The bite of the brown recluse spider (Loxosceles reclusa) can indeed, in a minority of cases, lead to local tissue necrosis (skin "rotting away"), but this is grossly exaggerated.

The brown recluse is mainly distributed in the south-central United States, and in China may only be found in the border regions of Yunnan.

Its venom contains sphingomyelinase D — this enzyme disrupts the integrity of cell membranes, leading to cell death.

About 90% of brown recluse bites result in only local redness, swelling, and mild pain, resolving on their own within 1–2 weeks without scarring.

Approximately 10% of bites develop into more severe "necrotic arachnidism" — 24–48 hours after the bite, a purple or black patch appears at the bite site, central tissue necrosis forms an ulcer, the surrounding area becomes red and swollen; the ulcer may take weeks to months to heal, and may leave a depressed scar after healing.

In very rare severe cases, the toxin can cause a systemic reaction (hemolytic anemia, kidney failure), but this is extremely rare (<1%).

Critical fact: The vast majority of cases diagnosed in the general community as "spider bite necrosis" are actually not spider bites — the more common cause is a bacterial infection (the appearance of a Methicillin-resistant Staphylococcus aureus (MRSA) skin infection is virtually indistinguishable from a necrotic spider bite).

If you do not live in the south-central United States and did not witness the spider biting you, your skin ulcer is overwhelmingly likely to be unrelated to the brown recluse spider.