic types do not contain venoms that are of medical importance to humans, Vetter says. They can bite, he adds. But the result is likely to be similar to a bee sting — meaning a sharp pain, followed by a swollen, red, painful lump at the bite site.

Spider Bites Usually Heal on Their Own. Here's When to Seek Medical Attention

If you think you were bitten by a black widow or brown recluse spider — either because you actually felt and saw the spider, or you’re experiencing the types of skin or systemic symptoms consistent with their bites — head to the emergency room. (6) Vetter says that recluse bites do usually heal on their own and don’t cause scarring wounds. But it’s better to play it safe.

If your spider bite isn’t causing any internal symptoms, or you’re sure it wasn’t a black widow or brown recluse that bit you, follow these steps:

1. Clean the bite with soap and water.
2. Swab the bite with alcohol to prevent an infection, Dr. Day recommends.
3. Apply ice or a cold compress to keep the swelling down.

If pain or itching develops at the bite site, it’s fine to take OTC pain meds or antihistamines for relief. But if pain spreads beyond the site of the bite, or if the swelling, inflammation, or redness are getting worse even a day or two after the bite occurred, get medical attention. You may have a secondary infection — something caused by bacteria getting into the bite — or you may have been bitten by a widow or recluse, Day says.

Why Do Spiders Bite People?

Unlike many other biting bugs, spiders are neither bloodsuckers nor flesh eaters. Vetter says they bite people for one reason only: self-defense.

“Biting is a last-ditch defensive response if a spider is being squashed,” he says. This can happen when someone rolls over in bed on top of a spider — or sits down on one, he says. “I’ve transferred spiders hundreds of times, and they’ll be running all over my arm and have no interest in biting,” he adds.

So while spider bites do occur, they’re far less common than most people assume.

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