The University of Arizona

The Bite of the Brown Recluse

By Shelley Littin, University Communications | August 29, 2013
The spindly brown recluse spiders can deliver one of the most toxic bites to humans of any arachnid, potentially causing blackened lesions or a dangerous systemic reaction that can be fatal in extremely rare cases. (Image: Wikimedia Commons)
The spindly brown recluse spiders can deliver one of the most toxic bites to humans of any arachnid, potentially causing blackened lesions or a dangerous systemic reaction that can be fatal in extremely rare cases. (Image: Wikimedia Commons)

The UA's Dr. Leslie Boyer, recently named an FDA Hero of Rare Diseases, shares insights on what happens to the body when bitten by a brown recluse spider.

The solitary nature of brown recluse spiders means that bites from them are extremely rare. Most people likely don't have any reaction, even if they are bitten by a brown recluse, Boyer said. (Image: Wikimedia Commons)
The solitary nature of brown recluse spiders means that bites from them are extremely rare. Most people likely don't have any reaction, even if they are bitten by a brown recluse, Boyer said. (Image: Wikimedia Commons)

We've seen the images: blackened lesions on someone's forearm, or large, twisted scars where skin has been removed – the sometimes gruesome signs of a brown recluse spider bite.

But what really happens when these small, solitary arachnids bite you? How often do these bites occur? Are brown recluses, in the grand scheme of things to worry about, at the top of the list? Should they even make the list?

Dr. Leslie Boyer, a member of the UA BIO5 Institute and director of the University of Arizona's VIPER Institute, which stands for Venom Immunochemistry, Pharmacology and Emergency Response, explains.

Brown recluse spiders, which live mostly in the southwestern and midwestern United States, are aptly named. They are small, brown, have long legs extending to about the size of a quarter and sport a tell-tale violin pattern on their backs. They also are extremely reclusive. The spiders usually live in dark areas, like underneath floorboards or in crawlspaces, basements or piles of discarded clothing on the floor.

"When someone is bitten by a brown recluse, the first thing that happens is the venom goes out of the fangs and into the skin," said Boyer, who was recently named a Hero of Rare Diseases by the Food and Drug Administration and also 2013 AZ Bioscience Researcher of the Year.

"Some of the time, it will penetrate the outer layer of skin and then through the dermis into the subcutaneous tissues where there is fat and a lot of your blood vessels. It starts to soak through and it spreads out a little," Boyer said.

Brown recluse venom contains a chemical protein that attacks the lipid molecules that make up the membrane surrounding each cell. The protein clips off the molecules' heads, causing the normally straight molecules to bend into rings. This affects the body in ways that are not yet fully understood.

"That's a very potent chemical that can affect more than one type of cell," Boyer said. "It causes a lot of inflammation in addition to causing damage to cells, which means that when the body's immune system recognizes that chemicals are damaging cells, many parts of your immune system kick in with white blood cells and antibodies. As those swarm to the site of the wound, you develop swelling, redness and pain."

"In the case of brown spider bites, we think that the reaction that occurs in the body is caused in large part by the body's own immune system," Boyer said. "The inflammation is so nasty that the tiny blood vessels, the arterioles that nourish the area of the bite, actually squeeze closed."

The necrosis, or tissue death, is the next phase, Boyer said, although a black color doesn't necessarily mean necrosis.

"A bruise will turn black and blue because there are broken blood cells, but that doesn't mean it's dead. We can tell that a patient has true necrosis when the center of the lesion stops hurting. Living nerves have pain," Boyer said.

Necrotic wounds are treated on a case-by-case basis, Boyer said.

"We've learned that it's probably not wise to cut it out," she said, addinf that it's usually better to allow the body to form scar tissue under the wound, to avoid the added damage of removing skin from the site of the bite. 

In very rare circumstances, the body can launch a more drastic systemic response to a brown recluse spider bite, which can destroy blood cells and cause other damage, and can be fatal. Researchers believe there is about one death from a brown recluse bite every five or 10 years.

Scary as the symptoms appear, it is thought that only a small group of those who are bitten by a brown recluse develop any symptoms at all. Most likely, you won't even notice a bite, Boyer said.

So how concerning is the threat of a brown recluse bite? It definitely shouldn’t be on the top of your list, Boyer said.

"To avoid spider bites, pay attention to your environment, know which creatures live near you and carefully inspect old boxes or seldom-used closets before using things that have been untouched for a while," she advised. "And if you must reach into a place that you know is good habitat, never put your hands where you can't see them."