Spiders are one of the hallmarks of Halloween. But did you know there is a spider living in North America with venom capable of destroying human flesh?

The brown recluse spider (Loxosceles reclusa) is native to the U.S. and has established itself in a number of states, including Alabama, Arkansas, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Mississippi, Missouri, Nebraska, Ohio, Oklahoma, Tennessee, and Texas.

The spider, which is considered among the most dangerous in North America, has also been spotted outside this range in other states. But these tend to be isolated cases where brown recluses have been inadvertently transported to these areas by humans.

This species is common within its range and can is often found in homes, but as the name suggests they tend to stay hidden and are not aggressive, Jerome Goddard, a professor of medical entomology at Mississippi State University’s Department of Biochemistry, Molecular Biology, Entomology, and Plant Pathology, told Newsweek.

As a result, brown recluse bites are relatively rare and when they do occur, it is usually because a spider is trapped against the skin and feels threatened, for example if someone rolls onto one while sleeping.

The venom of this spider can cause damage to local tissues and may produce a variety of symptoms. In many cases, the individual who is bitten experiences no notable effects.

“Brown recluse bite reactions may vary from no reaction at all, to a mild red wound, to a terrifying rotting flesh wound,” Goddard said.

The bite of brown recluse may feel like a pinprick and is usually painless until three to eight hours later, when it might become red, swollen and tender, according to Goddard.

The central area of a brown recluse bite eventually becomes pale or blue, not red, Goddard said. After 24 hours have passed, intense pain may develop. Later, a black scab may appear and, eventually, an area around the site may decay and slough away in a process known as “necrosis”—or death of body tissue—producing an ulcer.

Finally, the edges of the wound thicken and become raised, whereas the central area is filled by scar tissue. Healing may take months, and the victim could be left with a sunken scar.

“Their bites can produce nasty, slow-to-heal lesions that leave unsightly scars,” Goddard said.

The primary component of brown recluse venom that causes necrosis in the skin is likely an enzyme called sphingomyelinase D, which degrades fibrinogen (a clotting factor) and fibronectin (a protein that plays a role in tissue repair).

“Sphingomyelinase D also disrupts basement membrane structures, which act as a platform for cells to grow,” Goddard said. “All of this leads to local tissue death.”

The proportion of brown recluse bites that result in necrotic wounds is not entirely clear because many self-reported bites are actually something else, such as a staph infection, according to Goddard.

But the entomologist said his best estimate was that around 10 to 50 percent of brown recluse bites lead to necrosis in some form.

The lack of development of necrosis may be due to factors unique to the immune system of the individual that is bitten.

“Or like venomous snakes, perhaps brown recluse spiders may deliver ‘dry’ bites wherein they withhold or don’t inject much venom,” Goddard said.

In rare cases—perhaps less than one percent of incidents—brown recluse bites can lead to a potentially serious systemic illness roughly two to three days after the bite that affects the whole body.

This illness—known as “systemic loxoscelism”—is characterized by anemia, blood in the urine, fever, rash, nausea, vomiting and coma. In very rare cases, deaths have resulted from the systemic reaction of a brown recluse bite. The local necrotic wounds are not fatal.

The treatment of brown recluse bites is controversial and appears to be constantly changing, the Goddard said.

“A specific antidote—or antivenin—has shown success in patients prior to development of the necrotic lesion, but I don’t think it is widely available,” he said. “Also, some brown recluse bites are unremarkable, not leading to necrosis; therefore, treatment may not be needed in those cases.”

Some research has indicated that the application of ice to the bite site is effective. This may be because the necrotic enzyme sphingomyelinase D increases in activity as temperature rises.

At one time, early, total surgical excision of the bite site followed by skin-grafting was recommended. But more recent evidence no longer supports wound excision as a treatment, according to Goddard.

Some scientists and physicians have reported success in treating the individual with a medication known as dapsone. But some evidence shows that this drug is completely ineffective.