How to Identify and Treat Spider Bites

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? Well, it’s not likely.

Spider bites are uncommon, and doctors often misdiagnose them. But some spiders do bite people, and the symptoms can be severe. This article explains how to know when a spider has bitten you, why spiders bite people, and when to see a doctor.

Spider bites look different from one another, and plenty of people mistake other bites for spider bites.

“People wake up in the morning, 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 in Riverside. But most of the skin issues people pin on spiders are actually other types of bug bites or skin issues, he says.

Other researchers back Vetter up on this. “Spider bites are really rare,” says Jonathan Day, PhD, a professor emeritus of medical entomology at the University of Florida in Gainesville. He explains that most “spider bites” are more likely mosquito bites that were scratched and developed into an infection.

Spider Bites Can Look Very Different

“There’s no one true spider bite,” Vetter says. He suggests that different types of spider bites may provoke different reactions in different people. Even one type of spider could cause a range of reactions, “from a little pimple-like 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, they could never be mistaken for each other.

But the bites of common household spiders, hobo spiders, and other domestic varieties have some predictable characteristics.

How to Identify Which Spider Bit You

Broadly speaking, a spider’s bite resembles a beesting: A sharp prick of pain is followed by a red, inflamed skin lump that may hurt or itch but goes away after a few days.

There are characteristic signs and symptoms when it comes to venomous spider bites.

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

— “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 symptoms common to infections. The brown recluse’s bite is poisonous and can result in coma, kidney failure, or even death.

Chills
Fever
Nausea
Other flu-like symptoms
Coma
Kidney failure
Death

The brown recluse spider is limited in its geographic range: It’s only present in the central and southern United States.

The Centers for Disease Control and Prevention (CDC) also notes that while venomous spiders are dangerous, they aren’t usually aggressive, meaning they don't go out of their way to bite people.

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 cause immediate pain around the bite site. Within an hour, the pain 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:

Headaches
Muscle weakness
Difficulty breathing
Seizures
Numbness
Painful muscle cramps

Hobo spiders, wolf spiders, house spiders, and the bites of other domestic types do not contain venoms that harm humans, Vetter says, but they can bite. The result may feel like a beesting, causing a sharp pain, then a swollen, red, painful lump at the bite site.

If you suspect black widow or brown recluse spider bite — either because you actually felt and saw the spider, or you’re experiencing the above symptoms — head to the emergency room.

Vetter says that recluse bites 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, Dr. Day recommends these steps:

Clean the bite with soap and water.
Swab the bite with alcohol to prevent infection.
Apply ice or a cold compress to keep the swelling down.

If pain or itching develops at the bite site, taking OTC pain meds or antihistamines for relief is fine. But if pain spreads beyond the bite site, or if the swelling, inflammation, or redness are getting worse even a day or two after the bite occurs, 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.

Vetter says spiders bite people only in 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. “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.

The Takeaway

You might think you've been bitten by a spider, but that red mark could be something else. Most spider bites to humans are rare, and the risky bites usually come from black widows and brown recluses, whose bites can be serious. If you notice flu-like symptoms or unusual pain, it's a good idea to seek medical attention.

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The Takeaway